diff --git a/lib/compiler/build_runner.zig b/lib/compiler/build_runner.zig
index 523ef98824..b6d771302c 100644
--- a/lib/compiler/build_runner.zig
+++ b/lib/compiler/build_runner.zig
@@ -1,5 +1,8 @@
-const std = @import("std");
+const runner = @This();
 const builtin = @import("builtin");
+
+const std = @import("std");
+const Io = std.Io;
 const assert = std.debug.assert;
 const fmt = std.fmt;
 const mem = std.mem;
@@ -11,7 +14,6 @@ const WebServer = std.Build.WebServer;
 const Allocator = std.mem.Allocator;
 const fatal = std.process.fatal;
 const Writer = std.Io.Writer;
-const runner = @This();
 const tty = std.Io.tty;
 
 pub const root = @import("@build");
@@ -75,6 +77,7 @@ pub fn main() !void {
         .io = io,
         .arena = arena,
         .cache = .{
+            .io = io,
             .gpa = arena,
             .manifest_dir = try local_cache_directory.handle.makeOpenPath("h", .{}),
         },
@@ -84,7 +87,7 @@ pub fn main() !void {
         .zig_lib_directory = zig_lib_directory,
         .host = .{
             .query = .{},
-            .result = try std.zig.system.resolveTargetQuery(.{}),
+            .result = try std.zig.system.resolveTargetQuery(io, .{}),
         },
         .time_report = false,
     };
@@ -121,7 +124,7 @@ pub fn main() !void {
     var watch = false;
     var fuzz: ?std.Build.Fuzz.Mode = null;
     var debounce_interval_ms: u16 = 50;
-    var webui_listen: ?std.net.Address = null;
+    var webui_listen: ?Io.net.IpAddress = null;
 
     if (try std.zig.EnvVar.ZIG_BUILD_ERROR_STYLE.get(arena)) |str| {
         if (std.meta.stringToEnum(ErrorStyle, str)) |style| {
@@ -288,11 +291,11 @@ pub fn main() !void {
                     });
                 };
             } else if (mem.eql(u8, arg, "--webui")) {
-                webui_listen = std.net.Address.parseIp("::1", 0) catch unreachable;
+                if (webui_listen == null) webui_listen = .{ .ip6 = .loopback(0) };
             } else if (mem.startsWith(u8, arg, "--webui=")) {
                 const addr_str = arg["--webui=".len..];
                 if (std.mem.eql(u8, addr_str, "-")) fatal("web interface cannot listen on stdio", .{});
-                webui_listen = std.net.Address.parseIpAndPort(addr_str) catch |err| {
+                webui_listen = Io.net.IpAddress.parseLiteral(addr_str) catch |err| {
                     fatal("invalid web UI address '{s}': {s}", .{ addr_str, @errorName(err) });
                 };
             } else if (mem.eql(u8, arg, "--debug-log")) {
@@ -334,14 +337,10 @@ pub fn main() !void {
                 watch = true;
             } else if (mem.eql(u8, arg, "--time-report")) {
                 graph.time_report = true;
-                if (webui_listen == null) {
-                    webui_listen = std.net.Address.parseIp("::1", 0) catch unreachable;
-                }
+                if (webui_listen == null) webui_listen = .{ .ip6 = .loopback(0) };
             } else if (mem.eql(u8, arg, "--fuzz")) {
                 fuzz = .{ .forever = undefined };
-                if (webui_listen == null) {
-                    webui_listen = std.net.Address.parseIp("::1", 0) catch unreachable;
-                }
+                if (webui_listen == null) webui_listen = .{ .ip6 = .loopback(0) };
             } else if (mem.startsWith(u8, arg, "--fuzz=")) {
                 const value = arg["--fuzz=".len..];
                 if (value.len == 0) fatal("missing argument to --fuzz", .{});
@@ -550,13 +549,15 @@ pub fn main() !void {
 
     var w: Watch = w: {
         if (!watch) break :w undefined;
-        if (!Watch.have_impl) fatal("--watch not yet implemented for {s}", .{@tagName(builtin.os.tag)});
+        if (!Watch.have_impl) fatal("--watch not yet implemented for {t}", .{builtin.os.tag});
         break :w try .init();
     };
 
     try run.thread_pool.init(thread_pool_options);
     defer run.thread_pool.deinit();
 
+    const now = Io.Timestamp.now(io, .awake) catch |err| fatal("failed to collect timestamp: {t}", .{err});
+
     run.web_server = if (webui_listen) |listen_address| ws: {
         if (builtin.single_threaded) unreachable; // `fatal` above
         break :ws .init(.{
@@ -568,11 +569,12 @@ pub fn main() !void {
             .root_prog_node = main_progress_node,
             .watch = watch,
             .listen_address = listen_address,
+            .base_timestamp = now,
         });
     } else null;
 
     if (run.web_server) |*ws| {
-        ws.start() catch |err| fatal("failed to start web server: {s}", .{@errorName(err)});
+        ws.start() catch |err| fatal("failed to start web server: {t}", .{err});
     }
 
     rebuild: while (true) : (if (run.error_style.clearOnUpdate()) {
@@ -755,6 +757,7 @@ fn runStepNames(
     fuzz: ?std.Build.Fuzz.Mode,
 ) !void {
     const gpa = run.gpa;
+    const io = b.graph.io;
     const step_stack = &run.step_stack;
     const thread_pool = &run.thread_pool;
 
@@ -858,6 +861,7 @@ fn runStepNames(
         assert(mode == .limit);
         var f = std.Build.Fuzz.init(
             gpa,
+            io,
             thread_pool,
             step_stack.keys(),
             parent_prog_node,
diff --git a/lib/compiler/test_runner.zig b/lib/compiler/test_runner.zig
index 40f52fbd39..0d6f451947 100644
--- a/lib/compiler/test_runner.zig
+++ b/lib/compiler/test_runner.zig
@@ -2,6 +2,7 @@
 const builtin = @import("builtin");
 
 const std = @import("std");
+const Io = std.Io;
 const fatal = std.process.fatal;
 const testing = std.testing;
 const assert = std.debug.assert;
@@ -16,6 +17,7 @@ var fba: std.heap.FixedBufferAllocator = .init(&fba_buffer);
 var fba_buffer: [8192]u8 = undefined;
 var stdin_buffer: [4096]u8 = undefined;
 var stdout_buffer: [4096]u8 = undefined;
+var runner_threaded_io: Io.Threaded = .init_single_threaded;
 
 /// Keep in sync with logic in `std.Build.addRunArtifact` which decides whether
 /// the test runner will communicate with the build runner via `std.zig.Server`.
@@ -63,8 +65,6 @@ pub fn main() void {
         fuzz_abi.fuzzer_init(.fromSlice(cache_dir));
     }
 
-    fba.reset();
-
     if (listen) {
         return mainServer() catch @panic("internal test runner failure");
     } else {
@@ -74,7 +74,7 @@ pub fn main() void {
 
 fn mainServer() !void {
     @disableInstrumentation();
-    var stdin_reader = std.fs.File.stdin().readerStreaming(&stdin_buffer);
+    var stdin_reader = std.fs.File.stdin().readerStreaming(runner_threaded_io.io(), &stdin_buffer);
     var stdout_writer = std.fs.File.stdout().writerStreaming(&stdout_buffer);
     var server = try std.zig.Server.init(.{
         .in = &stdin_reader.interface,
@@ -131,7 +131,7 @@ fn mainServer() !void {
 
             .run_test => {
                 testing.allocator_instance = .{};
-                testing.io_instance = .init(fba.allocator());
+                testing.io_instance = .init(testing.allocator);
                 log_err_count = 0;
                 const index = try server.receiveBody_u32();
                 const test_fn = builtin.test_functions[index];
@@ -154,7 +154,6 @@ fn mainServer() !void {
                     },
                 };
                 testing.io_instance.deinit();
-                fba.reset();
                 const leak_count = testing.allocator_instance.detectLeaks();
                 testing.allocator_instance.deinitWithoutLeakChecks();
                 try server.serveTestResults(.{
@@ -234,10 +233,10 @@ fn mainTerminal() void {
     var leaks: usize = 0;
     for (test_fn_list, 0..) |test_fn, i| {
         testing.allocator_instance = .{};
-        testing.io_instance = .init(fba.allocator());
+        testing.io_instance = .init(testing.allocator);
         defer {
-            if (testing.allocator_instance.deinit() == .leak) leaks += 1;
             testing.io_instance.deinit();
+            if (testing.allocator_instance.deinit() == .leak) leaks += 1;
         }
         testing.log_level = .warn;
 
@@ -324,7 +323,7 @@ pub fn mainSimple() anyerror!void {
         .stage2_aarch64, .stage2_riscv64 => true,
         else => false,
     };
-    // is the backend capable of calling `std.Io.Writer.print`?
+    // is the backend capable of calling `Io.Writer.print`?
     const enable_print = switch (builtin.zig_backend) {
         .stage2_aarch64, .stage2_riscv64 => true,
         else => false,
diff --git a/lib/std/Build.zig b/lib/std/Build.zig
index d3df0c0d39..e9d2e81fba 100644
--- a/lib/std/Build.zig
+++ b/lib/std/Build.zig
@@ -1837,6 +1837,8 @@ pub fn runAllowFail(
     if (!process.can_spawn)
         return error.ExecNotSupported;
 
+    const io = b.graph.io;
+
     const max_output_size = 400 * 1024;
     var child = std.process.Child.init(argv, b.allocator);
     child.stdin_behavior = .Ignore;
@@ -1847,7 +1849,7 @@ pub fn runAllowFail(
     try Step.handleVerbose2(b, null, child.env_map, argv);
     try child.spawn();
 
-    var stdout_reader = child.stdout.?.readerStreaming(&.{});
+    var stdout_reader = child.stdout.?.readerStreaming(io, &.{});
     const stdout = stdout_reader.interface.allocRemaining(b.allocator, .limited(max_output_size)) catch {
         return error.ReadFailure;
     };
diff --git a/lib/std/Build/Cache.zig b/lib/std/Build/Cache.zig
index fe9714296d..8202c4dd15 100644
--- a/lib/std/Build/Cache.zig
+++ b/lib/std/Build/Cache.zig
@@ -3,8 +3,10 @@
 //! not to withstand attacks using specially-crafted input.
 
 const Cache = @This();
-const std = @import("std");
 const builtin = @import("builtin");
+
+const std = @import("std");
+const Io = std.Io;
 const crypto = std.crypto;
 const fs = std.fs;
 const assert = std.debug.assert;
@@ -15,6 +17,7 @@ const Allocator = std.mem.Allocator;
 const log = std.log.scoped(.cache);
 
 gpa: Allocator,
+io: Io,
 manifest_dir: fs.Dir,
 hash: HashHelper = .{},
 /// This value is accessed from multiple threads, protected by mutex.
@@ -661,9 +664,10 @@ pub const Manifest = struct {
         },
     } {
         const gpa = self.cache.gpa;
+        const io = self.cache.io;
         const input_file_count = self.files.entries.len;
         var tiny_buffer: [1]u8 = undefined; // allows allocRemaining to detect limit exceeded
-        var manifest_reader = self.manifest_file.?.reader(&tiny_buffer); // Reads positionally from zero.
+        var manifest_reader = self.manifest_file.?.reader(io, &tiny_buffer); // Reads positionally from zero.
         const limit: std.Io.Limit = .limited(manifest_file_size_max);
         const file_contents = manifest_reader.interface.allocRemaining(gpa, limit) catch |err| switch (err) {
             error.OutOfMemory => return error.OutOfMemory,
@@ -1337,7 +1341,8 @@ test "cache file and then recall it" {
     var digest2: HexDigest = undefined;
 
     {
-        var cache = Cache{
+        var cache: Cache = .{
+            .io = io,
             .gpa = testing.allocator,
             .manifest_dir = try tmp.dir.makeOpenPath(temp_manifest_dir, .{}),
         };
@@ -1402,7 +1407,8 @@ test "check that changing a file makes cache fail" {
     var digest2: HexDigest = undefined;
 
     {
-        var cache = Cache{
+        var cache: Cache = .{
+            .io = io,
             .gpa = testing.allocator,
             .manifest_dir = try tmp.dir.makeOpenPath(temp_manifest_dir, .{}),
         };
@@ -1451,6 +1457,8 @@ test "check that changing a file makes cache fail" {
 }
 
 test "no file inputs" {
+    const io = testing.io;
+
     var tmp = testing.tmpDir(.{});
     defer tmp.cleanup();
 
@@ -1459,7 +1467,8 @@ test "no file inputs" {
     var digest1: HexDigest = undefined;
     var digest2: HexDigest = undefined;
 
-    var cache = Cache{
+    var cache: Cache = .{
+        .io = io,
         .gpa = testing.allocator,
         .manifest_dir = try tmp.dir.makeOpenPath(temp_manifest_dir, .{}),
     };
@@ -1517,7 +1526,8 @@ test "Manifest with files added after initial hash work" {
     var digest3: HexDigest = undefined;
 
     {
-        var cache = Cache{
+        var cache: Cache = .{
+            .io = io,
             .gpa = testing.allocator,
             .manifest_dir = try tmp.dir.makeOpenPath(temp_manifest_dir, .{}),
         };
diff --git a/lib/std/Build/Fuzz.zig b/lib/std/Build/Fuzz.zig
index 8281fc4726..d342628871 100644
--- a/lib/std/Build/Fuzz.zig
+++ b/lib/std/Build/Fuzz.zig
@@ -1,4 +1,5 @@
 const std = @import("../std.zig");
+const Io = std.Io;
 const Build = std.Build;
 const Cache = Build.Cache;
 const Step = std.Build.Step;
@@ -14,6 +15,7 @@ const Fuzz = @This();
 const build_runner = @import("root");
 
 gpa: Allocator,
+io: Io,
 mode: Mode,
 
 /// Allocated into `gpa`.
@@ -75,6 +77,7 @@ const CoverageMap = struct {
 
 pub fn init(
     gpa: Allocator,
+    io: Io,
     thread_pool: *std.Thread.Pool,
     all_steps: []const *Build.Step,
     root_prog_node: std.Progress.Node,
@@ -111,6 +114,7 @@ pub fn init(
 
     return .{
         .gpa = gpa,
+        .io = io,
         .mode = mode,
         .run_steps = run_steps,
         .wait_group = .{},
@@ -484,6 +488,7 @@ fn addEntryPoint(fuzz: *Fuzz, coverage_id: u64, addr: u64) error{ AlreadyReporte
 
 pub fn waitAndPrintReport(fuzz: *Fuzz) void {
     assert(fuzz.mode == .limit);
+    const io = fuzz.io;
 
     fuzz.wait_group.wait();
     fuzz.wait_group.reset();
@@ -506,7 +511,7 @@ pub fn waitAndPrintReport(fuzz: *Fuzz) void {
 
         const fuzz_abi = std.Build.abi.fuzz;
         var rbuf: [0x1000]u8 = undefined;
-        var r = coverage_file.reader(&rbuf);
+        var r = coverage_file.reader(io, &rbuf);
 
         var header: fuzz_abi.SeenPcsHeader = undefined;
         r.interface.readSliceAll(std.mem.asBytes(&header)) catch |err| {
diff --git a/lib/std/Build/Step.zig b/lib/std/Build/Step.zig
index 72eb66e530..aa922ff37b 100644
--- a/lib/std/Build/Step.zig
+++ b/lib/std/Build/Step.zig
@@ -1,9 +1,11 @@
 const Step = @This();
+const builtin = @import("builtin");
+
 const std = @import("../std.zig");
+const Io = std.Io;
 const Build = std.Build;
 const Allocator = std.mem.Allocator;
 const assert = std.debug.assert;
-const builtin = @import("builtin");
 const Cache = Build.Cache;
 const Path = Cache.Path;
 const ArrayList = std.ArrayList;
@@ -327,7 +329,7 @@ pub fn cast(step: *Step, comptime T: type) ?*T {
 }
 
 /// For debugging purposes, prints identifying information about this Step.
-pub fn dump(step: *Step, w: *std.Io.Writer, tty_config: std.Io.tty.Config) void {
+pub fn dump(step: *Step, w: *Io.Writer, tty_config: Io.tty.Config) void {
     if (step.debug_stack_trace.instruction_addresses.len > 0) {
         w.print("name: '{s}'. creation stack trace:\n", .{step.name}) catch {};
         std.debug.writeStackTrace(&step.debug_stack_trace, w, tty_config) catch {};
@@ -382,7 +384,7 @@ pub fn addError(step: *Step, comptime fmt: []const u8, args: anytype) error{OutO
 
 pub const ZigProcess = struct {
     child: std.process.Child,
-    poller: std.Io.Poller(StreamEnum),
+    poller: Io.Poller(StreamEnum),
     progress_ipc_fd: if (std.Progress.have_ipc) ?std.posix.fd_t else void,
 
     pub const StreamEnum = enum { stdout, stderr };
@@ -458,7 +460,7 @@ pub fn evalZigProcess(
     const zp = try gpa.create(ZigProcess);
     zp.* = .{
         .child = child,
-        .poller = std.Io.poll(gpa, ZigProcess.StreamEnum, .{
+        .poller = Io.poll(gpa, ZigProcess.StreamEnum, .{
             .stdout = child.stdout.?,
             .stderr = child.stderr.?,
         }),
@@ -505,11 +507,12 @@ pub fn evalZigProcess(
 }
 
 /// Wrapper around `std.fs.Dir.updateFile` that handles verbose and error output.
-pub fn installFile(s: *Step, src_lazy_path: Build.LazyPath, dest_path: []const u8) !std.fs.Dir.PrevStatus {
+pub fn installFile(s: *Step, src_lazy_path: Build.LazyPath, dest_path: []const u8) !Io.Dir.PrevStatus {
     const b = s.owner;
+    const io = b.graph.io;
     const src_path = src_lazy_path.getPath3(b, s);
     try handleVerbose(b, null, &.{ "install", "-C", b.fmt("{f}", .{src_path}), dest_path });
-    return src_path.root_dir.handle.updateFile(src_path.sub_path, std.fs.cwd(), dest_path, .{}) catch |err| {
+    return Io.Dir.updateFile(src_path.root_dir.handle.adaptToNewApi(), io, src_path.sub_path, .cwd(), dest_path, .{}) catch |err| {
         return s.fail("unable to update file from '{f}' to '{s}': {s}", .{
             src_path, dest_path, @errorName(err),
         });
@@ -738,7 +741,7 @@ pub fn allocPrintCmd2(
     argv: []const []const u8,
 ) Allocator.Error![]u8 {
     const shell = struct {
-        fn escape(writer: *std.Io.Writer, string: []const u8, is_argv0: bool) !void {
+        fn escape(writer: *Io.Writer, string: []const u8, is_argv0: bool) !void {
             for (string) |c| {
                 if (switch (c) {
                     else => true,
@@ -772,7 +775,7 @@ pub fn allocPrintCmd2(
         }
     };
 
-    var aw: std.Io.Writer.Allocating = .init(gpa);
+    var aw: Io.Writer.Allocating = .init(gpa);
     defer aw.deinit();
     const writer = &aw.writer;
     if (opt_cwd) |cwd| writer.print("cd {s} && ", .{cwd}) catch return error.OutOfMemory;
diff --git a/lib/std/Build/Step/Options.zig b/lib/std/Build/Step/Options.zig
index d738b8edaa..8590fa9608 100644
--- a/lib/std/Build/Step/Options.zig
+++ b/lib/std/Build/Step/Options.zig
@@ -538,8 +538,10 @@ test Options {
     defer arena.deinit();
 
     var graph: std.Build.Graph = .{
+        .io = io,
         .arena = arena.allocator(),
         .cache = .{
+            .io = io,
             .gpa = arena.allocator(),
             .manifest_dir = std.fs.cwd(),
         },
diff --git a/lib/std/Build/Step/Run.zig b/lib/std/Build/Step/Run.zig
index cd29262612..314862e201 100644
--- a/lib/std/Build/Step/Run.zig
+++ b/lib/std/Build/Step/Run.zig
@@ -761,6 +761,7 @@ const IndexedOutput = struct {
 };
 fn make(step: *Step, options: Step.MakeOptions) !void {
     const b = step.owner;
+    const io = b.graph.io;
     const arena = b.allocator;
     const run: *Run = @fieldParentPtr("step", step);
     const has_side_effects = run.hasSideEffects();
@@ -834,7 +835,7 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
                 defer file.close();
 
                 var buf: [1024]u8 = undefined;
-                var file_reader = file.reader(&buf);
+                var file_reader = file.reader(io, &buf);
                 _ = file_reader.interface.streamRemaining(&result.writer) catch |err| switch (err) {
                     error.ReadFailed => return step.fail(
                         "failed to read from '{f}': {t}",
@@ -1067,6 +1068,7 @@ pub fn rerunInFuzzMode(
 ) !void {
     const step = &run.step;
     const b = step.owner;
+    const io = b.graph.io;
     const arena = b.allocator;
     var argv_list: std.ArrayList([]const u8) = .empty;
     for (run.argv.items) |arg| {
@@ -1093,7 +1095,7 @@ pub fn rerunInFuzzMode(
                 defer file.close();
 
                 var buf: [1024]u8 = undefined;
-                var file_reader = file.reader(&buf);
+                var file_reader = file.reader(io, &buf);
                 _ = file_reader.interface.streamRemaining(&result.writer) catch |err| switch (err) {
                     error.ReadFailed => return file_reader.err.?,
                     error.WriteFailed => return error.OutOfMemory,
@@ -2090,6 +2092,7 @@ fn sendRunFuzzTestMessage(
 
 fn evalGeneric(run: *Run, child: *std.process.Child) !EvalGenericResult {
     const b = run.step.owner;
+    const io = b.graph.io;
     const arena = b.allocator;
 
     try child.spawn();
@@ -2113,7 +2116,7 @@ fn evalGeneric(run: *Run, child: *std.process.Child) !EvalGenericResult {
             defer file.close();
             // TODO https://github.com/ziglang/zig/issues/23955
             var read_buffer: [1024]u8 = undefined;
-            var file_reader = file.reader(&read_buffer);
+            var file_reader = file.reader(io, &read_buffer);
             var write_buffer: [1024]u8 = undefined;
             var stdin_writer = child.stdin.?.writer(&write_buffer);
             _ = stdin_writer.interface.sendFileAll(&file_reader, .unlimited) catch |err| switch (err) {
@@ -2159,7 +2162,7 @@ fn evalGeneric(run: *Run, child: *std.process.Child) !EvalGenericResult {
             stdout_bytes = try poller.toOwnedSlice(.stdout);
             stderr_bytes = try poller.toOwnedSlice(.stderr);
         } else {
-            var stdout_reader = stdout.readerStreaming(&.{});
+            var stdout_reader = stdout.readerStreaming(io, &.{});
             stdout_bytes = stdout_reader.interface.allocRemaining(arena, run.stdio_limit) catch |err| switch (err) {
                 error.OutOfMemory => return error.OutOfMemory,
                 error.ReadFailed => return stdout_reader.err.?,
@@ -2167,7 +2170,7 @@ fn evalGeneric(run: *Run, child: *std.process.Child) !EvalGenericResult {
             };
         }
     } else if (child.stderr) |stderr| {
-        var stderr_reader = stderr.readerStreaming(&.{});
+        var stderr_reader = stderr.readerStreaming(io, &.{});
         stderr_bytes = stderr_reader.interface.allocRemaining(arena, run.stdio_limit) catch |err| switch (err) {
             error.OutOfMemory => return error.OutOfMemory,
             error.ReadFailed => return stderr_reader.err.?,
diff --git a/lib/std/Build/Step/UpdateSourceFiles.zig b/lib/std/Build/Step/UpdateSourceFiles.zig
index 674e2a01c6..6f1559bd68 100644
--- a/lib/std/Build/Step/UpdateSourceFiles.zig
+++ b/lib/std/Build/Step/UpdateSourceFiles.zig
@@ -3,11 +3,13 @@
 //! not be used during the normal build process, but as a utility run by a
 //! developer with intention to update source files, which will then be
 //! committed to version control.
+const UpdateSourceFiles = @This();
+
 const std = @import("std");
+const Io = std.Io;
 const Step = std.Build.Step;
 const fs = std.fs;
 const ArrayList = std.ArrayList;
-const UpdateSourceFiles = @This();
 
 step: Step,
 output_source_files: std.ArrayListUnmanaged(OutputSourceFile),
@@ -70,22 +72,21 @@ pub fn addBytesToSource(usf: *UpdateSourceFiles, bytes: []const u8, sub_path: []
 fn make(step: *Step, options: Step.MakeOptions) !void {
     _ = options;
     const b = step.owner;
+    const io = b.graph.io;
     const usf: *UpdateSourceFiles = @fieldParentPtr("step", step);
 
     var any_miss = false;
     for (usf.output_source_files.items) |output_source_file| {
         if (fs.path.dirname(output_source_file.sub_path)) |dirname| {
             b.build_root.handle.makePath(dirname) catch |err| {
-                return step.fail("unable to make path '{f}{s}': {s}", .{
-                    b.build_root, dirname, @errorName(err),
-                });
+                return step.fail("unable to make path '{f}{s}': {t}", .{ b.build_root, dirname, err });
             };
         }
         switch (output_source_file.contents) {
             .bytes => |bytes| {
                 b.build_root.handle.writeFile(.{ .sub_path = output_source_file.sub_path, .data = bytes }) catch |err| {
-                    return step.fail("unable to write file '{f}{s}': {s}", .{
-                        b.build_root, output_source_file.sub_path, @errorName(err),
+                    return step.fail("unable to write file '{f}{s}': {t}", .{
+                        b.build_root, output_source_file.sub_path, err,
                     });
                 };
                 any_miss = true;
@@ -94,15 +95,16 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
                 if (!step.inputs.populated()) try step.addWatchInput(file_source);
 
                 const source_path = file_source.getPath2(b, step);
-                const prev_status = fs.Dir.updateFile(
-                    fs.cwd(),
+                const prev_status = Io.Dir.updateFile(
+                    .cwd(),
+                    io,
                     source_path,
-                    b.build_root.handle,
+                    b.build_root.handle.adaptToNewApi(),
                     output_source_file.sub_path,
                     .{},
                 ) catch |err| {
-                    return step.fail("unable to update file from '{s}' to '{f}{s}': {s}", .{
-                        source_path, b.build_root, output_source_file.sub_path, @errorName(err),
+                    return step.fail("unable to update file from '{s}' to '{f}{s}': {t}", .{
+                        source_path, b.build_root, output_source_file.sub_path, err,
                     });
                 };
                 any_miss = any_miss or prev_status == .stale;
diff --git a/lib/std/Build/Step/WriteFile.zig b/lib/std/Build/Step/WriteFile.zig
index b1cfb3b42a..1a531604e3 100644
--- a/lib/std/Build/Step/WriteFile.zig
+++ b/lib/std/Build/Step/WriteFile.zig
@@ -2,6 +2,7 @@
 //! the local cache which has a set of files that have either been generated
 //! during the build, or are copied from the source package.
 const std = @import("std");
+const Io = std.Io;
 const Step = std.Build.Step;
 const fs = std.fs;
 const ArrayList = std.ArrayList;
@@ -174,6 +175,7 @@ fn maybeUpdateName(write_file: *WriteFile) void {
 fn make(step: *Step, options: Step.MakeOptions) !void {
     _ = options;
     const b = step.owner;
+    const io = b.graph.io;
     const arena = b.allocator;
     const gpa = arena;
     const write_file: *WriteFile = @fieldParentPtr("step", step);
@@ -264,40 +266,27 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
     };
     defer cache_dir.close();
 
-    const cwd = fs.cwd();
-
     for (write_file.files.items) |file| {
         if (fs.path.dirname(file.sub_path)) |dirname| {
             cache_dir.makePath(dirname) catch |err| {
-                return step.fail("unable to make path '{f}{s}{c}{s}': {s}", .{
-                    b.cache_root, cache_path, fs.path.sep, dirname, @errorName(err),
+                return step.fail("unable to make path '{f}{s}{c}{s}': {t}", .{
+                    b.cache_root, cache_path, fs.path.sep, dirname, err,
                 });
             };
         }
         switch (file.contents) {
             .bytes => |bytes| {
                 cache_dir.writeFile(.{ .sub_path = file.sub_path, .data = bytes }) catch |err| {
-                    return step.fail("unable to write file '{f}{s}{c}{s}': {s}", .{
-                        b.cache_root, cache_path, fs.path.sep, file.sub_path, @errorName(err),
+                    return step.fail("unable to write file '{f}{s}{c}{s}': {t}", .{
+                        b.cache_root, cache_path, fs.path.sep, file.sub_path, err,
                     });
                 };
             },
             .copy => |file_source| {
                 const source_path = file_source.getPath2(b, step);
-                const prev_status = fs.Dir.updateFile(
-                    cwd,
-                    source_path,
-                    cache_dir,
-                    file.sub_path,
-                    .{},
-                ) catch |err| {
-                    return step.fail("unable to update file from '{s}' to '{f}{s}{c}{s}': {s}", .{
-                        source_path,
-                        b.cache_root,
-                        cache_path,
-                        fs.path.sep,
-                        file.sub_path,
-                        @errorName(err),
+                const prev_status = Io.Dir.updateFile(.cwd(), io, source_path, cache_dir.adaptToNewApi(), file.sub_path, .{}) catch |err| {
+                    return step.fail("unable to update file from '{s}' to '{f}{s}{c}{s}': {t}", .{
+                        source_path, b.cache_root, cache_path, fs.path.sep, file.sub_path, err,
                     });
                 };
                 // At this point we already will mark the step as a cache miss.
@@ -331,10 +320,11 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
             switch (entry.kind) {
                 .directory => try cache_dir.makePath(dest_path),
                 .file => {
-                    const prev_status = fs.Dir.updateFile(
-                        src_entry_path.root_dir.handle,
+                    const prev_status = Io.Dir.updateFile(
+                        src_entry_path.root_dir.handle.adaptToNewApi(),
+                        io,
                         src_entry_path.sub_path,
-                        cache_dir,
+                        cache_dir.adaptToNewApi(),
                         dest_path,
                         .{},
                     ) catch |err| {
diff --git a/lib/std/Build/WebServer.zig b/lib/std/Build/WebServer.zig
index 135fbe7f0a..95338c9f08 100644
--- a/lib/std/Build/WebServer.zig
+++ b/lib/std/Build/WebServer.zig
@@ -3,14 +3,15 @@ thread_pool: *std.Thread.Pool,
 graph: *const Build.Graph,
 all_steps: []const *Build.Step,
 listen_address: net.IpAddress,
-ttyconf: std.Io.tty.Config,
+ttyconf: Io.tty.Config,
 root_prog_node: std.Progress.Node,
 watch: bool,
 
 tcp_server: ?net.Server,
 serve_thread: ?std.Thread,
 
-base_timestamp: i128,
+/// Uses `Io.Clock.awake`.
+base_timestamp: i96,
 /// The "step name" data which trails `abi.Hello`, for the steps in `all_steps`.
 step_names_trailing: []u8,
 
@@ -53,15 +54,17 @@ pub const Options = struct {
     thread_pool: *std.Thread.Pool,
     graph: *const std.Build.Graph,
     all_steps: []const *Build.Step,
-    ttyconf: std.Io.tty.Config,
+    ttyconf: Io.tty.Config,
     root_prog_node: std.Progress.Node,
     watch: bool,
     listen_address: net.IpAddress,
+    base_timestamp: Io.Timestamp,
 };
 pub fn init(opts: Options) WebServer {
-    // The upcoming `std.Io` interface should allow us to use `Io.async` and `Io.concurrent`
+    // The upcoming `Io` interface should allow us to use `Io.async` and `Io.concurrent`
     // instead of threads, so that the web server can function in single-threaded builds.
     comptime assert(!builtin.single_threaded);
+    assert(opts.base_timestamp.clock == .awake);
 
     const all_steps = opts.all_steps;
 
@@ -106,7 +109,7 @@ pub fn init(opts: Options) WebServer {
         .tcp_server = null,
         .serve_thread = null,
 
-        .base_timestamp = std.time.nanoTimestamp(),
+        .base_timestamp = opts.base_timestamp.nanoseconds,
         .step_names_trailing = step_names_trailing,
 
         .step_status_bits = step_status_bits,
@@ -147,32 +150,34 @@ pub fn deinit(ws: *WebServer) void {
 pub fn start(ws: *WebServer) error{AlreadyReported}!void {
     assert(ws.tcp_server == null);
     assert(ws.serve_thread == null);
+    const io = ws.graph.io;
 
-    ws.tcp_server = ws.listen_address.listen(.{ .reuse_address = true }) catch |err| {
+    ws.tcp_server = ws.listen_address.listen(io, .{ .reuse_address = true }) catch |err| {
         log.err("failed to listen to port {d}: {s}", .{ ws.listen_address.getPort(), @errorName(err) });
         return error.AlreadyReported;
     };
     ws.serve_thread = std.Thread.spawn(.{}, serve, .{ws}) catch |err| {
         log.err("unable to spawn web server thread: {s}", .{@errorName(err)});
-        ws.tcp_server.?.deinit();
+        ws.tcp_server.?.deinit(io);
         ws.tcp_server = null;
         return error.AlreadyReported;
     };
 
-    log.info("web interface listening at http://{f}/", .{ws.tcp_server.?.listen_address});
+    log.info("web interface listening at http://{f}/", .{ws.tcp_server.?.socket.address});
     if (ws.listen_address.getPort() == 0) {
-        log.info("hint: pass '--webui={f}' to use the same port next time", .{ws.tcp_server.?.listen_address});
+        log.info("hint: pass '--webui={f}' to use the same port next time", .{ws.tcp_server.?.socket.address});
     }
 }
 fn serve(ws: *WebServer) void {
+    const io = ws.graph.io;
     while (true) {
-        const connection = ws.tcp_server.?.accept() catch |err| {
+        var stream = ws.tcp_server.?.accept(io) catch |err| {
             log.err("failed to accept connection: {s}", .{@errorName(err)});
             return;
         };
-        _ = std.Thread.spawn(.{}, accept, .{ ws, connection }) catch |err| {
+        _ = std.Thread.spawn(.{}, accept, .{ ws, stream }) catch |err| {
             log.err("unable to spawn connection thread: {s}", .{@errorName(err)});
-            connection.stream.close();
+            stream.close(io);
             continue;
         };
     }
@@ -227,6 +232,7 @@ pub fn finishBuild(ws: *WebServer, opts: struct {
 
         ws.fuzz = Fuzz.init(
             ws.gpa,
+            ws.graph.io,
             ws.thread_pool,
             ws.all_steps,
             ws.root_prog_node,
@@ -241,17 +247,25 @@ pub fn finishBuild(ws: *WebServer, opts: struct {
 }
 
 pub fn now(s: *const WebServer) i64 {
-    return @intCast(std.time.nanoTimestamp() - s.base_timestamp);
+    const io = s.graph.io;
+    const base: Io.Timestamp = .{ .nanoseconds = s.base_timestamp, .clock = .awake };
+    const ts = Io.Timestamp.now(io, base.clock) catch base;
+    return @intCast(base.durationTo(ts).toNanoseconds());
 }
 
-fn accept(ws: *WebServer, connection: net.Server.Connection) void {
-    defer connection.stream.close();
-
+fn accept(ws: *WebServer, stream: net.Stream) void {
+    const io = ws.graph.io;
+    defer {
+        // `net.Stream.close` wants to helpfully overwrite `stream` with
+        // `undefined`, but it cannot do so since it is an immutable parameter.
+        var copy = stream;
+        copy.close(io);
+    }
     var send_buffer: [4096]u8 = undefined;
     var recv_buffer: [4096]u8 = undefined;
-    var connection_reader = connection.stream.reader(&recv_buffer);
-    var connection_writer = connection.stream.writer(&send_buffer);
-    var server: http.Server = .init(connection_reader.interface(), &connection_writer.interface);
+    var connection_reader = stream.reader(io, &recv_buffer);
+    var connection_writer = stream.writer(io, &send_buffer);
+    var server: http.Server = .init(&connection_reader.interface, &connection_writer.interface);
 
     while (true) {
         var request = server.receiveHead() catch |err| switch (err) {
@@ -466,12 +480,9 @@ pub fn serveFile(
         },
     });
 }
-pub fn serveTarFile(
-    ws: *WebServer,
-    request: *http.Server.Request,
-    paths: []const Cache.Path,
-) !void {
+pub fn serveTarFile(ws: *WebServer, request: *http.Server.Request, paths: []const Cache.Path) !void {
     const gpa = ws.gpa;
+    const io = ws.graph.io;
 
     var send_buffer: [0x4000]u8 = undefined;
     var response = try request.respondStreaming(&send_buffer, .{
@@ -496,7 +507,7 @@ pub fn serveTarFile(
         defer file.close();
         const stat = try file.stat();
         var read_buffer: [1024]u8 = undefined;
-        var file_reader: std.fs.File.Reader = .initSize(file, &read_buffer, stat.size);
+        var file_reader: Io.File.Reader = .initSize(file.adaptToNewApi(), io, &read_buffer, stat.size);
 
         // TODO: this logic is completely bogus -- obviously so, because `path.root_dir.path` can
         // be cwd-relative. This is also related to why linkification doesn't work in the fuzzer UI:
@@ -566,7 +577,7 @@ fn buildClientWasm(ws: *WebServer, arena: Allocator, optimize: std.builtin.Optim
     child.stderr_behavior = .Pipe;
     try child.spawn();
 
-    var poller = std.Io.poll(gpa, enum { stdout, stderr }, .{
+    var poller = Io.poll(gpa, enum { stdout, stderr }, .{
         .stdout = child.stdout.?,
         .stderr = child.stderr.?,
     });
@@ -842,7 +853,10 @@ const cache_control_header: http.Header = .{
 };
 
 const builtin = @import("builtin");
+
 const std = @import("std");
+const Io = std.Io;
+const net = std.Io.net;
 const assert = std.debug.assert;
 const mem = std.mem;
 const log = std.log.scoped(.web_server);
@@ -852,6 +866,5 @@ const Cache = Build.Cache;
 const Fuzz = Build.Fuzz;
 const abi = Build.abi;
 const http = std.http;
-const net = std.Io.net;
 
 const WebServer = @This();
diff --git a/lib/std/Io.zig b/lib/std/Io.zig
index 5cdb9b0f01..e569c4ec94 100644
--- a/lib/std/Io.zig
+++ b/lib/std/Io.zig
@@ -654,6 +654,10 @@ pub const VTable = struct {
     conditionWait: *const fn (?*anyopaque, cond: *Condition, mutex: *Mutex) Cancelable!void,
     conditionWake: *const fn (?*anyopaque, cond: *Condition, wake: Condition.Wake) void,
 
+    dirMake: *const fn (?*anyopaque, dir: Dir, sub_path: []const u8, mode: Dir.Mode) Dir.MakeError!void,
+    dirStat: *const fn (?*anyopaque, dir: Dir) Dir.StatError!Dir.Stat,
+    dirStatPath: *const fn (?*anyopaque, dir: Dir, sub_path: []const u8) Dir.StatError!File.Stat,
+    fileStat: *const fn (?*anyopaque, file: File) File.StatError!File.Stat,
     createFile: *const fn (?*anyopaque, dir: Dir, sub_path: []const u8, flags: File.CreateFlags) File.OpenError!File,
     fileOpen: *const fn (?*anyopaque, dir: Dir, sub_path: []const u8, flags: File.OpenFlags) File.OpenError!File,
     fileClose: *const fn (?*anyopaque, File) void,
@@ -804,6 +808,10 @@ pub const Timestamp = struct {
         assert(lhs.clock == rhs.clock);
         return std.math.compare(lhs.nanoseconds, op, rhs.nanoseconds);
     }
+
+    pub fn toSeconds(t: Timestamp) i64 {
+        return @intCast(@divTrunc(t.nanoseconds, std.time.ns_per_s));
+    }
 };
 
 pub const Duration = struct {
@@ -831,6 +839,10 @@ pub const Duration = struct {
         return @intCast(@divTrunc(d.nanoseconds, std.time.ns_per_s));
     }
 
+    pub fn toNanoseconds(d: Duration) i96 {
+        return d.nanoseconds;
+    }
+
     pub fn sleep(duration: Duration, io: Io) SleepError!void {
         return io.vtable.sleep(io.userdata, .{ .duration = .{ .duration = duration, .clock = .awake } });
     }
diff --git a/lib/std/Io/Dir.zig b/lib/std/Io/Dir.zig
index a2ae96210e..3c9772076b 100644
--- a/lib/std/Io/Dir.zig
+++ b/lib/std/Io/Dir.zig
@@ -6,6 +6,9 @@ const File = Io.File;
 
 handle: Handle,
 
+pub const Mode = Io.File.Mode;
+pub const default_mode: Mode = 0o755;
+
 pub fn cwd() Dir {
     return .{ .handle = std.fs.cwd().fd };
 }
@@ -47,8 +50,9 @@ pub const UpdateFileError = File.OpenError;
 
 /// Check the file size, mtime, and mode of `source_path` and `dest_path`. If
 /// they are equal, does nothing. Otherwise, atomically copies `source_path` to
-/// `dest_path`. The destination file gains the mtime, atime, and mode of the
-/// source file so that the next call to `updateFile` will not need a copy.
+/// `dest_path`, creating the parent directory hierarchy as needed. The
+/// destination file gains the mtime, atime, and mode of the source file so
+/// that the next call to `updateFile` will not need a copy.
 ///
 /// Returns the previous status of the file before updating.
 ///
@@ -65,7 +69,7 @@ pub fn updateFile(
     options: std.fs.Dir.CopyFileOptions,
 ) !PrevStatus {
     var src_file = try source_dir.openFile(io, source_path, .{});
-    defer src_file.close();
+    defer src_file.close(io);
 
     const src_stat = try src_file.stat(io);
     const actual_mode = options.override_mode orelse src_stat.mode;
@@ -93,13 +97,13 @@ pub fn updateFile(
     }
 
     var buffer: [1000]u8 = undefined; // Used only when direct fd-to-fd is not available.
-    var atomic_file = try dest_dir.atomicFile(io, dest_path, .{
+    var atomic_file = try std.fs.Dir.atomicFile(.adaptFromNewApi(dest_dir), dest_path, .{
         .mode = actual_mode,
         .write_buffer = &buffer,
     });
     defer atomic_file.deinit();
 
-    var src_reader: File.Reader = .initSize(io, src_file, &.{}, src_stat.size);
+    var src_reader: File.Reader = .initSize(src_file, io, &.{}, src_stat.size);
     const dest_writer = &atomic_file.file_writer.interface;
 
     _ = dest_writer.sendFileAll(&src_reader, .unlimited) catch |err| switch (err) {
@@ -111,3 +115,154 @@ pub fn updateFile(
     try atomic_file.renameIntoPlace();
     return .stale;
 }
+
+pub const ReadFileError = File.OpenError || File.Reader.Error;
+
+/// Read all of file contents using a preallocated buffer.
+///
+/// The returned slice has the same pointer as `buffer`. If the length matches `buffer.len`
+/// the situation is ambiguous. It could either mean that the entire file was read, and
+/// it exactly fits the buffer, or it could mean the buffer was not big enough for the
+/// entire file.
+///
+/// * On Windows, `file_path` should be encoded as [WTF-8](https://simonsapin.github.io/wtf-8/).
+/// * On WASI, `file_path` should be encoded as valid UTF-8.
+/// * On other platforms, `file_path` is an opaque sequence of bytes with no particular encoding.
+pub fn readFile(dir: Dir, io: Io, file_path: []const u8, buffer: []u8) ReadFileError![]u8 {
+    var file = try dir.openFile(io, file_path, .{});
+    defer file.close(io);
+
+    var reader = file.reader(io, &.{});
+    const n = reader.interface.readSliceShort(buffer) catch |err| switch (err) {
+        error.ReadFailed => return reader.err.?,
+    };
+
+    return buffer[0..n];
+}
+
+pub const MakeError = error{
+    /// In WASI, this error may occur when the file descriptor does
+    /// not hold the required rights to create a new directory relative to it.
+    AccessDenied,
+    PermissionDenied,
+    DiskQuota,
+    PathAlreadyExists,
+    SymLinkLoop,
+    LinkQuotaExceeded,
+    NameTooLong,
+    FileNotFound,
+    SystemResources,
+    NoSpaceLeft,
+    NotDir,
+    ReadOnlyFileSystem,
+    /// WASI-only; file paths must be valid UTF-8.
+    InvalidUtf8,
+    /// Windows-only; file paths provided by the user must be valid WTF-8.
+    /// https://simonsapin.github.io/wtf-8/
+    InvalidWtf8,
+    BadPathName,
+    NoDevice,
+    /// On Windows, `\\server` or `\\server\share` was not found.
+    NetworkNotFound,
+} || Io.Cancelable || Io.UnexpectedError;
+
+/// Creates a single directory with a relative or absolute path.
+///
+/// * On Windows, `sub_path` should be encoded as [WTF-8](https://simonsapin.github.io/wtf-8/).
+/// * On WASI, `sub_path` should be encoded as valid UTF-8.
+/// * On other platforms, `sub_path` is an opaque sequence of bytes with no particular encoding.
+///
+/// Related:
+/// * `makePath`
+/// * `makeDirAbsolute`
+pub fn makeDir(dir: Dir, io: Io, sub_path: []const u8) MakeError!void {
+    return io.vtable.dirMake(io.userdata, dir, sub_path, default_mode);
+}
+
+pub const MakePathError = MakeError || StatPathError;
+
+/// Calls makeDir iteratively to make an entire path, creating any parent
+/// directories that do not exist.
+///
+/// Returns success if the path already exists and is a directory.
+///
+/// This function is not atomic, and if it returns an error, the file system
+/// may have been modified regardless.
+///
+/// Fails on an empty path with `error.BadPathName` as that is not a path that
+/// can be created.
+///
+/// On Windows, `sub_path` should be encoded as [WTF-8](https://simonsapin.github.io/wtf-8/).
+/// On WASI, `sub_path` should be encoded as valid UTF-8.
+/// On other platforms, `sub_path` is an opaque sequence of bytes with no particular encoding.
+///
+/// Paths containing `..` components are handled differently depending on the platform:
+/// - On Windows, `..` are resolved before the path is passed to NtCreateFile, meaning
+///   a `sub_path` like "first/../second" will resolve to "second" and only a
+///   `./second` directory will be created.
+/// - On other platforms, `..` are not resolved before the path is passed to `mkdirat`,
+///   meaning a `sub_path` like "first/../second" will create both a `./first`
+///   and a `./second` directory.
+pub fn makePath(dir: Dir, io: Io, sub_path: []const u8) MakePathError!void {
+    _ = try makePathStatus(dir, io, sub_path);
+}
+
+pub const MakePathStatus = enum { existed, created };
+
+/// Same as `makePath` except returns whether the path already existed or was
+/// successfully created.
+pub fn makePathStatus(dir: Dir, io: Io, sub_path: []const u8) MakePathError!MakePathStatus {
+    var it = try std.fs.path.componentIterator(sub_path);
+    var status: MakePathStatus = .existed;
+    var component = it.last() orelse return error.BadPathName;
+    while (true) {
+        if (makeDir(dir, io, component.path)) |_| {
+            status = .created;
+        } else |err| switch (err) {
+            error.PathAlreadyExists => {
+                // stat the file and return an error if it's not a directory
+                // this is important because otherwise a dangling symlink
+                // could cause an infinite loop
+                check_dir: {
+                    // workaround for windows, see https://github.com/ziglang/zig/issues/16738
+                    const fstat = statPath(dir, io, component.path) catch |stat_err| switch (stat_err) {
+                        error.IsDir => break :check_dir,
+                        else => |e| return e,
+                    };
+                    if (fstat.kind != .directory) return error.NotDir;
+                }
+            },
+            error.FileNotFound => |e| {
+                component = it.previous() orelse return e;
+                continue;
+            },
+            else => |e| return e,
+        }
+        component = it.next() orelse return status;
+    }
+}
+
+pub const Stat = File.Stat;
+pub const StatError = File.StatError;
+
+pub fn stat(dir: Dir, io: Io) StatError!Stat {
+    return io.vtable.dirStat(io.userdata, dir);
+}
+
+pub const StatPathError = File.OpenError || File.StatError;
+
+/// Returns metadata for a file inside the directory.
+///
+/// On Windows, this requires three syscalls. On other operating systems, it
+/// only takes one.
+///
+/// Symlinks are followed.
+///
+/// `sub_path` may be absolute, in which case `self` is ignored.
+///
+/// * On Windows, `sub_path` should be encoded as [WTF-8](https://simonsapin.github.io/wtf-8/).
+/// * On WASI, `sub_path` should be encoded as valid UTF-8.
+/// * On other platforms, `sub_path` is an opaque sequence of bytes with no particular encoding.
+pub fn statPath(dir: Dir, io: Io, sub_path: []const u8) StatPathError!File.Stat {
+    return io.vtable.dirStatPath(io.userdata, dir, sub_path);
+}
diff --git a/lib/std/Io/File.zig b/lib/std/Io/File.zig
index 242d96d52f..7d87fab973 100644
--- a/lib/std/Io/File.zig
+++ b/lib/std/Io/File.zig
@@ -446,7 +446,11 @@ pub const Reader = struct {
 
     fn stream(io_reader: *Io.Reader, w: *Io.Writer, limit: Io.Limit) Io.Reader.StreamError!usize {
         const r: *Reader = @alignCast(@fieldParentPtr("interface", io_reader));
-        switch (r.mode) {
+        return streamMode(r, w, limit, r.mode);
+    }
+
+    pub fn streamMode(r: *Reader, w: *Io.Writer, limit: Io.Limit, mode: Reader.Mode) Io.Reader.StreamError!usize {
+        switch (mode) {
             .positional, .streaming => return w.sendFile(r, limit) catch |write_err| switch (write_err) {
                 error.Unimplemented => {
                     r.mode = r.mode.toReading();
diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
index 0727730b93..0f217c7f80 100644
--- a/lib/std/Io/Threaded.zig
+++ b/lib/std/Io/Threaded.zig
@@ -63,7 +63,17 @@ const Closure = struct {
 
 pub const InitError = std.Thread.CpuCountError || Allocator.Error;
 
-pub fn init(gpa: Allocator) Pool {
+/// Related:
+/// * `init_single_threaded`
+pub fn init(
+    /// Must be threadsafe. Only used for the following functions:
+    /// * `Io.VTable.async`
+    /// * `Io.VTable.concurrent`
+    /// * `Io.VTable.groupAsync`
+    /// If these functions are avoided, then `Allocator.failing` may be passed
+    /// here.
+    gpa: Allocator,
+) Pool {
     var pool: Pool = .{
         .allocator = gpa,
         .threads = .empty,
@@ -77,6 +87,20 @@ pub fn init(gpa: Allocator) Pool {
     return pool;
 }
 
+/// Statically initialize such that any call to the following functions will
+/// fail with `error.OutOfMemory`:
+/// * `Io.VTable.async`
+/// * `Io.VTable.concurrent`
+/// * `Io.VTable.groupAsync`
+/// When initialized this way, `deinit` is safe, but unnecessary to call.
+pub const init_single_threaded: Pool = .{
+    .allocator = .failing,
+    .threads = .empty,
+    .stack_size = std.Thread.SpawnConfig.default_stack_size,
+    .cpu_count = 1,
+    .concurrent_count = 0,
+};
+
 pub fn deinit(pool: *Pool) void {
     const gpa = pool.allocator;
     pool.join();
@@ -136,6 +160,10 @@ pub fn io(pool: *Pool) Io {
             .conditionWait = conditionWait,
             .conditionWake = conditionWake,
 
+            .dirMake = dirMake,
+            .dirStat = dirStat,
+            .dirStatPath = dirStatPath,
+            .fileStat = fileStat,
             .createFile = createFile,
             .fileOpen = fileOpen,
             .fileClose = fileClose,
@@ -520,10 +548,11 @@ fn groupAsync(
 
 fn groupWait(userdata: ?*anyopaque, group: *Io.Group, token: *anyopaque) void {
     const pool: *Pool = @ptrCast(@alignCast(userdata));
-    _ = pool;
+    const gpa = pool.allocator;
 
     if (builtin.single_threaded) return;
 
+    // TODO these primitives are too high level, need to check cancel on EINTR
     const group_state: *std.atomic.Value(usize) = @ptrCast(&group.state);
     const reset_event: *ResetEvent = @ptrCast(&group.context);
     std.Thread.WaitGroup.waitStateless(group_state, reset_event);
@@ -531,8 +560,9 @@ fn groupWait(userdata: ?*anyopaque, group: *Io.Group, token: *anyopaque) void {
     var node: *std.SinglyLinkedList.Node = @ptrCast(@alignCast(token));
     while (true) {
         const gc: *GroupClosure = @fieldParentPtr("node", node);
-        gc.closure.requestCancel();
-        node = node.next orelse break;
+        const node_next = node.next;
+        gc.free(gpa);
+        node = node_next orelse break;
     }
 }
 
@@ -724,6 +754,41 @@ fn conditionWake(userdata: ?*anyopaque, cond: *Io.Condition, wake: Io.Condition.
     }
 }
 
+fn dirMake(userdata: ?*anyopaque, dir: Io.Dir, sub_path: []const u8, mode: Io.Dir.Mode) Io.Dir.MakeError!void {
+    const pool: *Pool = @ptrCast(@alignCast(userdata));
+    try pool.checkCancel();
+
+    _ = dir;
+    _ = sub_path;
+    _ = mode;
+    @panic("TODO");
+}
+
+fn dirStat(userdata: ?*anyopaque, dir: Io.Dir) Io.Dir.StatError!Io.Dir.Stat {
+    const pool: *Pool = @ptrCast(@alignCast(userdata));
+    try pool.checkCancel();
+
+    _ = dir;
+    @panic("TODO");
+}
+
+fn dirStatPath(userdata: ?*anyopaque, dir: Io.Dir, sub_path: []const u8) Io.Dir.StatError!Io.File.Stat {
+    const pool: *Pool = @ptrCast(@alignCast(userdata));
+    try pool.checkCancel();
+
+    _ = dir;
+    _ = sub_path;
+    @panic("TODO");
+}
+
+fn fileStat(userdata: ?*anyopaque, file: Io.File) Io.File.StatError!Io.File.Stat {
+    const pool: *Pool = @ptrCast(@alignCast(userdata));
+    try pool.checkCancel();
+
+    _ = file;
+    @panic("TODO");
+}
+
 fn createFile(
     userdata: ?*anyopaque,
     dir: Io.Dir,
diff --git a/lib/std/Io/Writer.zig b/lib/std/Io/Writer.zig
index 2bb3c12b15..32aae1673e 100644
--- a/lib/std/Io/Writer.zig
+++ b/lib/std/Io/Writer.zig
@@ -2827,6 +2827,8 @@ pub const Allocating = struct {
 };
 
 test "discarding sendFile" {
+    const io = testing.io;
+
     var tmp_dir = testing.tmpDir(.{});
     defer tmp_dir.cleanup();
 
@@ -2837,7 +2839,7 @@ test "discarding sendFile" {
     try file_writer.interface.writeByte('h');
     try file_writer.interface.flush();
 
-    var file_reader = file_writer.moveToReader();
+    var file_reader = file_writer.moveToReader(io);
     try file_reader.seekTo(0);
 
     var w_buffer: [256]u8 = undefined;
@@ -2847,6 +2849,8 @@ test "discarding sendFile" {
 }
 
 test "allocating sendFile" {
+    const io = testing.io;
+
     var tmp_dir = testing.tmpDir(.{});
     defer tmp_dir.cleanup();
 
@@ -2857,7 +2861,7 @@ test "allocating sendFile" {
     try file_writer.interface.writeAll("abcd");
     try file_writer.interface.flush();
 
-    var file_reader = file_writer.moveToReader();
+    var file_reader = file_writer.moveToReader(io);
     try file_reader.seekTo(0);
     try file_reader.interface.fill(2);
 
diff --git a/lib/std/Io/net.zig b/lib/std/Io/net.zig
index e305145575..dc1080ab38 100644
--- a/lib/std/Io/net.zig
+++ b/lib/std/Io/net.zig
@@ -57,6 +57,39 @@ pub const IpAddress = union(enum) {
 
     pub const Family = @typeInfo(IpAddress).@"union".tag_type.?;
 
+    pub const ParseLiteralError = error{ InvalidAddress, InvalidPort };
+
+    /// Parse an IP address which may include a port.
+    ///
+    /// For IPv4, this is written `address:port`.
+    ///
+    /// For IPv6, RFC 3986 defines this as an "IP literal", and the port is
+    /// differentiated from the address by surrounding the address part in
+    /// brackets "[addr]:port". Even if the port is not given, the brackets are
+    /// mandatory.
+    pub fn parseLiteral(text: []const u8) ParseLiteralError!IpAddress {
+        if (text.len == 0) return error.InvalidAddress;
+        if (text[0] == '[') {
+            const addr_end = std.mem.indexOfScalar(u8, text, ']') orelse
+                return error.InvalidAddress;
+            const addr_text = text[1..addr_end];
+            const port: u16 = p: {
+                if (addr_end == text.len - 1) break :p 0;
+                if (text[addr_end + 1] != ':') return error.InvalidAddress;
+                break :p std.fmt.parseInt(u16, text[addr_end + 2 ..], 10) catch return error.InvalidPort;
+            };
+            return parseIp6(addr_text, port) catch error.InvalidAddress;
+        }
+        if (std.mem.indexOfScalar(u8, text, ':')) |i| {
+            const addr = Ip4Address.parse(text[0..i], 0) catch return error.InvalidAddress;
+            return .{ .ip4 = .{
+                .bytes = addr.bytes,
+                .port = std.fmt.parseInt(u16, text[i + 1 ..], 10) catch return error.InvalidPort,
+            } };
+        }
+        return parseIp4(text, 0) catch error.InvalidAddress;
+    }
+
     /// Parse the given IP address string into an `IpAddress` value.
     ///
     /// This is a pure function but it cannot handle IPv6 addresses that have
diff --git a/lib/std/Io/net/HostName.zig b/lib/std/Io/net/HostName.zig
index d8c3a3a903..72b5d39038 100644
--- a/lib/std/Io/net/HostName.zig
+++ b/lib/std/Io/net/HostName.zig
@@ -77,7 +77,9 @@ pub const LookupError = error{
     InvalidDnsAAAARecord,
     InvalidDnsCnameRecord,
     NameServerFailure,
-} || Io.Timestamp.Error || IpAddress.BindError || Io.File.OpenError || Io.File.Reader.Error || Io.Cancelable;
+    /// Failed to open or read "/etc/hosts" or "/etc/resolv.conf".
+    DetectingNetworkConfigurationFailed,
+} || Io.Timestamp.Error || IpAddress.BindError || Io.Cancelable;
 
 pub const LookupResult = struct {
     /// How many `LookupOptions.addresses_buffer` elements are populated.
@@ -428,14 +430,25 @@ fn lookupHosts(host_name: HostName, io: Io, options: LookupOptions) !LookupResul
         error.AccessDenied,
         => return .empty,
 
-        else => |e| return e,
+        error.Canceled => |e| return e,
+
+        else => {
+            // TODO populate optional diagnostic struct
+            return error.DetectingNetworkConfigurationFailed;
+        },
     };
     defer file.close(io);
 
     var line_buf: [512]u8 = undefined;
     var file_reader = file.reader(io, &line_buf);
     return lookupHostsReader(host_name, options, &file_reader.interface) catch |err| switch (err) {
-        error.ReadFailed => return file_reader.err.?,
+        error.ReadFailed => switch (file_reader.err.?) {
+            error.Canceled => |e| return e,
+            else => {
+                // TODO populate optional diagnostic struct
+                return error.DetectingNetworkConfigurationFailed;
+            },
+        },
     };
 }
 
diff --git a/lib/std/Io/net/test.zig b/lib/std/Io/net/test.zig
index 01c4b213a8..f1fb6cd57c 100644
--- a/lib/std/Io/net/test.zig
+++ b/lib/std/Io/net/test.zig
@@ -211,11 +211,11 @@ test "listen on a port, send bytes, receive bytes" {
     const t = try std.Thread.spawn(.{}, S.clientFn, .{server.socket.address});
     defer t.join();
 
-    var client = try server.accept(io);
-    defer client.stream.close(io);
+    var stream = try server.accept(io);
+    defer stream.close(io);
     var buf: [16]u8 = undefined;
-    var stream_reader = client.stream.reader(io, &.{});
-    const n = try stream_reader.interface().readSliceShort(&buf);
+    var stream_reader = stream.reader(io, &.{});
+    const n = try stream_reader.interface.readSliceShort(&buf);
 
     try testing.expectEqual(@as(usize, 12), n);
     try testing.expectEqualSlices(u8, "Hello world!", buf[0..n]);
@@ -267,10 +267,9 @@ fn testServer(server: *net.Server) anyerror!void {
 
     const io = testing.io;
 
-    var client = try server.accept(io);
-
-    const stream = client.stream.writer(io);
-    try stream.print("hello from server\n", .{});
+    var stream = try server.accept(io);
+    var writer = stream.writer(io, &.{});
+    try writer.interface.print("hello from server\n", .{});
 }
 
 test "listen on a unix socket, send bytes, receive bytes" {
@@ -310,11 +309,11 @@ test "listen on a unix socket, send bytes, receive bytes" {
     const t = try std.Thread.spawn(.{}, S.clientFn, .{socket_path});
     defer t.join();
 
-    var client = try server.accept(io);
-    defer client.stream.close(io);
+    var stream = try server.accept(io);
+    defer stream.close(io);
     var buf: [16]u8 = undefined;
-    var stream_reader = client.stream.reader(io, &.{});
-    const n = try stream_reader.interface().readSliceShort(&buf);
+    var stream_reader = stream.reader(io, &.{});
+    const n = try stream_reader.interface.readSliceShort(&buf);
 
     try testing.expectEqual(@as(usize, 12), n);
     try testing.expectEqualSlices(u8, "Hello world!", buf[0..n]);
@@ -366,10 +365,10 @@ test "non-blocking tcp server" {
     const socket_file = try net.tcpConnectToAddress(server.socket.address);
     defer socket_file.close();
 
-    var client = try server.accept(io);
-    defer client.stream.close(io);
-    const stream = client.stream.writer(io);
-    try stream.print("hello from server\n", .{});
+    var stream = try server.accept(io);
+    defer stream.close(io);
+    var writer = stream.writer(io, .{});
+    try writer.interface.print("hello from server\n", .{});
 
     var buf: [100]u8 = undefined;
     const len = try socket_file.read(&buf);
diff --git a/lib/std/crypto/tls/Client.zig b/lib/std/crypto/tls/Client.zig
index b697d624fa..f6e334af8e 100644
--- a/lib/std/crypto/tls/Client.zig
+++ b/lib/std/crypto/tls/Client.zig
@@ -105,6 +105,14 @@ pub const Options = struct {
         /// Verify that the server certificate is authorized by a given ca bundle.
         bundle: Certificate.Bundle,
     },
+    write_buffer: []u8,
+    read_buffer: []u8,
+    /// Cryptographically secure random bytes. The pointer is not captured; data is only
+    /// read during `init`.
+    entropy: *const [176]u8,
+    /// Current time according to the wall clock / calendar, in seconds.
+    realtime_now_seconds: i64,
+
     /// If non-null, ssl secrets are logged to this stream. Creating such a log file allows
     /// other programs with access to that file to decrypt all traffic over this connection.
     ///
@@ -120,8 +128,6 @@ pub const Options = struct {
     /// application layer itself verifies that the amount of data received equals
     /// the amount of data expected, such as HTTP with the Content-Length header.
     allow_truncation_attacks: bool = false,
-    write_buffer: []u8,
-    read_buffer: []u8,
     /// Populated when `error.TlsAlert` is returned from `init`.
     alert: ?*tls.Alert = null,
 };
@@ -189,14 +195,12 @@ pub fn init(input: *Reader, output: *Writer, options: Options) InitError!Client
     };
     const host_len: u16 = @intCast(host.len);
 
-    var random_buffer: [176]u8 = undefined;
-    crypto.random.bytes(&random_buffer);
-    const client_hello_rand = random_buffer[0..32].*;
+    const client_hello_rand = options.entropy[0..32].*;
     var key_seq: u64 = 0;
     var server_hello_rand: [32]u8 = undefined;
-    const legacy_session_id = random_buffer[32..64].*;
+    const legacy_session_id = options.entropy[32..64].*;
 
-    var key_share = KeyShare.init(random_buffer[64..176].*) catch |err| switch (err) {
+    var key_share = KeyShare.init(options.entropy[64..176].*) catch |err| switch (err) {
         // Only possible to happen if the seed is all zeroes.
         error.IdentityElement => return error.InsufficientEntropy,
     };
@@ -321,7 +325,7 @@ pub fn init(input: *Reader, output: *Writer, options: Options) InitError!Client
     var handshake_cipher: tls.HandshakeCipher = undefined;
     var main_cert_pub_key: CertificatePublicKey = undefined;
     var tls12_negotiated_group: ?tls.NamedGroup = null;
-    const now_sec = std.time.timestamp();
+    const now_sec = options.realtime_now_seconds;
 
     var cleartext_fragment_start: usize = 0;
     var cleartext_fragment_end: usize = 0;
diff --git a/lib/std/debug/SelfInfo/Windows.zig b/lib/std/debug/SelfInfo/Windows.zig
index f84836a6d4..ea2fa96199 100644
--- a/lib/std/debug/SelfInfo/Windows.zig
+++ b/lib/std/debug/SelfInfo/Windows.zig
@@ -434,7 +434,7 @@ const Module = struct {
             };
             errdefer pdb_file.close();
 
-            const pdb_reader = try arena.create(std.fs.File.Reader);
+            const pdb_reader = try arena.create(Io.File.Reader);
             pdb_reader.* = pdb_file.reader(try arena.alloc(u8, 4096));
 
             var pdb = Pdb.init(gpa, pdb_reader) catch |err| switch (err) {
@@ -544,6 +544,7 @@ fn findModule(si: *SelfInfo, gpa: Allocator, address: usize) error{ MissingDebug
 }
 
 const std = @import("std");
+const Io = std.Io;
 const Allocator = std.mem.Allocator;
 const Dwarf = std.debug.Dwarf;
 const Pdb = std.debug.Pdb;
diff --git a/lib/std/elf.zig b/lib/std/elf.zig
index 746d24f61a..c5f8a7ea80 100644
--- a/lib/std/elf.zig
+++ b/lib/std/elf.zig
@@ -710,7 +710,7 @@ pub const ProgramHeaderIterator = struct {
         const offset = it.phoff + size * it.index;
         try it.file_reader.seekTo(offset);
 
-        return takeProgramHeader(&it.file_reader.interface, it.is_64, it.endian);
+        return try takeProgramHeader(&it.file_reader.interface, it.is_64, it.endian);
     }
 };
 
@@ -731,7 +731,7 @@ pub const ProgramHeaderBufferIterator = struct {
         const offset = it.phoff + size * it.index;
         var reader = Io.Reader.fixed(it.buf[offset..]);
 
-        return takeProgramHeader(&reader, it.is_64, it.endian);
+        return try takeProgramHeader(&reader, it.is_64, it.endian);
     }
 };
 
@@ -771,7 +771,7 @@ pub const SectionHeaderIterator = struct {
         const offset = it.shoff + size * it.index;
         try it.file_reader.seekTo(offset);
 
-        return takeSectionHeader(&it.file_reader.interface, it.is_64, it.endian);
+        return try takeSectionHeader(&it.file_reader.interface, it.is_64, it.endian);
     }
 };
 
@@ -793,7 +793,7 @@ pub const SectionHeaderBufferIterator = struct {
         if (offset > it.buf.len) return error.EndOfStream;
         var reader = Io.Reader.fixed(it.buf[@intCast(offset)..]);
 
-        return takeSectionHeader(&reader, it.is_64, it.endian);
+        return try takeSectionHeader(&reader, it.is_64, it.endian);
     }
 };
 
@@ -826,12 +826,12 @@ pub const DynamicSectionIterator = struct {
 
     file_reader: *Io.File.Reader,
 
-    pub fn next(it: *SectionHeaderIterator) !?Elf64_Dyn {
+    pub fn next(it: *DynamicSectionIterator) !?Elf64_Dyn {
         if (it.offset >= it.end_offset) return null;
         const size: u64 = if (it.is_64) @sizeOf(Elf64_Dyn) else @sizeOf(Elf32_Dyn);
         defer it.offset += size;
         try it.file_reader.seekTo(it.offset);
-        return takeDynamicSection(&it.file_reader.interface, it.is_64, it.endian);
+        return try takeDynamicSection(&it.file_reader.interface, it.is_64, it.endian);
     }
 };
 
diff --git a/lib/std/fs/Dir.zig b/lib/std/fs/Dir.zig
index 3e3577bbf0..c2ddf98627 100644
--- a/lib/std/fs/Dir.zig
+++ b/lib/std/fs/Dir.zig
@@ -1,6 +1,11 @@
+//! Deprecated in favor of `Io.Dir`.
 const Dir = @This();
+
 const builtin = @import("builtin");
+const native_os = builtin.os.tag;
+
 const std = @import("../std.zig");
+const Io = std.Io;
 const File = std.fs.File;
 const AtomicFile = std.fs.AtomicFile;
 const base64_encoder = fs.base64_encoder;
@@ -12,7 +17,6 @@ const Allocator = std.mem.Allocator;
 const assert = std.debug.assert;
 const linux = std.os.linux;
 const windows = std.os.windows;
-const native_os = builtin.os.tag;
 const have_flock = @TypeOf(posix.system.flock) != void;
 
 fd: Handle,
@@ -1189,84 +1193,41 @@ pub fn createFileW(self: Dir, sub_path_w: []const u16, flags: File.CreateFlags)
     return file;
 }
 
-pub const MakeError = posix.MakeDirError;
+/// Deprecated in favor of `Io.Dir.MakeError`.
+pub const MakeError = Io.Dir.MakeError;
 
-/// Creates a single directory with a relative or absolute path.
-/// To create multiple directories to make an entire path, see `makePath`.
-/// To operate on only absolute paths, see `makeDirAbsolute`.
-/// On Windows, `sub_path` should be encoded as [WTF-8](https://wtf-8.codeberg.page/).
-/// On WASI, `sub_path` should be encoded as valid UTF-8.
-/// On other platforms, `sub_path` is an opaque sequence of bytes with no particular encoding.
+/// Deprecated in favor of `Io.Dir.makeDir`.
 pub fn makeDir(self: Dir, sub_path: []const u8) MakeError!void {
-    try posix.mkdirat(self.fd, sub_path, default_mode);
+    var threaded: Io.Threaded = .init_single_threaded;
+    const io = threaded.io();
+    return Io.Dir.makeDir(.{ .handle = self.fd }, io, sub_path);
 }
 
-/// Same as `makeDir`, but `sub_path` is null-terminated.
-/// To create multiple directories to make an entire path, see `makePath`.
-/// To operate on only absolute paths, see `makeDirAbsoluteZ`.
+/// Deprecated in favor of `Io.Dir.makeDir`.
 pub fn makeDirZ(self: Dir, sub_path: [*:0]const u8) MakeError!void {
     try posix.mkdiratZ(self.fd, sub_path, default_mode);
 }
 
-/// Creates a single directory with a relative or absolute null-terminated WTF-16 LE-encoded path.
-/// To create multiple directories to make an entire path, see `makePath`.
-/// To operate on only absolute paths, see `makeDirAbsoluteW`.
+/// Deprecated in favor of `Io.Dir.makeDir`.
 pub fn makeDirW(self: Dir, sub_path: [*:0]const u16) MakeError!void {
     try posix.mkdiratW(self.fd, mem.span(sub_path), default_mode);
 }
 
-/// Calls makeDir iteratively to make an entire path
-/// (i.e. creating any parent directories that do not exist).
-/// Returns success if the path already exists and is a directory.
-/// This function is not atomic, and if it returns an error, the file system may
-/// have been modified regardless.
-/// On Windows, `sub_path` should be encoded as [WTF-8](https://wtf-8.codeberg.page/).
-/// On WASI, `sub_path` should be encoded as valid UTF-8.
-/// On other platforms, `sub_path` is an opaque sequence of bytes with no particular encoding.
-/// Fails on an empty path with `error.BadPathName` as that is not a path that can be created.
-///
-/// Paths containing `..` components are handled differently depending on the platform:
-/// - On Windows, `..` are resolved before the path is passed to NtCreateFile, meaning
-///   a `sub_path` like "first/../second" will resolve to "second" and only a
-///   `./second` directory will be created.
-/// - On other platforms, `..` are not resolved before the path is passed to `mkdirat`,
-///   meaning a `sub_path` like "first/../second" will create both a `./first`
-///   and a `./second` directory.
-pub fn makePath(self: Dir, sub_path: []const u8) (MakeError || StatFileError)!void {
+/// Deprecated in favor of `Io.Dir.makePath`.
+pub fn makePath(self: Dir, sub_path: []const u8) MakePathError!void {
     _ = try self.makePathStatus(sub_path);
 }
 
-pub const MakePathStatus = enum { existed, created };
-/// Same as `makePath` except returns whether the path already existed or was successfully created.
-pub fn makePathStatus(self: Dir, sub_path: []const u8) (MakeError || StatFileError)!MakePathStatus {
-    var it = try fs.path.componentIterator(sub_path);
-    var status: MakePathStatus = .existed;
-    var component = it.last() orelse return error.BadPathName;
-    while (true) {
-        if (self.makeDir(component.path)) |_| {
-            status = .created;
-        } else |err| switch (err) {
-            error.PathAlreadyExists => {
-                // stat the file and return an error if it's not a directory
-                // this is important because otherwise a dangling symlink
-                // could cause an infinite loop
-                check_dir: {
-                    // workaround for windows, see https://github.com/ziglang/zig/issues/16738
-                    const fstat = self.statFile(component.path) catch |stat_err| switch (stat_err) {
-                        error.IsDir => break :check_dir,
-                        else => |e| return e,
-                    };
-                    if (fstat.kind != .directory) return error.NotDir;
-                }
-            },
-            error.FileNotFound => |e| {
-                component = it.previous() orelse return e;
-                continue;
-            },
-            else => |e| return e,
-        }
-        component = it.next() orelse return status;
-    }
+/// Deprecated in favor of `Io.Dir.MakePathStatus`.
+pub const MakePathStatus = Io.Dir.MakePathStatus;
+/// Deprecated in favor of `Io.Dir.MakePathError`.
+pub const MakePathError = Io.Dir.MakePathError;
+
+/// Deprecated in favor of `Io.Dir.makePathStatus`.
+pub fn makePathStatus(self: Dir, sub_path: []const u8) MakePathError!MakePathStatus {
+    var threaded: Io.Threaded = .init_single_threaded;
+    const io = threaded.io();
+    return Io.Dir.makePathStatus(.{ .handle = self.fd }, io, sub_path);
 }
 
 /// Windows only. Calls makeOpenDirAccessMaskW iteratively to make an entire path
@@ -2052,20 +2013,11 @@ pub fn readLinkW(self: Dir, sub_path_w: []const u16, buffer: []u8) ![]u8 {
     return windows.ReadLink(self.fd, sub_path_w, buffer);
 }
 
-/// Read all of file contents using a preallocated buffer.
-/// The returned slice has the same pointer as `buffer`. If the length matches `buffer.len`
-/// the situation is ambiguous. It could either mean that the entire file was read, and
-/// it exactly fits the buffer, or it could mean the buffer was not big enough for the
-/// entire file.
-/// On Windows, `file_path` should be encoded as [WTF-8](https://wtf-8.codeberg.page/).
-/// On WASI, `file_path` should be encoded as valid UTF-8.
-/// On other platforms, `file_path` is an opaque sequence of bytes with no particular encoding.
+/// Deprecated in favor of `Io.Dir.readFile`.
 pub fn readFile(self: Dir, file_path: []const u8, buffer: []u8) ![]u8 {
-    var file = try self.openFile(file_path, .{});
-    defer file.close();
-
-    const end_index = try file.readAll(buffer);
-    return buffer[0..end_index];
+    var threaded: Io.Threaded = .init_single_threaded;
+    const io = threaded.io();
+    return Io.Dir.readFile(.{ .handle = self.fd }, io, file_path, buffer);
 }
 
 pub const ReadFileAllocError = File.OpenError || File.ReadError || Allocator.Error || error{
@@ -2091,7 +2043,7 @@ pub fn readFileAlloc(
     /// Used to allocate the result.
     gpa: Allocator,
     /// If reached or exceeded, `error.StreamTooLong` is returned instead.
-    limit: std.Io.Limit,
+    limit: Io.Limit,
 ) ReadFileAllocError![]u8 {
     return readFileAllocOptions(dir, sub_path, gpa, limit, .of(u8), null);
 }
@@ -2101,6 +2053,8 @@ pub fn readFileAlloc(
 ///
 /// If the file size is already known, a better alternative is to initialize a
 /// `File.Reader`.
+///
+/// TODO move this function to Io.Dir
 pub fn readFileAllocOptions(
     dir: Dir,
     /// On Windows, should be encoded as [WTF-8](https://wtf-8.codeberg.page/).
@@ -2110,13 +2064,16 @@ pub fn readFileAllocOptions(
     /// Used to allocate the result.
     gpa: Allocator,
     /// If reached or exceeded, `error.StreamTooLong` is returned instead.
-    limit: std.Io.Limit,
+    limit: Io.Limit,
     comptime alignment: std.mem.Alignment,
     comptime sentinel: ?u8,
 ) ReadFileAllocError!(if (sentinel) |s| [:s]align(alignment.toByteUnits()) u8 else []align(alignment.toByteUnits()) u8) {
+    var threaded: Io.Threaded = .init_single_threaded;
+    const io = threaded.io();
+
     var file = try dir.openFile(sub_path, .{});
     defer file.close();
-    var file_reader = file.reader(&.{});
+    var file_reader = file.reader(io, &.{});
     return file_reader.interface.allocRemainingAlignedSentinel(gpa, limit, alignment, sentinel) catch |err| switch (err) {
         error.ReadFailed => return file_reader.err.?,
         error.OutOfMemory, error.StreamTooLong => |e| return e,
@@ -2647,6 +2604,8 @@ pub const CopyFileError = File.OpenError || File.StatError ||
 /// [WTF-8](https://wtf-8.codeberg.page/). On WASI, both paths should be
 /// encoded as valid UTF-8. On other platforms, both paths are an opaque
 /// sequence of bytes with no particular encoding.
+///
+/// TODO move this function to Io.Dir
 pub fn copyFile(
     source_dir: Dir,
     source_path: []const u8,
@@ -2654,11 +2613,15 @@ pub fn copyFile(
     dest_path: []const u8,
     options: CopyFileOptions,
 ) CopyFileError!void {
-    var file_reader: File.Reader = .init(try source_dir.openFile(source_path, .{}), &.{});
-    defer file_reader.file.close();
+    var threaded: Io.Threaded = .init_single_threaded;
+    const io = threaded.io();
+
+    const file = try source_dir.openFile(source_path, .{});
+    var file_reader: File.Reader = .init(.{ .handle = file.handle }, io, &.{});
+    defer file_reader.file.close(io);
 
     const mode = options.override_mode orelse blk: {
-        const st = try file_reader.file.stat();
+        const st = try file_reader.file.stat(io);
         file_reader.size = st.size;
         break :blk st.mode;
     };
@@ -2708,6 +2671,7 @@ pub fn atomicFile(self: Dir, dest_path: []const u8, options: AtomicFileOptions)
 pub const Stat = File.Stat;
 pub const StatError = File.StatError;
 
+/// Deprecated in favor of `Io.Dir.stat`.
 pub fn stat(self: Dir) StatError!Stat {
     const file: File = .{ .handle = self.fd };
     return file.stat();
@@ -2715,17 +2679,7 @@ pub fn stat(self: Dir) StatError!Stat {
 
 pub const StatFileError = File.OpenError || File.StatError || posix.FStatAtError;
 
-/// Returns metadata for a file inside the directory.
-///
-/// On Windows, this requires three syscalls. On other operating systems, it
-/// only takes one.
-///
-/// Symlinks are followed.
-///
-/// `sub_path` may be absolute, in which case `self` is ignored.
-/// On Windows, `sub_path` should be encoded as [WTF-8](https://wtf-8.codeberg.page/).
-/// On WASI, `sub_path` should be encoded as valid UTF-8.
-/// On other platforms, `sub_path` is an opaque sequence of bytes with no particular encoding.
+/// Deprecated in favor of `Io.Dir.statPath`.
 pub fn statFile(self: Dir, sub_path: []const u8) StatFileError!Stat {
     if (native_os == .windows) {
         var file = try self.openFile(sub_path, .{});
@@ -2799,3 +2753,11 @@ pub fn setPermissions(self: Dir, permissions: Permissions) SetPermissionsError!v
     const file: File = .{ .handle = self.fd };
     try file.setPermissions(permissions);
 }
+
+pub fn adaptToNewApi(dir: Dir) Io.Dir {
+    return .{ .handle = dir.fd };
+}
+
+pub fn adaptFromNewApi(dir: Io.Dir) Dir {
+    return .{ .fd = dir.handle };
+}
diff --git a/lib/std/fs/File.zig b/lib/std/fs/File.zig
index 7c94fc1df6..3de4d9bcb4 100644
--- a/lib/std/fs/File.zig
+++ b/lib/std/fs/File.zig
@@ -858,10 +858,12 @@ pub const Writer = struct {
         };
     }
 
-    pub fn moveToReader(w: *Writer) Reader {
+    /// TODO when this logic moves from fs.File to Io.File the io parameter should be deleted
+    pub fn moveToReader(w: *Writer, io: std.Io) Reader {
         defer w.* = undefined;
         return .{
-            .file = w.file,
+            .io = io,
+            .file = .{ .handle = w.file.handle },
             .mode = w.mode,
             .pos = w.pos,
             .interface = Reader.initInterface(w.interface.buffer),
@@ -1350,15 +1352,15 @@ pub const Writer = struct {
 ///
 /// Positional is more threadsafe, since the global seek position is not
 /// affected.
-pub fn reader(file: File, buffer: []u8) Reader {
-    return .init(file, buffer);
+pub fn reader(file: File, io: std.Io, buffer: []u8) Reader {
+    return .init(.{ .handle = file.handle }, io, buffer);
 }
 
 /// Positional is more threadsafe, since the global seek position is not
 /// affected, but when such syscalls are not available, preemptively
 /// initializing in streaming mode skips a failed syscall.
-pub fn readerStreaming(file: File, buffer: []u8) Reader {
-    return .initStreaming(file, buffer);
+pub fn readerStreaming(file: File, io: std.Io, buffer: []u8) Reader {
+    return .initStreaming(.{ .handle = file.handle }, io, buffer);
 }
 
 /// Defaults to positional reading; falls back to streaming.
@@ -1538,3 +1540,11 @@ pub fn downgradeLock(file: File) LockError!void {
         };
     }
 }
+
+pub fn adaptToNewApi(file: File) std.Io.File {
+    return .{ .handle = file.handle };
+}
+
+pub fn adaptFromNewApi(file: std.Io.File) File {
+    return .{ .handle = file.handle };
+}
diff --git a/lib/std/fs/test.zig b/lib/std/fs/test.zig
index e9c341d39a..e60e3c7911 100644
--- a/lib/std/fs/test.zig
+++ b/lib/std/fs/test.zig
@@ -1,10 +1,12 @@
-const std = @import("../std.zig");
 const builtin = @import("builtin");
+const native_os = builtin.os.tag;
+
+const std = @import("../std.zig");
+const Io = std.Io;
 const testing = std.testing;
 const fs = std.fs;
 const mem = std.mem;
 const wasi = std.os.wasi;
-const native_os = builtin.os.tag;
 const windows = std.os.windows;
 const posix = std.posix;
 
@@ -73,6 +75,7 @@ const PathType = enum {
 };
 
 const TestContext = struct {
+    io: Io,
     path_type: PathType,
     path_sep: u8,
     arena: ArenaAllocator,
@@ -83,6 +86,7 @@ const TestContext = struct {
     pub fn init(path_type: PathType, path_sep: u8, allocator: mem.Allocator, transform_fn: *const PathType.TransformFn) TestContext {
         const tmp = tmpDir(.{ .iterate = true });
         return .{
+            .io = testing.io,
             .path_type = path_type,
             .path_sep = path_sep,
             .arena = ArenaAllocator.init(allocator),
@@ -1319,6 +1323,8 @@ test "max file name component lengths" {
 }
 
 test "writev, readv" {
+    const io = testing.io;
+
     var tmp = tmpDir(.{});
     defer tmp.cleanup();
 
@@ -1327,78 +1333,55 @@ test "writev, readv" {
 
     var buf1: [line1.len]u8 = undefined;
     var buf2: [line2.len]u8 = undefined;
-    var write_vecs = [_]posix.iovec_const{
-        .{
-            .base = line1,
-            .len = line1.len,
-        },
-        .{
-            .base = line2,
-            .len = line2.len,
-        },
-    };
-    var read_vecs = [_]posix.iovec{
-        .{
-            .base = &buf2,
-            .len = buf2.len,
-        },
-        .{
-            .base = &buf1,
-            .len = buf1.len,
-        },
-    };
+    var write_vecs: [2][]const u8 = .{ line1, line2 };
+    var read_vecs: [2][]u8 = .{ &buf2, &buf1 };
 
     var src_file = try tmp.dir.createFile("test.txt", .{ .read = true });
     defer src_file.close();
 
-    try src_file.writevAll(&write_vecs);
+    var writer = src_file.writerStreaming(&.{});
+
+    try writer.interface.writeVecAll(&write_vecs);
+    try writer.interface.flush();
     try testing.expectEqual(@as(u64, line1.len + line2.len), try src_file.getEndPos());
-    try src_file.seekTo(0);
-    const read = try src_file.readvAll(&read_vecs);
-    try testing.expectEqual(@as(usize, line1.len + line2.len), read);
+
+    var reader = writer.moveToReader(io);
+    try reader.seekTo(0);
+    try reader.interface.readVecAll(&read_vecs);
     try testing.expectEqualStrings(&buf1, "line2\n");
     try testing.expectEqualStrings(&buf2, "line1\n");
+    try testing.expectError(error.EndOfStream, reader.interface.readSliceAll(&buf1));
 }
 
 test "pwritev, preadv" {
+    const io = testing.io;
+
     var tmp = tmpDir(.{});
     defer tmp.cleanup();
 
     const line1 = "line1\n";
     const line2 = "line2\n";
-
+    var lines: [2][]const u8 = .{ line1, line2 };
     var buf1: [line1.len]u8 = undefined;
     var buf2: [line2.len]u8 = undefined;
-    var write_vecs = [_]posix.iovec_const{
-        .{
-            .base = line1,
-            .len = line1.len,
-        },
-        .{
-            .base = line2,
-            .len = line2.len,
-        },
-    };
-    var read_vecs = [_]posix.iovec{
-        .{
-            .base = &buf2,
-            .len = buf2.len,
-        },
-        .{
-            .base = &buf1,
-            .len = buf1.len,
-        },
-    };
+    var read_vecs: [2][]u8 = .{ &buf2, &buf1 };
 
     var src_file = try tmp.dir.createFile("test.txt", .{ .read = true });
     defer src_file.close();
 
-    try src_file.pwritevAll(&write_vecs, 16);
+    var writer = src_file.writer(&.{});
+
+    try writer.seekTo(16);
+    try writer.interface.writeVecAll(&lines);
+    try writer.interface.flush();
     try testing.expectEqual(@as(u64, 16 + line1.len + line2.len), try src_file.getEndPos());
-    const read = try src_file.preadvAll(&read_vecs, 16);
-    try testing.expectEqual(@as(usize, line1.len + line2.len), read);
+
+    var reader = writer.moveToReader(io);
+    try reader.seekTo(16);
+    try reader.interface.readVecAll(&read_vecs);
     try testing.expectEqualStrings(&buf1, "line2\n");
     try testing.expectEqualStrings(&buf2, "line1\n");
+    try testing.expectError(error.EndOfStream, reader.interface.readSliceAll(&buf1));
 }
 
 test "setEndPos" {
@@ -1406,6 +1389,8 @@ test "setEndPos" {
     if (native_os == .wasi and builtin.link_libc) return error.SkipZigTest;
     if (builtin.cpu.arch.isMIPS64() and (builtin.abi == .gnuabin32 or builtin.abi == .muslabin32)) return error.SkipZigTest; // https://github.com/ziglang/zig/issues/23806
 
+    const io = testing.io;
+
     var tmp = tmpDir(.{});
     defer tmp.cleanup();
 
@@ -1416,11 +1401,13 @@ test "setEndPos" {
 
     const initial_size = try f.getEndPos();
     var buffer: [32]u8 = undefined;
+    var reader = f.reader(io, &.{});
 
     {
         try f.setEndPos(initial_size);
         try testing.expectEqual(initial_size, try f.getEndPos());
-        try testing.expectEqual(initial_size, try f.preadAll(&buffer, 0));
+        try reader.seekTo(0);
+        try testing.expectEqual(initial_size, reader.interface.readSliceShort(&buffer));
         try testing.expectEqualStrings("ninebytes", buffer[0..@intCast(initial_size)]);
     }
 
@@ -1428,7 +1415,8 @@ test "setEndPos" {
         const larger = initial_size + 4;
         try f.setEndPos(larger);
         try testing.expectEqual(larger, try f.getEndPos());
-        try testing.expectEqual(larger, try f.preadAll(&buffer, 0));
+        try reader.seekTo(0);
+        try testing.expectEqual(larger, reader.interface.readSliceShort(&buffer));
         try testing.expectEqualStrings("ninebytes\x00\x00\x00\x00", buffer[0..@intCast(larger)]);
     }
 
@@ -1436,25 +1424,21 @@ test "setEndPos" {
         const smaller = initial_size - 5;
         try f.setEndPos(smaller);
         try testing.expectEqual(smaller, try f.getEndPos());
-        try testing.expectEqual(smaller, try f.preadAll(&buffer, 0));
+        try reader.seekTo(0);
+        try testing.expectEqual(smaller, try reader.interface.readSliceShort(&buffer));
         try testing.expectEqualStrings("nine", buffer[0..@intCast(smaller)]);
     }
 
     try f.setEndPos(0);
     try testing.expectEqual(0, try f.getEndPos());
-    try testing.expectEqual(0, try f.preadAll(&buffer, 0));
+    try reader.seekTo(0);
+    try testing.expectEqual(0, try reader.interface.readSliceShort(&buffer));
 
     // Invalid file length should error gracefully. Actual limit is host
     // and file-system dependent, but 1PB should fail on filesystems like
     // EXT4 and NTFS.  But XFS or Btrfs support up to 8EiB files.
-    f.setEndPos(0x4_0000_0000_0000) catch |err| if (err != error.FileTooBig) {
-        return err;
-    };
-
-    f.setEndPos(std.math.maxInt(u63)) catch |err| if (err != error.FileTooBig) {
-        return err;
-    };
-
+    try testing.expectError(error.FileTooBig, f.setEndPos(0x4_0000_0000_0000));
+    try testing.expectError(error.FileTooBig, f.setEndPos(std.math.maxInt(u63)));
     try testing.expectError(error.FileTooBig, f.setEndPos(std.math.maxInt(u63) + 1));
     try testing.expectError(error.FileTooBig, f.setEndPos(std.math.maxInt(u64)));
 }
@@ -1560,31 +1544,6 @@ test "sendfile with buffered data" {
     try std.testing.expectEqualSlices(u8, "AAAA", written_buf[0..amt]);
 }
 
-test "copyRangeAll" {
-    var tmp = tmpDir(.{});
-    defer tmp.cleanup();
-
-    try tmp.dir.makePath("os_test_tmp");
-
-    var dir = try tmp.dir.openDir("os_test_tmp", .{});
-    defer dir.close();
-
-    var src_file = try dir.createFile("file1.txt", .{ .read = true });
-    defer src_file.close();
-
-    const data = "u6wj+JmdF3qHsFPE BUlH2g4gJCmEz0PP";
-    try src_file.writeAll(data);
-
-    var dest_file = try dir.createFile("file2.txt", .{ .read = true });
-    defer dest_file.close();
-
-    var written_buf: [100]u8 = undefined;
-    _ = try src_file.copyRangeAll(0, dest_file, 0, data.len);
-
-    const amt = try dest_file.preadAll(&written_buf, 0);
-    try testing.expectEqualStrings(data, written_buf[0..amt]);
-}
-
 test "copyFile" {
     try testWithAllSupportedPathTypes(struct {
         fn impl(ctx: *TestContext) !void {
@@ -1708,8 +1667,8 @@ test "open file with exclusive lock twice, make sure second lock waits" {
                 }
             };
 
-            var started = std.Thread.ResetEvent{};
-            var locked = std.Thread.ResetEvent{};
+            var started: std.Thread.ResetEvent = .unset;
+            var locked: std.Thread.ResetEvent = .unset;
 
             const t = try std.Thread.spawn(.{}, S.checkFn, .{
                 &ctx.dir,
@@ -1773,7 +1732,7 @@ test "read from locked file" {
                 const f = try ctx.dir.createFile(filename, .{ .read = true });
                 defer f.close();
                 var buffer: [1]u8 = undefined;
-                _ = try f.readAll(&buffer);
+                _ = try f.read(&buffer);
             }
             {
                 const f = try ctx.dir.createFile(filename, .{
@@ -1785,9 +1744,9 @@ test "read from locked file" {
                 defer f2.close();
                 var buffer: [1]u8 = undefined;
                 if (builtin.os.tag == .windows) {
-                    try std.testing.expectError(error.LockViolation, f2.readAll(&buffer));
+                    try std.testing.expectError(error.LockViolation, f2.read(&buffer));
                 } else {
-                    try std.testing.expectEqual(0, f2.readAll(&buffer));
+                    try std.testing.expectEqual(0, f2.read(&buffer));
                 }
             }
         }
@@ -1944,6 +1903,7 @@ test "'.' and '..' in fs.Dir functions" {
 
     try testWithAllSupportedPathTypes(struct {
         fn impl(ctx: *TestContext) !void {
+            const io = ctx.io;
             const subdir_path = try ctx.transformPath("./subdir");
             const file_path = try ctx.transformPath("./subdir/../file");
             const copy_path = try ctx.transformPath("./subdir/../copy");
@@ -1966,7 +1926,8 @@ test "'.' and '..' in fs.Dir functions" {
             try ctx.dir.deleteFile(rename_path);
 
             try ctx.dir.writeFile(.{ .sub_path = update_path, .data = "something" });
-            const prev_status = try ctx.dir.updateFile(file_path, ctx.dir, update_path, .{});
+            var dir = ctx.dir.adaptToNewApi();
+            const prev_status = try dir.updateFile(io, file_path, dir, update_path, .{});
             try testing.expectEqual(fs.Dir.PrevStatus.stale, prev_status);
 
             try ctx.dir.deleteDir(subdir_path);
@@ -2005,13 +1966,6 @@ test "'.' and '..' in absolute functions" {
     renamed_file.close();
     try fs.deleteFileAbsolute(renamed_file_path);
 
-    const update_file_path = try fs.path.join(allocator, &.{ subdir_path, "../update" });
-    const update_file = try fs.createFileAbsolute(update_file_path, .{});
-    try update_file.writeAll("something");
-    update_file.close();
-    const prev_status = try fs.updateFileAbsolute(created_file_path, update_file_path, .{});
-    try testing.expectEqual(fs.Dir.PrevStatus.stale, prev_status);
-
     try fs.deleteDirAbsolute(subdir_path);
 }
 
@@ -2079,6 +2033,7 @@ test "invalid UTF-8/WTF-8 paths" {
 
     try testWithAllSupportedPathTypes(struct {
         fn impl(ctx: *TestContext) !void {
+            const io = ctx.io;
             // This is both invalid UTF-8 and WTF-8, since \xFF is an invalid start byte
             const invalid_path = try ctx.transformPath("\xFF");
 
@@ -2129,7 +2084,8 @@ test "invalid UTF-8/WTF-8 paths" {
             try testing.expectError(expected_err, ctx.dir.access(invalid_path, .{}));
             try testing.expectError(expected_err, ctx.dir.accessZ(invalid_path, .{}));
 
-            try testing.expectError(expected_err, ctx.dir.updateFile(invalid_path, ctx.dir, invalid_path, .{}));
+            var dir = ctx.dir.adaptToNewApi();
+            try testing.expectError(expected_err, dir.updateFile(io, invalid_path, dir, invalid_path, .{}));
             try testing.expectError(expected_err, ctx.dir.copyFile(invalid_path, ctx.dir, invalid_path, .{}));
 
             try testing.expectError(expected_err, ctx.dir.statFile(invalid_path));
@@ -2144,7 +2100,6 @@ test "invalid UTF-8/WTF-8 paths" {
             try testing.expectError(expected_err, fs.renameZ(ctx.dir, invalid_path, ctx.dir, invalid_path));
 
             if (native_os != .wasi and ctx.path_type != .relative) {
-                try testing.expectError(expected_err, fs.updateFileAbsolute(invalid_path, invalid_path, .{}));
                 try testing.expectError(expected_err, fs.copyFileAbsolute(invalid_path, invalid_path, .{}));
                 try testing.expectError(expected_err, fs.makeDirAbsolute(invalid_path));
                 try testing.expectError(expected_err, fs.makeDirAbsoluteZ(invalid_path));
@@ -2175,6 +2130,8 @@ test "invalid UTF-8/WTF-8 paths" {
 }
 
 test "read file non vectored" {
+    const io = std.testing.io;
+
     var tmp_dir = testing.tmpDir(.{});
     defer tmp_dir.cleanup();
 
@@ -2188,7 +2145,7 @@ test "read file non vectored" {
         try file_writer.interface.flush();
     }
 
-    var file_reader: std.fs.File.Reader = .init(file, &.{});
+    var file_reader: std.Io.File.Reader = .initAdapted(file, io, &.{});
 
     var write_buffer: [100]u8 = undefined;
     var w: std.Io.Writer = .fixed(&write_buffer);
@@ -2205,6 +2162,8 @@ test "read file non vectored" {
 }
 
 test "seek keeping partial buffer" {
+    const io = std.testing.io;
+
     var tmp_dir = testing.tmpDir(.{});
     defer tmp_dir.cleanup();
 
@@ -2219,7 +2178,7 @@ test "seek keeping partial buffer" {
     }
 
     var read_buffer: [3]u8 = undefined;
-    var file_reader: std.fs.File.Reader = .init(file, &read_buffer);
+    var file_reader: Io.File.Reader = .initAdapted(file, io, &read_buffer);
 
     try testing.expectEqual(0, file_reader.logicalPos());
 
@@ -2246,13 +2205,15 @@ test "seek keeping partial buffer" {
 }
 
 test "seekBy" {
+    const io = testing.io;
+
     var tmp_dir = testing.tmpDir(.{});
     defer tmp_dir.cleanup();
 
     try tmp_dir.dir.writeFile(.{ .sub_path = "blah.txt", .data = "let's test seekBy" });
     const f = try tmp_dir.dir.openFile("blah.txt", .{ .mode = .read_only });
     defer f.close();
-    var reader = f.readerStreaming(&.{});
+    var reader = f.readerStreaming(io, &.{});
     try reader.seekBy(2);
 
     var buffer: [20]u8 = undefined;
diff --git a/lib/std/http/Client.zig b/lib/std/http/Client.zig
index a701c09a90..dbd547611f 100644
--- a/lib/std/http/Client.zig
+++ b/lib/std/http/Client.zig
@@ -247,6 +247,7 @@ pub const Connection = struct {
             port: u16,
             stream: Io.net.Stream,
         ) error{OutOfMemory}!*Plain {
+            const io = client.io;
             const gpa = client.allocator;
             const alloc_len = allocLen(client, remote_host.bytes.len);
             const base = try gpa.alignedAlloc(u8, .of(Plain), alloc_len);
@@ -260,8 +261,8 @@ pub const Connection = struct {
             plain.* = .{
                 .connection = .{
                     .client = client,
-                    .stream_writer = stream.writer(socket_write_buffer),
-                    .stream_reader = stream.reader(socket_read_buffer),
+                    .stream_writer = stream.writer(io, socket_write_buffer),
+                    .stream_reader = stream.reader(io, socket_read_buffer),
                     .pool_node = .{},
                     .port = port,
                     .host_len = @intCast(remote_host.bytes.len),
@@ -300,6 +301,7 @@ pub const Connection = struct {
             port: u16,
             stream: Io.net.Stream,
         ) error{ OutOfMemory, TlsInitializationFailed }!*Tls {
+            const io = client.io;
             const gpa = client.allocator;
             const alloc_len = allocLen(client, remote_host.bytes.len);
             const base = try gpa.alignedAlloc(u8, .of(Tls), alloc_len);
@@ -316,11 +318,14 @@ pub const Connection = struct {
             assert(base.ptr + alloc_len == socket_read_buffer.ptr + socket_read_buffer.len);
             @memcpy(host_buffer, remote_host.bytes);
             const tls: *Tls = @ptrCast(base);
+            var random_buffer: [176]u8 = undefined;
+            std.crypto.random.bytes(&random_buffer);
+            const now_ts = if (Io.Timestamp.now(io, .real)) |ts| ts.toSeconds() else |_| return error.TlsInitializationFailed;
             tls.* = .{
                 .connection = .{
                     .client = client,
-                    .stream_writer = stream.writer(tls_write_buffer),
-                    .stream_reader = stream.reader(socket_read_buffer),
+                    .stream_writer = stream.writer(io, tls_write_buffer),
+                    .stream_reader = stream.reader(io, socket_read_buffer),
                     .pool_node = .{},
                     .port = port,
                     .host_len = @intCast(remote_host.bytes.len),
@@ -338,6 +343,8 @@ pub const Connection = struct {
                         .ssl_key_log = client.ssl_key_log,
                         .read_buffer = tls_read_buffer,
                         .write_buffer = socket_write_buffer,
+                        .entropy = &random_buffer,
+                        .realtime_now_seconds = now_ts,
                         // This is appropriate for HTTPS because the HTTP headers contain
                         // the content length which is used to detect truncation attacks.
                         .allow_truncation_attacks = true,
@@ -1390,16 +1397,8 @@ pub const basic_authorization = struct {
 };
 
 pub const ConnectTcpError = error{
-    ConnectionRefused,
-    NetworkUnreachable,
-    ConnectionTimedOut,
-    ConnectionResetByPeer,
-    TemporaryNameServerFailure,
-    NameServerFailure,
-    UnknownHostName,
-    UnexpectedConnectFailure,
     TlsInitializationFailed,
-} || Allocator.Error || Io.Cancelable;
+} || Allocator.Error || HostName.ConnectError;
 
 /// Reuses a `Connection` if one matching `host` and `port` is already open.
 ///
@@ -1424,6 +1423,7 @@ pub const ConnectTcpOptions = struct {
 };
 
 pub fn connectTcpOptions(client: *Client, options: ConnectTcpOptions) ConnectTcpError!*Connection {
+    const io = client.io;
     const host = options.host;
     const port = options.port;
     const protocol = options.protocol;
@@ -1437,22 +1437,17 @@ pub fn connectTcpOptions(client: *Client, options: ConnectTcpOptions) ConnectTcp
         .protocol = protocol,
     })) |conn| return conn;
 
-    const stream = host.connect(client.io, port, .{ .mode = .stream }) catch |err| switch (err) {
-        error.ConnectionRefused => return error.ConnectionRefused,
-        error.NetworkUnreachable => return error.NetworkUnreachable,
-        error.ConnectionTimedOut => return error.ConnectionTimedOut,
-        error.ConnectionResetByPeer => return error.ConnectionResetByPeer,
-        error.NameServerFailure => return error.NameServerFailure,
-        error.UnknownHostName => return error.UnknownHostName,
-        error.Canceled => return error.Canceled,
-        //else => return error.UnexpectedConnectFailure,
-    };
-    errdefer stream.close();
+    var stream = try host.connect(io, port, .{ .mode = .stream });
+    errdefer stream.close(io);
 
     switch (protocol) {
         .tls => {
             if (disable_tls) return error.TlsInitializationFailed;
-            const tc = try Connection.Tls.create(client, proxied_host, proxied_port, stream);
+            const tc = Connection.Tls.create(client, proxied_host, proxied_port, stream) catch |err| switch (err) {
+                error.OutOfMemory => |e| return e,
+                error.Unexpected => |e| return e,
+                error.UnsupportedClock => return error.TlsInitializationFailed,
+            };
             client.connection_pool.addUsed(&tc.connection);
             return &tc.connection;
         },
diff --git a/lib/std/os/linux/IoUring.zig b/lib/std/os/linux/IoUring.zig
index 25d4d88fd0..eaaa7643a9 100644
--- a/lib/std/os/linux/IoUring.zig
+++ b/lib/std/os/linux/IoUring.zig
@@ -3,7 +3,7 @@ const std = @import("std");
 const builtin = @import("builtin");
 const assert = std.debug.assert;
 const mem = std.mem;
-const net = std.net;
+const net = std.Io.net;
 const posix = std.posix;
 const linux = std.os.linux;
 const testing = std.testing;
@@ -2361,19 +2361,22 @@ test "sendmsg/recvmsg" {
     };
     defer ring.deinit();
 
-    var address_server = try net.Address.parseIp4("127.0.0.1", 0);
+    var address_server: linux.sockaddr.in = .{
+        .port = 0,
+        .addr = @bitCast([4]u8{ 127, 0, 0, 1 }),
+    };
 
-    const server = try posix.socket(address_server.any.family, posix.SOCK.DGRAM, 0);
+    const server = try posix.socket(address_server.family, posix.SOCK.DGRAM, 0);
     defer posix.close(server);
     try posix.setsockopt(server, posix.SOL.SOCKET, posix.SO.REUSEPORT, &mem.toBytes(@as(c_int, 1)));
     try posix.setsockopt(server, posix.SOL.SOCKET, posix.SO.REUSEADDR, &mem.toBytes(@as(c_int, 1)));
-    try posix.bind(server, &address_server.any, address_server.getOsSockLen());
+    try posix.bind(server, addrAny(&address_server), @sizeOf(linux.sockaddr.in));
 
     // set address_server to the OS-chosen IP/port.
-    var slen: posix.socklen_t = address_server.getOsSockLen();
-    try posix.getsockname(server, &address_server.any, &slen);
+    var slen: posix.socklen_t = @sizeOf(linux.sockaddr.in);
+    try posix.getsockname(server, addrAny(&address_server), &slen);
 
-    const client = try posix.socket(address_server.any.family, posix.SOCK.DGRAM, 0);
+    const client = try posix.socket(address_server.family, posix.SOCK.DGRAM, 0);
     defer posix.close(client);
 
     const buffer_send = [_]u8{42} ** 128;
@@ -2381,8 +2384,8 @@ test "sendmsg/recvmsg" {
         posix.iovec_const{ .base = &buffer_send, .len = buffer_send.len },
     };
     const msg_send: posix.msghdr_const = .{
-        .name = &address_server.any,
-        .namelen = address_server.getOsSockLen(),
+        .name = addrAny(&address_server),
+        .namelen = @sizeOf(linux.sockaddr.in),
         .iov = &iovecs_send,
         .iovlen = 1,
         .control = null,
@@ -2398,11 +2401,13 @@ test "sendmsg/recvmsg" {
     var iovecs_recv = [_]posix.iovec{
         posix.iovec{ .base = &buffer_recv, .len = buffer_recv.len },
     };
-    const addr = [_]u8{0} ** 4;
-    var address_recv = net.Address.initIp4(addr, 0);
+    var address_recv: linux.sockaddr.in = .{
+        .port = 0,
+        .addr = 0,
+    };
     var msg_recv: posix.msghdr = .{
-        .name = &address_recv.any,
-        .namelen = address_recv.getOsSockLen(),
+        .name = addrAny(&address_recv),
+        .namelen = @sizeOf(linux.sockaddr.in),
         .iov = &iovecs_recv,
         .iovlen = 1,
         .control = null,
@@ -2441,6 +2446,8 @@ test "sendmsg/recvmsg" {
 test "timeout (after a relative time)" {
     if (!is_linux) return error.SkipZigTest;
 
+    const io = testing.io;
+
     var ring = IoUring.init(1, 0) catch |err| switch (err) {
         error.SystemOutdated => return error.SkipZigTest,
         error.PermissionDenied => return error.SkipZigTest,
@@ -2452,12 +2459,12 @@ test "timeout (after a relative time)" {
     const margin = 5;
     const ts: linux.kernel_timespec = .{ .sec = 0, .nsec = ms * 1000000 };
 
-    const started = std.time.milliTimestamp();
+    const started = try std.Io.Timestamp.now(io, .awake);
     const sqe = try ring.timeout(0x55555555, &ts, 0, 0);
     try testing.expectEqual(linux.IORING_OP.TIMEOUT, sqe.opcode);
     try testing.expectEqual(@as(u32, 1), try ring.submit());
     const cqe = try ring.copy_cqe();
-    const stopped = std.time.milliTimestamp();
+    const stopped = try std.Io.Timestamp.now(io, .awake);
 
     try testing.expectEqual(linux.io_uring_cqe{
         .user_data = 0x55555555,
@@ -2466,7 +2473,8 @@ test "timeout (after a relative time)" {
     }, cqe);
 
     // Tests should not depend on timings: skip test if outside margin.
-    if (!std.math.approxEqAbs(f64, ms, @as(f64, @floatFromInt(stopped - started)), margin)) return error.SkipZigTest;
+    const ms_elapsed = started.durationTo(stopped).toMilliseconds();
+    if (ms_elapsed > margin) return error.SkipZigTest;
 }
 
 test "timeout (after a number of completions)" {
@@ -2861,19 +2869,22 @@ test "shutdown" {
     };
     defer ring.deinit();
 
-    var address = try net.Address.parseIp4("127.0.0.1", 0);
+    var address: linux.sockaddr.in = .{
+        .port = 0,
+        .addr = @bitCast([4]u8{ 127, 0, 0, 1 }),
+    };
 
     // Socket bound, expect shutdown to work
     {
-        const server = try posix.socket(address.any.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
+        const server = try posix.socket(address.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
         defer posix.close(server);
         try posix.setsockopt(server, posix.SOL.SOCKET, posix.SO.REUSEADDR, &mem.toBytes(@as(c_int, 1)));
-        try posix.bind(server, &address.any, address.getOsSockLen());
+        try posix.bind(server, addrAny(&address), @sizeOf(linux.sockaddr.in));
         try posix.listen(server, 1);
 
         // set address to the OS-chosen IP/port.
-        var slen: posix.socklen_t = address.getOsSockLen();
-        try posix.getsockname(server, &address.any, &slen);
+        var slen: posix.socklen_t = @sizeOf(linux.sockaddr.in);
+        try posix.getsockname(server, addrAny(&address), &slen);
 
         const shutdown_sqe = try ring.shutdown(0x445445445, server, linux.SHUT.RD);
         try testing.expectEqual(linux.IORING_OP.SHUTDOWN, shutdown_sqe.opcode);
@@ -2898,7 +2909,7 @@ test "shutdown" {
 
     // Socket not bound, expect to fail with ENOTCONN
     {
-        const server = try posix.socket(address.any.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
+        const server = try posix.socket(address.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
         defer posix.close(server);
 
         const shutdown_sqe = ring.shutdown(0x445445445, server, linux.SHUT.RD) catch |err| switch (err) {
@@ -2966,22 +2977,11 @@ test "renameat" {
     }, cqe);
 
     // Validate that the old file doesn't exist anymore
-    {
-        _ = tmp.dir.openFile(old_path, .{}) catch |err| switch (err) {
-            error.FileNotFound => {},
-            else => std.debug.panic("unexpected error: {}", .{err}),
-        };
-    }
+    try testing.expectError(error.FileNotFound, tmp.dir.openFile(old_path, .{}));
 
     // Validate that the new file exists with the proper content
-    {
-        const new_file = try tmp.dir.openFile(new_path, .{});
-        defer new_file.close();
-
-        var new_file_data: [16]u8 = undefined;
-        const bytes_read = try new_file.readAll(&new_file_data);
-        try testing.expectEqualStrings("hello", new_file_data[0..bytes_read]);
-    }
+    var new_file_data: [16]u8 = undefined;
+    try testing.expectEqualStrings("hello", try tmp.dir.readFile(new_path, &new_file_data));
 }
 
 test "unlinkat" {
@@ -3179,12 +3179,8 @@ test "linkat" {
     }, cqe);
 
     // Validate the second file
-    const second_file = try tmp.dir.openFile(second_path, .{});
-    defer second_file.close();
-
     var second_file_data: [16]u8 = undefined;
-    const bytes_read = try second_file.readAll(&second_file_data);
-    try testing.expectEqualStrings("hello", second_file_data[0..bytes_read]);
+    try testing.expectEqualStrings("hello", try tmp.dir.readFile(second_path, &second_file_data));
 }
 
 test "provide_buffers: read" {
@@ -3588,7 +3584,10 @@ const SocketTestHarness = struct {
 
 fn createSocketTestHarness(ring: *IoUring) !SocketTestHarness {
     // Create a TCP server socket
-    var address = try net.Address.parseIp4("127.0.0.1", 0);
+    var address: linux.sockaddr.in = .{
+        .port = 0,
+        .addr = @bitCast([4]u8{ 127, 0, 0, 1 }),
+    };
     const listener_socket = try createListenerSocket(&address);
     errdefer posix.close(listener_socket);
 
@@ -3598,9 +3597,9 @@ fn createSocketTestHarness(ring: *IoUring) !SocketTestHarness {
     _ = try ring.accept(0xaaaaaaaa, listener_socket, &accept_addr, &accept_addr_len, 0);
 
     // Create a TCP client socket
-    const client = try posix.socket(address.any.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
+    const client = try posix.socket(address.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
     errdefer posix.close(client);
-    _ = try ring.connect(0xcccccccc, client, &address.any, address.getOsSockLen());
+    _ = try ring.connect(0xcccccccc, client, addrAny(&address), @sizeOf(linux.sockaddr.in));
 
     try testing.expectEqual(@as(u32, 2), try ring.submit());
 
@@ -3636,18 +3635,18 @@ fn createSocketTestHarness(ring: *IoUring) !SocketTestHarness {
     };
 }
 
-fn createListenerSocket(address: *net.Address) !posix.socket_t {
+fn createListenerSocket(address: *linux.sockaddr.in) !posix.socket_t {
     const kernel_backlog = 1;
-    const listener_socket = try posix.socket(address.any.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
+    const listener_socket = try posix.socket(address.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
     errdefer posix.close(listener_socket);
 
     try posix.setsockopt(listener_socket, posix.SOL.SOCKET, posix.SO.REUSEADDR, &mem.toBytes(@as(c_int, 1)));
-    try posix.bind(listener_socket, &address.any, address.getOsSockLen());
+    try posix.bind(listener_socket, addrAny(address), @sizeOf(linux.sockaddr.in));
     try posix.listen(listener_socket, kernel_backlog);
 
     // set address to the OS-chosen IP/port.
-    var slen: posix.socklen_t = address.getOsSockLen();
-    try posix.getsockname(listener_socket, &address.any, &slen);
+    var slen: posix.socklen_t = @sizeOf(linux.sockaddr.in);
+    try posix.getsockname(listener_socket, addrAny(address), &slen);
 
     return listener_socket;
 }
@@ -3662,7 +3661,10 @@ test "accept multishot" {
     };
     defer ring.deinit();
 
-    var address = try net.Address.parseIp4("127.0.0.1", 0);
+    var address: linux.sockaddr.in = .{
+        .port = 0,
+        .addr = @bitCast([4]u8{ 127, 0, 0, 1 }),
+    };
     const listener_socket = try createListenerSocket(&address);
     defer posix.close(listener_socket);
 
@@ -3676,9 +3678,9 @@ test "accept multishot" {
     var nr: usize = 4; // number of clients to connect
     while (nr > 0) : (nr -= 1) {
         // connect client
-        const client = try posix.socket(address.any.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
+        const client = try posix.socket(address.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
         errdefer posix.close(client);
-        try posix.connect(client, &address.any, address.getOsSockLen());
+        try posix.connect(client, addrAny(&address), @sizeOf(linux.sockaddr.in));
 
         // test accept completion
         var cqe = try ring.copy_cqe();
@@ -3756,7 +3758,10 @@ test "accept_direct" {
         else => return err,
     };
     defer ring.deinit();
-    var address = try net.Address.parseIp4("127.0.0.1", 0);
+    var address: linux.sockaddr.in = .{
+        .port = 0,
+        .addr = @bitCast([4]u8{ 127, 0, 0, 1 }),
+    };
 
     // register direct file descriptors
     var registered_fds = [_]posix.fd_t{-1} ** 2;
@@ -3779,8 +3784,8 @@ test "accept_direct" {
             try testing.expectEqual(@as(u32, 1), try ring.submit());
 
             // connect
-            const client = try posix.socket(address.any.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
-            try posix.connect(client, &address.any, address.getOsSockLen());
+            const client = try posix.socket(address.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
+            try posix.connect(client, addrAny(&address), @sizeOf(linux.sockaddr.in));
             defer posix.close(client);
 
             // accept completion
@@ -3813,8 +3818,8 @@ test "accept_direct" {
             _ = try ring.accept_direct(accept_userdata, listener_socket, null, null, 0);
             try testing.expectEqual(@as(u32, 1), try ring.submit());
             // connect
-            const client = try posix.socket(address.any.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
-            try posix.connect(client, &address.any, address.getOsSockLen());
+            const client = try posix.socket(address.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
+            try posix.connect(client, addrAny(&address), @sizeOf(linux.sockaddr.in));
             defer posix.close(client);
             // completion with error
             const cqe_accept = try ring.copy_cqe();
@@ -3837,7 +3842,10 @@ test "accept_multishot_direct" {
     };
     defer ring.deinit();
 
-    var address = try net.Address.parseIp4("127.0.0.1", 0);
+    var address: linux.sockaddr.in = .{
+        .port = 0,
+        .addr = @bitCast([4]u8{ 127, 0, 0, 1 }),
+    };
 
     var registered_fds = [_]posix.fd_t{-1} ** 2;
     try ring.register_files(registered_fds[0..]);
@@ -3855,8 +3863,8 @@ test "accept_multishot_direct" {
 
         for (registered_fds) |_| {
             // connect
-            const client = try posix.socket(address.any.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
-            try posix.connect(client, &address.any, address.getOsSockLen());
+            const client = try posix.socket(address.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
+            try posix.connect(client, addrAny(&address), @sizeOf(linux.sockaddr.in));
             defer posix.close(client);
 
             // accept completion
@@ -3870,8 +3878,8 @@ test "accept_multishot_direct" {
         // Multishot is terminated (more flag is not set).
         {
             // connect
-            const client = try posix.socket(address.any.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
-            try posix.connect(client, &address.any, address.getOsSockLen());
+            const client = try posix.socket(address.family, posix.SOCK.STREAM | posix.SOCK.CLOEXEC, 0);
+            try posix.connect(client, addrAny(&address), @sizeOf(linux.sockaddr.in));
             defer posix.close(client);
             // completion with error
             const cqe_accept = try ring.copy_cqe();
@@ -3944,7 +3952,10 @@ test "socket_direct/socket_direct_alloc/close_direct" {
     try testing.expect(cqe_socket.res == 2); // returns registered file index
 
     // use sockets from registered_fds in connect operation
-    var address = try net.Address.parseIp4("127.0.0.1", 0);
+    var address: linux.sockaddr.in = .{
+        .port = 0,
+        .addr = @bitCast([4]u8{ 127, 0, 0, 1 }),
+    };
     const listener_socket = try createListenerSocket(&address);
     defer posix.close(listener_socket);
     const accept_userdata: u64 = 0xaaaaaaaa;
@@ -3954,7 +3965,7 @@ test "socket_direct/socket_direct_alloc/close_direct" {
         // prepare accept
         _ = try ring.accept(accept_userdata, listener_socket, null, null, 0);
         // prepare connect with fixed socket
-        const connect_sqe = try ring.connect(connect_userdata, @intCast(fd_index), &address.any, address.getOsSockLen());
+        const connect_sqe = try ring.connect(connect_userdata, @intCast(fd_index), addrAny(&address), @sizeOf(linux.sockaddr.in));
         connect_sqe.flags |= linux.IOSQE_FIXED_FILE; // fd is fixed file index
         // submit both
         try testing.expectEqual(@as(u32, 2), try ring.submit());
@@ -4483,12 +4494,15 @@ test "bind/listen/connect" {
     // LISTEN is higher required operation
     if (!probe.is_supported(.LISTEN)) return error.SkipZigTest;
 
-    var addr = net.Address.initIp4([4]u8{ 127, 0, 0, 1 }, 0);
-    const proto: u32 = if (addr.any.family == linux.AF.UNIX) 0 else linux.IPPROTO.TCP;
+    var addr: linux.sockaddr.in = .{
+        .port = 0,
+        .addr = @bitCast([4]u8{ 127, 0, 0, 1 }),
+    };
+    const proto: u32 = if (addr.family == linux.AF.UNIX) 0 else linux.IPPROTO.TCP;
 
     const listen_fd = brk: {
         // Create socket
-        _ = try ring.socket(1, addr.any.family, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, proto, 0);
+        _ = try ring.socket(1, addr.family, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, proto, 0);
         try testing.expectEqual(1, try ring.submit());
         var cqe = try ring.copy_cqe();
         try testing.expectEqual(1, cqe.user_data);
@@ -4500,7 +4514,7 @@ test "bind/listen/connect" {
         var optval: u32 = 1;
         (try ring.setsockopt(2, listen_fd, linux.SOL.SOCKET, linux.SO.REUSEADDR, mem.asBytes(&optval))).link_next();
         (try ring.setsockopt(3, listen_fd, linux.SOL.SOCKET, linux.SO.REUSEPORT, mem.asBytes(&optval))).link_next();
-        (try ring.bind(4, listen_fd, &addr.any, addr.getOsSockLen(), 0)).link_next();
+        (try ring.bind(4, listen_fd, addrAny(&addr), @sizeOf(linux.sockaddr.in), 0)).link_next();
         _ = try ring.listen(5, listen_fd, 1, 0);
         // Submit 4 operations
         try testing.expectEqual(4, try ring.submit());
@@ -4521,15 +4535,15 @@ test "bind/listen/connect" {
         try testing.expectEqual(1, optval);
 
         // Read system assigned port into addr
-        var addr_len: posix.socklen_t = addr.getOsSockLen();
-        try posix.getsockname(listen_fd, &addr.any, &addr_len);
+        var addr_len: posix.socklen_t = @sizeOf(linux.sockaddr.in);
+        try posix.getsockname(listen_fd, addrAny(&addr), &addr_len);
 
         break :brk listen_fd;
     };
 
     const connect_fd = brk: {
         // Create connect socket
-        _ = try ring.socket(6, addr.any.family, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, proto, 0);
+        _ = try ring.socket(6, addr.family, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, proto, 0);
         try testing.expectEqual(1, try ring.submit());
         const cqe = try ring.copy_cqe();
         try testing.expectEqual(6, cqe.user_data);
@@ -4542,7 +4556,7 @@ test "bind/listen/connect" {
 
     // Prepare accept/connect operations
     _ = try ring.accept(7, listen_fd, null, null, 0);
-    _ = try ring.connect(8, connect_fd, &addr.any, addr.getOsSockLen());
+    _ = try ring.connect(8, connect_fd, addrAny(&addr), @sizeOf(linux.sockaddr.in));
     try testing.expectEqual(2, try ring.submit());
     // Get listener accepted socket
     var accept_fd: posix.socket_t = 0;
@@ -4604,3 +4618,7 @@ fn testSendRecv(ring: *IoUring, send_fd: posix.socket_t, recv_fd: posix.socket_t
     try testing.expectEqualSlices(u8, buffer_send, buffer_recv[0..buffer_send.len]);
     try testing.expectEqualSlices(u8, buffer_send, buffer_recv[buffer_send.len..]);
 }
+
+fn addrAny(addr: *linux.sockaddr.in) *linux.sockaddr {
+    return @ptrCast(addr);
+}
diff --git a/lib/std/posix.zig b/lib/std/posix.zig
index 93676f2a74..c5e61749b2 100644
--- a/lib/std/posix.zig
+++ b/lib/std/posix.zig
@@ -3000,31 +3000,7 @@ pub fn mkdiratW(dir_fd: fd_t, sub_path_w: []const u16, mode: mode_t) MakeDirErro
     windows.CloseHandle(sub_dir_handle);
 }
 
-pub const MakeDirError = error{
-    /// In WASI, this error may occur when the file descriptor does
-    /// not hold the required rights to create a new directory relative to it.
-    AccessDenied,
-    PermissionDenied,
-    DiskQuota,
-    PathAlreadyExists,
-    SymLinkLoop,
-    LinkQuotaExceeded,
-    NameTooLong,
-    FileNotFound,
-    SystemResources,
-    NoSpaceLeft,
-    NotDir,
-    ReadOnlyFileSystem,
-    /// WASI-only; file paths must be valid UTF-8.
-    InvalidUtf8,
-    /// Windows-only; file paths provided by the user must be valid WTF-8.
-    /// https://wtf-8.codeberg.page/
-    InvalidWtf8,
-    BadPathName,
-    NoDevice,
-    /// On Windows, `\\server` or `\\server\share` was not found.
-    NetworkNotFound,
-} || UnexpectedError;
+pub const MakeDirError = std.Io.Dir.MakeError;
 
 /// Create a directory.
 /// `mode` is ignored on Windows and WASI.
diff --git a/lib/std/posix/test.zig b/lib/std/posix/test.zig
index f8206b8a5a..b5ce476441 100644
--- a/lib/std/posix/test.zig
+++ b/lib/std/posix/test.zig
@@ -731,11 +731,8 @@ test "dup & dup2" {
         try dup2ed.writeAll("dup2");
     }
 
-    var file = try tmp.dir.openFile("os_dup_test", .{});
-    defer file.close();
-
-    var buf: [7]u8 = undefined;
-    try testing.expectEqualStrings("dupdup2", buf[0..try file.readAll(&buf)]);
+    var buffer: [8]u8 = undefined;
+    try testing.expectEqualStrings("dupdup2", try tmp.dir.readFile("os_dup_test", &buffer));
 }
 
 test "writev longer than IOV_MAX" {
diff --git a/lib/std/process/Child.zig b/lib/std/process/Child.zig
index 50157d52d9..c018d77424 100644
--- a/lib/std/process/Child.zig
+++ b/lib/std/process/Child.zig
@@ -1,5 +1,9 @@
-const std = @import("../std.zig");
+const ChildProcess = @This();
+
 const builtin = @import("builtin");
+const native_os = builtin.os.tag;
+
+const std = @import("../std.zig");
 const unicode = std.unicode;
 const fs = std.fs;
 const process = std.process;
@@ -11,9 +15,7 @@ const mem = std.mem;
 const EnvMap = std.process.EnvMap;
 const maxInt = std.math.maxInt;
 const assert = std.debug.assert;
-const native_os = builtin.os.tag;
 const Allocator = std.mem.Allocator;
-const ChildProcess = @This();
 const ArrayList = std.ArrayList;
 
 pub const Id = switch (native_os) {
@@ -317,16 +319,23 @@ pub fn waitForSpawn(self: *ChildProcess) SpawnError!void {
 
     const err_pipe = self.err_pipe orelse return;
     self.err_pipe = null;
-
     // Wait for the child to report any errors in or before `execvpe`.
-    if (readIntFd(err_pipe)) |child_err_int| {
-        posix.close(err_pipe);
+    const report = readIntFd(err_pipe);
+    posix.close(err_pipe);
+    if (report) |child_err_int| {
         const child_err: SpawnError = @errorCast(@errorFromInt(child_err_int));
         self.term = child_err;
         return child_err;
-    } else |_| {
-        // Write end closed by CLOEXEC at the time of the `execvpe` call, indicating success!
-        posix.close(err_pipe);
+    } else |read_err| switch (read_err) {
+        error.EndOfStream => {
+            // Write end closed by CLOEXEC at the time of the `execvpe` call,
+            // indicating success.
+        },
+        else => {
+            // Problem reading the error from the error reporting pipe. We
+            // don't know if the child is alive or dead. Better to assume it is
+            // alive so the resource does not risk being leaked.
+        },
     }
 }
 
@@ -1014,8 +1023,14 @@ fn writeIntFd(fd: i32, value: ErrInt) !void {
 
 fn readIntFd(fd: i32) !ErrInt {
     var buffer: [8]u8 = undefined;
-    var fr: std.fs.File.Reader = .initStreaming(.{ .handle = fd }, &buffer);
-    return @intCast(fr.interface.takeInt(u64, .little) catch return error.SystemResources);
+    var i: usize = 0;
+    while (i < buffer.len) {
+        const n = try std.posix.read(fd, buffer[i..]);
+        if (n == 0) return error.EndOfStream;
+        i += n;
+    }
+    const int = mem.readInt(u64, &buffer, .little);
+    return @intCast(int);
 }
 
 const ErrInt = std.meta.Int(.unsigned, @sizeOf(anyerror) * 8);
diff --git a/lib/std/tar/Writer.zig b/lib/std/tar/Writer.zig
index bffdb8ee7c..a48e8cc407 100644
--- a/lib/std/tar/Writer.zig
+++ b/lib/std/tar/Writer.zig
@@ -1,7 +1,9 @@
+const Writer = @This();
+
 const std = @import("std");
+const Io = std.Io;
 const assert = std.debug.assert;
 const testing = std.testing;
-const Writer = @This();
 
 const block_size = @sizeOf(Header);
 
@@ -14,7 +16,7 @@ pub const Options = struct {
     mtime: u64 = 0,
 };
 
-underlying_writer: *std.Io.Writer,
+underlying_writer: *Io.Writer,
 prefix: []const u8 = "",
 mtime_now: u64 = 0,
 
@@ -36,12 +38,12 @@ pub fn writeDir(w: *Writer, sub_path: []const u8, options: Options) Error!void {
     try w.writeHeader(.directory, sub_path, "", 0, options);
 }
 
-pub const WriteFileError = std.Io.Writer.FileError || Error || std.fs.File.Reader.SizeError;
+pub const WriteFileError = Io.Writer.FileError || Error || Io.File.Reader.SizeError;
 
 pub fn writeFile(
     w: *Writer,
     sub_path: []const u8,
-    file_reader: *std.fs.File.Reader,
+    file_reader: *Io.File.Reader,
     stat_mtime: i128,
 ) WriteFileError!void {
     const size = try file_reader.getSize();
@@ -58,7 +60,7 @@ pub fn writeFile(
     try w.writePadding64(size);
 }
 
-pub const WriteFileStreamError = Error || std.Io.Reader.StreamError;
+pub const WriteFileStreamError = Error || Io.Reader.StreamError;
 
 /// Writes file reading file content from `reader`. Reads exactly `size` bytes
 /// from `reader`, or returns `error.EndOfStream`.
@@ -66,7 +68,7 @@ pub fn writeFileStream(
     w: *Writer,
     sub_path: []const u8,
     size: u64,
-    reader: *std.Io.Reader,
+    reader: *Io.Reader,
     options: Options,
 ) WriteFileStreamError!void {
     try w.writeHeader(.regular, sub_path, "", size, options);
@@ -136,15 +138,15 @@ fn writeExtendedHeader(w: *Writer, typeflag: Header.FileType, buffers: []const [
     try w.writePadding(len);
 }
 
-fn writePadding(w: *Writer, bytes: usize) std.Io.Writer.Error!void {
+fn writePadding(w: *Writer, bytes: usize) Io.Writer.Error!void {
     return writePaddingPos(w, bytes % block_size);
 }
 
-fn writePadding64(w: *Writer, bytes: u64) std.Io.Writer.Error!void {
+fn writePadding64(w: *Writer, bytes: u64) Io.Writer.Error!void {
     return writePaddingPos(w, @intCast(bytes % block_size));
 }
 
-fn writePaddingPos(w: *Writer, pos: usize) std.Io.Writer.Error!void {
+fn writePaddingPos(w: *Writer, pos: usize) Io.Writer.Error!void {
     if (pos == 0) return;
     try w.underlying_writer.splatByteAll(0, block_size - pos);
 }
@@ -153,7 +155,7 @@ fn writePaddingPos(w: *Writer, pos: usize) std.Io.Writer.Error!void {
 /// "reasonable system must not assume that such a block exists when reading an
 /// archive". Therefore, the Zig standard library recommends to not call this
 /// function.
-pub fn finishPedantically(w: *Writer) std.Io.Writer.Error!void {
+pub fn finishPedantically(w: *Writer) Io.Writer.Error!void {
     try w.underlying_writer.splatByteAll(0, block_size * 2);
 }
 
@@ -248,7 +250,7 @@ pub const Header = extern struct {
         try octal(&w.checksum, checksum);
     }
 
-    pub fn write(h: *Header, bw: *std.Io.Writer) error{ OctalOverflow, WriteFailed }!void {
+    pub fn write(h: *Header, bw: *Io.Writer) error{ OctalOverflow, WriteFailed }!void {
         try h.updateChecksum();
         try bw.writeAll(std.mem.asBytes(h));
     }
@@ -396,14 +398,14 @@ test "write files" {
     {
         const root = "root";
 
-        var output: std.Io.Writer.Allocating = .init(testing.allocator);
+        var output: Io.Writer.Allocating = .init(testing.allocator);
         var w: Writer = .{ .underlying_writer = &output.writer };
         defer output.deinit();
         try w.setRoot(root);
         for (files) |file|
             try w.writeFileBytes(file.path, file.content, .{});
 
-        var input: std.Io.Reader = .fixed(output.written());
+        var input: Io.Reader = .fixed(output.written());
         var it: std.tar.Iterator = .init(&input, .{
             .file_name_buffer = &file_name_buffer,
             .link_name_buffer = &link_name_buffer,
@@ -424,7 +426,7 @@ test "write files" {
             try testing.expectEqual('/', actual.name[root.len..][0]);
             try testing.expectEqualStrings(expected.path, actual.name[root.len + 1 ..]);
 
-            var content: std.Io.Writer.Allocating = .init(testing.allocator);
+            var content: Io.Writer.Allocating = .init(testing.allocator);
             defer content.deinit();
             try it.streamRemaining(actual, &content.writer);
             try testing.expectEqualSlices(u8, expected.content, content.written());
@@ -432,15 +434,15 @@ test "write files" {
     }
     // without root
     {
-        var output: std.Io.Writer.Allocating = .init(testing.allocator);
+        var output: Io.Writer.Allocating = .init(testing.allocator);
         var w: Writer = .{ .underlying_writer = &output.writer };
         defer output.deinit();
         for (files) |file| {
-            var content: std.Io.Reader = .fixed(file.content);
+            var content: Io.Reader = .fixed(file.content);
             try w.writeFileStream(file.path, file.content.len, &content, .{});
         }
 
-        var input: std.Io.Reader = .fixed(output.written());
+        var input: Io.Reader = .fixed(output.written());
         var it: std.tar.Iterator = .init(&input, .{
             .file_name_buffer = &file_name_buffer,
             .link_name_buffer = &link_name_buffer,
@@ -452,7 +454,7 @@ test "write files" {
             const expected = files[i];
             try testing.expectEqualStrings(expected.path, actual.name);
 
-            var content: std.Io.Writer.Allocating = .init(testing.allocator);
+            var content: Io.Writer.Allocating = .init(testing.allocator);
             defer content.deinit();
             try it.streamRemaining(actual, &content.writer);
             try testing.expectEqualSlices(u8, expected.content, content.written());
diff --git a/lib/std/zig.zig b/lib/std/zig.zig
index 04e5c2b221..6d3ccc9b22 100644
--- a/lib/std/zig.zig
+++ b/lib/std/zig.zig
@@ -559,7 +559,7 @@ test isUnderscore {
 /// If the source can be UTF-16LE encoded, this function asserts that `gpa`
 /// will align a byte-sized allocation to at least 2. Allocators that don't do
 /// this are rare.
-pub fn readSourceFileToEndAlloc(gpa: Allocator, file_reader: *std.fs.File.Reader) ![:0]u8 {
+pub fn readSourceFileToEndAlloc(gpa: Allocator, file_reader: *Io.File.Reader) ![:0]u8 {
     var buffer: std.ArrayList(u8) = .empty;
     defer buffer.deinit(gpa);
 
diff --git a/lib/std/zig/system.zig b/lib/std/zig/system.zig
index 43d64205a7..7318612151 100644
--- a/lib/std/zig/system.zig
+++ b/lib/std/zig/system.zig
@@ -442,6 +442,7 @@ pub fn resolveTargetQuery(io: Io, query: Target.Query) DetectError!Target {
         error.DeviceBusy,
         error.InputOutput,
         error.LockViolation,
+        error.FileSystem,
 
         error.UnableToOpenElfFile,
         error.UnhelpfulFile,
@@ -542,16 +543,15 @@ fn detectNativeCpuAndFeatures(cpu_arch: Target.Cpu.Arch, os: Target.Os, query: T
     return null;
 }
 
-pub const AbiAndDynamicLinkerFromFileError = error{};
-
-pub fn abiAndDynamicLinkerFromFile(
+fn abiAndDynamicLinkerFromFile(
     file_reader: *Io.File.Reader,
     header: *const elf.Header,
     cpu: Target.Cpu,
     os: Target.Os,
     ld_info_list: []const LdInfo,
     query: Target.Query,
-) AbiAndDynamicLinkerFromFileError!Target {
+) !Target {
+    const io = file_reader.io;
     var result: Target = .{
         .cpu = cpu,
         .os = os,
@@ -623,8 +623,8 @@ pub fn abiAndDynamicLinkerFromFile(
         try file_reader.seekTo(shstr.sh_offset);
         try file_reader.interface.readSliceAll(shstrtab);
         const dynstr: ?struct { offset: u64, size: u64 } = find_dyn_str: {
-            var it = header.iterateSectionHeaders(&file_reader.interface);
-            while (it.next()) |shdr| {
+            var it = header.iterateSectionHeaders(file_reader);
+            while (try it.next()) |shdr| {
                 const end = mem.findScalarPos(u8, shstrtab, shdr.sh_name, 0) orelse continue;
                 const sh_name = shstrtab[shdr.sh_name..end :0];
                 if (mem.eql(u8, sh_name, ".dynstr")) break :find_dyn_str .{
@@ -645,7 +645,7 @@ pub fn abiAndDynamicLinkerFromFile(
 
                 var it = mem.tokenizeScalar(u8, rpath_list, ':');
                 while (it.next()) |rpath| {
-                    if (glibcVerFromRPath(rpath)) |ver| {
+                    if (glibcVerFromRPath(io, rpath)) |ver| {
                         result.os.version_range.linux.glibc = ver;
                         return result;
                     } else |err| switch (err) {
@@ -660,7 +660,7 @@ pub fn abiAndDynamicLinkerFromFile(
             // There is no DT_RUNPATH so we try to find libc.so.6 inside the same
             // directory as the dynamic linker.
             if (fs.path.dirname(dl_path)) |rpath| {
-                if (glibcVerFromRPath(rpath)) |ver| {
+                if (glibcVerFromRPath(io, rpath)) |ver| {
                     result.os.version_range.linux.glibc = ver;
                     return result;
                 } else |err| switch (err) {
@@ -725,7 +725,7 @@ pub fn abiAndDynamicLinkerFromFile(
         @memcpy(path_buf[index..][0..abi.len], abi);
         index += abi.len;
         const rpath = path_buf[0..index];
-        if (glibcVerFromRPath(rpath)) |ver| {
+        if (glibcVerFromRPath(io, rpath)) |ver| {
             result.os.version_range.linux.glibc = ver;
             return result;
         } else |err| switch (err) {
@@ -842,18 +842,13 @@ fn glibcVerFromRPath(io: Io, rpath: []const u8) !std.SemanticVersion {
         error.InvalidElfMagic,
         error.InvalidElfEndian,
         error.InvalidElfClass,
-        error.InvalidElfFile,
         error.InvalidElfVersion,
         error.InvalidGnuLibCVersion,
         error.EndOfStream,
         => return error.GLibCNotFound,
 
-        error.SystemResources,
-        error.UnableToReadElfFile,
-        error.Unexpected,
-        error.FileSystem,
-        error.ProcessNotFound,
-        => |e| return e,
+        error.ReadFailed => return file_reader.err.?,
+        else => |e| return e,
     };
 }
 
@@ -867,8 +862,8 @@ fn glibcVerFromSoFile(file_reader: *Io.File.Reader) !std.SemanticVersion {
     try file_reader.seekTo(shstr.sh_offset);
     try file_reader.interface.readSliceAll(shstrtab);
     const dynstr: struct { offset: u64, size: u64 } = find_dyn_str: {
-        var it = header.iterateSectionHeaders(&file_reader.interface);
-        while (it.next()) |shdr| {
+        var it = header.iterateSectionHeaders(file_reader);
+        while (try it.next()) |shdr| {
             const end = mem.findScalarPos(u8, shstrtab, shdr.sh_name, 0) orelse continue;
             const sh_name = shstrtab[shdr.sh_name..end :0];
             if (mem.eql(u8, sh_name, ".dynstr")) break :find_dyn_str .{
@@ -882,19 +877,25 @@ fn glibcVerFromSoFile(file_reader: *Io.File.Reader) !std.SemanticVersion {
     // strings that start with "GLIBC_2." indicate the existence of such a glibc version,
     // and furthermore, that the system-installed glibc is at minimum that version.
     var max_ver: std.SemanticVersion = .{ .major = 2, .minor = 2, .patch = 5 };
-
+    var offset: u64 = 0;
     try file_reader.seekTo(dynstr.offset);
-    while (file_reader.interface.takeSentinel(0)) |s| {
-        if (mem.startsWith(u8, s, "GLIBC_2.")) {
-            const chopped = s["GLIBC_".len..];
-            const ver = Target.Query.parseVersion(chopped) catch |err| switch (err) {
-                error.Overflow => return error.InvalidGnuLibCVersion,
-                error.InvalidVersion => return error.InvalidGnuLibCVersion,
-            };
-            switch (ver.order(max_ver)) {
-                .gt => max_ver = ver,
-                .lt, .eq => continue,
+    while (offset < dynstr.size) {
+        if (file_reader.interface.takeSentinel(0)) |s| {
+            if (mem.startsWith(u8, s, "GLIBC_2.")) {
+                const chopped = s["GLIBC_".len..];
+                const ver = Target.Query.parseVersion(chopped) catch |err| switch (err) {
+                    error.Overflow => return error.InvalidGnuLibCVersion,
+                    error.InvalidVersion => return error.InvalidGnuLibCVersion,
+                };
+                switch (ver.order(max_ver)) {
+                    .gt => max_ver = ver,
+                    .lt, .eq => continue,
+                }
             }
+            offset += s.len + 1;
+        } else |err| switch (err) {
+            error.EndOfStream, error.StreamTooLong => break,
+            error.ReadFailed => |e| return e,
         }
     }
 
@@ -1091,22 +1092,12 @@ fn detectAbiAndDynamicLinker(io: Io, cpu: Target.Cpu, os: Target.Os, query: Targ
         error.ProcessFdQuotaExceeded,
         error.SystemFdQuotaExceeded,
         error.ProcessNotFound,
+        error.Canceled,
         => |e| return e,
 
         error.ReadFailed => return file_reader.err.?,
 
-        error.UnableToReadElfFile,
-        error.InvalidElfClass,
-        error.InvalidElfVersion,
-        error.InvalidElfEndian,
-        error.InvalidElfFile,
-        error.InvalidElfMagic,
-        error.Unexpected,
-        error.EndOfStream,
-        error.NameTooLong,
-        error.StaticElfFile,
-        // Finally, we fall back on the standard path.
-        => |e| {
+        else => |e| {
             std.log.warn("encountered {t}; falling back to default ABI and dynamic linker", .{e});
             return defaultAbiAndDynamicLinker(cpu, os, query);
         },
diff --git a/test/src/Cases.zig b/test/src/Cases.zig
index 7edb66aede..9f134075b4 100644
--- a/test/src/Cases.zig
+++ b/test/src/Cases.zig
@@ -455,8 +455,7 @@ pub fn lowerToBuildSteps(
     parent_step: *std.Build.Step,
     options: CaseTestOptions,
 ) void {
-    const host = std.zig.system.resolveTargetQuery(.{}) catch |err|
-        std.debug.panic("unable to detect native host: {s}\n", .{@errorName(err)});
+    const host = b.resolveTargetQuery(.{});
     const cases_dir_path = b.build_root.join(b.allocator, &.{ "test", "cases" }) catch @panic("OOM");
 
     for (self.cases.items) |case| {
@@ -587,7 +586,7 @@ pub fn lowerToBuildSteps(
             },
             .Execution => |expected_stdout| no_exec: {
                 const run = if (case.target.result.ofmt == .c) run_step: {
-                    if (getExternalExecutor(&host, &case.target.result, .{ .link_libc = true }) != .native) {
+                    if (getExternalExecutor(&host.result, &case.target.result, .{ .link_libc = true }) != .native) {
                         // We wouldn't be able to run the compiled C code.
                         break :no_exec;
                     }
@@ -972,14 +971,6 @@ const TestManifest = struct {
     }
 };
 
-fn resolveTargetQuery(query: std.Target.Query) std.Build.ResolvedTarget {
-    return .{
-        .query = query,
-        .target = std.zig.system.resolveTargetQuery(query) catch
-            @panic("unable to resolve target query"),
-    };
-}
-
 fn knownFileExtension(filename: []const u8) bool {
     // List taken from `Compilation.classifyFileExt` in the compiler.
     for ([_][]const u8{