diff --git a/src/Zcu.zig b/src/Zcu.zig
index 1cbfe0175d..10fec614d1 100644
--- a/src/Zcu.zig
+++ b/src/Zcu.zig
@@ -2817,12 +2817,18 @@ comptime {
 }
 
 pub fn loadZirCache(gpa: Allocator, cache_file: std.fs.File) !Zir {
-    var header: Zir.Header = undefined;
-    if (try cache_file.readAll(std.mem.asBytes(&header)) < @sizeOf(Zir.Header)) return error.EndOfStream;
-    return loadZirCacheBody(gpa, header, cache_file);
+    var cache_fr = cache_file.reader();
+    var cache_br = cache_fr.interface().unbuffered();
+    return result: {
+        const header = cache_br.takeStruct(Zir.Header) catch |err| break :result err;
+        break :result loadZirCacheBody(gpa, header.*, &cache_br);
+    } catch |err| switch (err) {
+        error.ReadFailed => return cache_fr.err.?,
+        else => |e| return e,
+    };
 }
 
-pub fn loadZirCacheBody(gpa: Allocator, header: Zir.Header, cache_file: std.fs.File) !Zir {
+pub fn loadZirCacheBody(gpa: Allocator, header: Zir.Header, cache_br: *std.io.BufferedReader) !Zir {
     var instructions: std.MultiArrayList(Zir.Inst) = .{};
     errdefer instructions.deinit(gpa);
 
@@ -2850,16 +2856,11 @@ pub fn loadZirCacheBody(gpa: Allocator, header: Zir.Header, cache_file: std.fs.F
         if (data_has_safety_tag)
             @ptrCast(safety_buffer)
         else
-            zir.instructions.items(.data),
+            @ptrCast(zir.instructions.items(.data)),
         zir.string_bytes,
         @ptrCast(zir.extra),
     };
-    var cache_fr = cache_file.reader();
-    var cache_br = cache_fr.interface().unbuffered();
-    cache_br.readVecAll(&vecs) catch |err| switch (err) {
-        error.ReadFailed => return cache_fr.err.?,
-        error.EndOfStream => return error.UnexpectedFileSize,
-    };
+    try cache_br.readVecAll(&vecs);
     if (data_has_safety_tag) {
         const tags = zir.instructions.items(.tag);
         for (zir.instructions.items(.data), 0..) |*data, i| {
@@ -2871,7 +2872,6 @@ pub fn loadZirCacheBody(gpa: Allocator, header: Zir.Header, cache_file: std.fs.F
             };
         }
     }
-
     return zir;
 }
 
@@ -2947,7 +2947,7 @@ pub fn saveZoirCache(cache_file: std.fs.File, stat: std.fs.File.Stat, zoir: Zoir
     };
 }
 
-pub fn loadZoirCacheBody(gpa: Allocator, header: Zoir.Header, cache_file: std.fs.File) !Zoir {
+pub fn loadZoirCacheBody(gpa: Allocator, header: Zoir.Header, cache_br: *std.io.BufferedReader) !Zoir {
     var zoir: Zoir = .{
         .nodes = .empty,
         .extra = &.{},
@@ -2983,12 +2983,7 @@ pub fn loadZoirCacheBody(gpa: Allocator, header: Zoir.Header, cache_file: std.fs
         @ptrCast(zoir.compile_errors),
         @ptrCast(zoir.error_notes),
     };
-    var cache_fr = cache_file.reader();
-    var cache_br = cache_fr.interface().unbuffered();
-    cache_br.readVecAll(&vecs) catch |err| switch (err) {
-        error.ReadFailed => return cache_fr.err.?,
-        error.EndOfStream => return error.UnexpectedFileSize,
-    };
+    try cache_br.readVecAll(&vecs);
     return zoir;
 }
 
diff --git a/src/Zcu/PerThread.zig b/src/Zcu/PerThread.zig
index e8c331b5cc..2fbead2f37 100644
--- a/src/Zcu/PerThread.zig
+++ b/src/Zcu/PerThread.zig
@@ -349,6 +349,7 @@ fn loadZirZoirCache(
 
     // First we read the header to determine the lengths of arrays.
     const header = (cache_br.takeStruct(Header) catch |err| switch (err) {
+        error.ReadFailed => return cache_fr.err.?,
         // This can happen if Zig bails out of this function between creating
         // the cached file and writing it.
         error.EndOfStream => return .invalid,
@@ -365,17 +366,15 @@ fn loadZirZoirCache(
     }
 
     switch (mode) {
-        .zig => {
-            file.zir = Zcu.loadZirCacheBody(gpa, header, cache_file) catch |err| switch (err) {
-                error.UnexpectedFileSize => return .truncated,
-                else => |e| return e,
-            };
+        .zig => file.zir = Zcu.loadZirCacheBody(gpa, header, &cache_br) catch |err| switch (err) {
+            error.ReadFailed => return cache_fr.err.?,
+            error.EndOfStream => return .truncated,
+            else => |e| return e,
         },
-        .zon => {
-            file.zoir = Zcu.loadZoirCacheBody(gpa, header, cache_file) catch |err| switch (err) {
-                error.UnexpectedFileSize => return .truncated,
-                else => |e| return e,
-            };
+        .zon => file.zoir = Zcu.loadZoirCacheBody(gpa, header, &cache_br) catch |err| switch (err) {
+            error.ReadFailed => return cache_fr.err.?,
+            error.EndOfStream => return .truncated,
+            else => |e| return e,
         },
     }
 
diff --git a/src/main.zig b/src/main.zig
index 70805c1165..e810cbf402 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -6359,7 +6359,8 @@ fn cmdDumpZir(
 
     const zir = try Zcu.loadZirCache(arena, f);
 
-    var stdout_bw = fs.File.stdout().writer().buffered(&stdio_buffer);
+    var stdout_fw = fs.File.stdout().writer();
+    var stdout_bw = stdout_fw.interface().buffered(&stdio_buffer);
     {
         const instruction_bytes = zir.instructions.len *
             // Here we don't use @sizeOf(Zir.Inst.Data) because it would include