diff --git a/src/bun.js/webcore/FileSink.zig b/src/bun.js/webcore/FileSink.zig
index 89309b3bd5..200ef8c845 100644
--- a/src/bun.js/webcore/FileSink.zig
+++ b/src/bun.js/webcore/FileSink.zig
@@ -62,12 +62,33 @@ pub fn memoryCost(this: *const FileSink) usize {
 
 fn Bun__ForceFileSinkToBeSynchronousForProcessObjectStdio(_: *jsc.JSGlobalObject, jsvalue: jsc.JSValue) callconv(.C) void {
     var this: *FileSink = @alignCast(@ptrCast(JSSink.fromJS(jsvalue) orelse return));
-    this.force_sync = true;
+
     if (comptime !Environment.isWindows) {
+        this.force_sync = true;
         this.writer.force_sync = true;
         if (this.fd != bun.invalid_fd) {
             _ = bun.sys.updateNonblocking(this.fd, false);
         }
+    } else {
+        if (this.writer.source) |*source| {
+            switch (source.*) {
+                .pipe => |pipe| {
+                    if (uv.uv_stream_set_blocking(@ptrCast(pipe), 1) == .zero) {
+                        return;
+                    }
+                },
+                .tty => |tty| {
+                    if (uv.uv_stream_set_blocking(@ptrCast(tty), 1) == .zero) {
+                        return;
+                    }
+                },
+
+                else => {},
+            }
+        }
+
+        // Fallback to WriteFile() if it fails.
+        this.force_sync = true;
     }
 }
 
diff --git a/src/io/PipeReader.zig b/src/io/PipeReader.zig
index 50717375c9..4ff47c2bda 100644
--- a/src/io/PipeReader.zig
+++ b/src/io/PipeReader.zig
@@ -824,13 +824,19 @@ pub const WindowsBufferedReader = struct {
 
     fn _onReadChunk(this: *WindowsBufferedReader, buf: []u8, hasMore: ReadState) bool {
         if (this.maxbuf) |m| m.onReadBytes(buf.len);
-        this.flags.has_inflight_read = false;
         if (hasMore == .eof) {
             this.flags.received_eof = true;
         }
 
-        const onReadChunkFn = this.vtable.onReadChunk orelse return true;
-        return onReadChunkFn(this.parent, buf, hasMore);
+        const onReadChunkFn = this.vtable.onReadChunk orelse {
+            this.flags.has_inflight_read = false;
+            return true;
+        };
+        const result = onReadChunkFn(this.parent, buf, hasMore);
+        // Clear has_inflight_read after the callback completes to prevent
+        // libuv from starting a new read while we're still processing data
+        this.flags.has_inflight_read = false;
+        return result;
     }
 
     fn finish(this: *WindowsBufferedReader) void {
@@ -928,7 +934,10 @@ pub const WindowsBufferedReader = struct {
         switch (nread_int) {
             0 => {
                 // EAGAIN or EWOULDBLOCK or canceled  (buf is not safe to access here)
-                return this.onRead(.{ .result = 0 }, "", .drained);
+                // With libuv 1.51.0+, calling onRead(.drained) here causes a race condition
+                // where subsequent reads return truncated data (see logs showing 6024 instead
+                // of 74468 bytes). Just ignore 0-byte reads and let libuv continue.
+                return;
             },
             uv.UV_EOF => {
                 _ = this.stopReading();
diff --git a/test/js/bun/spawn/spawn-stdin-readable-stream.test.ts b/test/js/bun/spawn/spawn-stdin-readable-stream.test.ts
index 2f26d71b8f..9790f76fce 100644
--- a/test/js/bun/spawn/spawn-stdin-readable-stream.test.ts
+++ b/test/js/bun/spawn/spawn-stdin-readable-stream.test.ts
@@ -169,11 +169,12 @@ describe("spawn stdin ReadableStream", () => {
     const chunkSize = 64 * 1024; // 64KB chunks
     const numChunks = 16; // 1MB total
     let pushedChunks = 0;
+    const chunk = Buffer.alloc(chunkSize, "x");
 
     const stream = new ReadableStream({
       pull(controller) {
         if (pushedChunks < numChunks) {
-          controller.enqueue("x".repeat(chunkSize));
+          controller.enqueue(chunk);
           pushedChunks++;
         } else {
           controller.close();
@@ -182,7 +183,16 @@ describe("spawn stdin ReadableStream", () => {
     });
 
     await using proc = spawn({
-      cmd: [bunExe(), "-e", "process.stdin.pipe(process.stdout)"],
+      cmd: [
+        bunExe(),
+        "-e",
+        `
+        let length = 0;
+        process.stdin.on('data', (data) => length += data.length);
+        process.once('beforeExit', () => console.error(length));
+        process.stdin.pipe(process.stdout)
+`,
+      ],
       stdin: stream,
       stdout: "pipe",
       env: bunEnv,
@@ -190,7 +200,7 @@ describe("spawn stdin ReadableStream", () => {
 
     const text = await proc.stdout.text();
     expect(text.length).toBe(chunkSize * numChunks);
-    expect(text).toBe("x".repeat(chunkSize * numChunks));
+    expect(text).toBe(chunk.toString().repeat(numChunks));
     expect(await proc.exited).toBe(0);
   });
 
diff --git a/test/regression/issue/11297/11297.fixture.ts b/test/regression/issue/11297/11297.fixture.ts
index bc6ad09128..39aefc5719 100644
--- a/test/regression/issue/11297/11297.fixture.ts
+++ b/test/regression/issue/11297/11297.fixture.ts
@@ -4,7 +4,16 @@ const string = Buffer.alloc(1024 * 1024, "zombo.com\n").toString();
 process.exitCode = 1;
 
 const proc = Bun.spawn({
-  cmd: [bunExe(), "-e", "process.stdin.pipe(process.stdout)"],
+  cmd: [
+    bunExe(),
+    "-e",
+    `
+let length = 0;
+process.stdin.on('data', (data) => length += data.length);
+process.once('beforeExit', () => console.error(length));
+process.stdin.pipe(process.stdout)
+    `,
+  ],
   stdio: ["pipe", "pipe", "inherit"],
 });