diff --git a/src/runtime/webcore/FileSink.rs b/src/runtime/webcore/FileSink.rs
index b6294f7e5e92..4dd4c29f8670 100644
--- a/src/runtime/webcore/FileSink.rs
+++ b/src/runtime/webcore/FileSink.rs
@@ -978,7 +978,7 @@ impl FileSink {
 
     pub fn write(&self, data: &streams::Result) -> streams::Writable {
         if self.done.get() {
-            return streams::Writable::Done;
+            return streams::Writable::Owned(0);
         }
         // SAFETY(JsCell): `IOWriter::write` buffers/writes to fd; does not call JS.
         let rc = self.writer.with_mut(|w| w.write(data.slice()));
@@ -992,7 +992,7 @@ impl FileSink {
 
     pub fn write_latin1(&self, data: &streams::Result) -> streams::Writable {
         if self.done.get() {
-            return streams::Writable::Done;
+            return streams::Writable::Owned(0);
         }
         // SAFETY(JsCell): `IOWriter::write_latin1` buffers/writes; no JS.
         let rc = self.writer.with_mut(|w| w.write_latin1(data.slice()));
@@ -1001,7 +1001,7 @@ impl FileSink {
 
     pub fn write_utf16(&self, data: &streams::Result) -> streams::Writable {
         if self.done.get() {
-            return streams::Writable::Done;
+            return streams::Writable::Owned(0);
         }
         // SAFETY(JsCell): `IOWriter::write_utf16` buffers/writes; no JS.
         let rc = self.writer.with_mut(|w| w.write_utf16(data.slice16()));
@@ -1017,6 +1017,7 @@ impl FileSink {
         // goes via the stored `*mut FileSink` backref, not this borrow.
         match self.writer.with_mut(|w| w.flush()) {
             WriteResult::Done(written) => {
+                self.done.set(true);
                 self.written.set(self.written.get() + written as usize); // @truncate
                 self.writer.with_mut(|w| w.end());
                 sys::Result::Ok(())
@@ -1036,6 +1037,7 @@ impl FileSink {
                 sys::Result::Ok(())
             }
             WriteResult::Wrote(written) => {
+                self.done.set(true);
                 self.written.set(self.written.get() + written as usize); // @truncate
                 self.writer.with_mut(|w| w.end());
                 sys::Result::Ok(())
@@ -1097,6 +1099,7 @@ impl FileSink {
 
         match flush_result {
             WriteResult::Done(written) => {
+                self.done.set(true);
                 self.update_ref(false);
                 self.writer.with_mut(|w| w.end());
                 sys::Result::Ok(JSValue::js_number(written as f64))
@@ -1125,6 +1128,7 @@ impl FileSink {
                 sys::Result::Ok(unsafe { (*promise_result).to_js() })
             }
             WriteResult::Wrote(written) => {
+                self.done.set(true);
                 self.writer.with_mut(|w| w.end());
                 sys::Result::Ok(JSValue::js_number(written as f64))
             }
diff --git a/test/js/bun/util/filesink.test.ts b/test/js/bun/util/filesink.test.ts
index e8a208a3a268..f9d416c9a2d1 100644
--- a/test/js/bun/util/filesink.test.ts
+++ b/test/js/bun/util/filesink.test.ts
@@ -309,6 +309,46 @@ it.skipIf(!isPosix)("writing after end() fails during flush does not crash", asy
   await 1;
 });
 
+it("write() and flush() after end() return 0 and undefined, not boolean true", async () => {
+  const path = join(tmpdirSync(), "filesink-write-after-end.txt");
+  const writer = Bun.file(path).writer();
+
+  // write() may return a number synchronously (POSIX regular file) or a
+  // Promise<number> (Windows regular-file uv_fs_write); await either.
+  expect(await writer.write("hello")).toBe(5);
+  await Promise.resolve(writer.end());
+
+  // write()/flush() are typed `number | Promise<number>`. After end() they
+  // returned the boolean `true` because end_from_js() never set `done` on the
+  // synchronous success paths, so the call fell through to the already-ended
+  // IOWriter. Now they return 0 / undefined like every other sink.
+  expect(writer.write("world")).toBe(0);
+  expect(writer.flush()).toBeUndefined();
+
+  // The dropped write must not appear in the file.
+  expect(await Bun.file(path).text()).toBe("hello");
+
+  // Calling end() again is idempotent and must not throw.
+  await Promise.resolve(writer.end()).catch(() => {});
+});
+
+it("write() to an exited subprocess's stdin returns 0, not boolean true", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", "process.exit(0)"],
+    env: bunEnv,
+    stdin: "pipe",
+    stdout: "ignore",
+    stderr: "ignore",
+  });
+  await proc.exited;
+
+  // The stdin FileSink's `done` is set by on_attached_process_exit. Writing
+  // to it returns 0 (nothing written), matching the HTTP/network sinks and
+  // the declared `number | Promise<number>` type.
+  expect(proc.stdin.write("late\n")).toBe(0);
+  await Promise.resolve(proc.stdin.end()).catch(() => {});
+});
+
 // On Windows the libuv write completion path re-enters JS (promise resolution)
 // while a `&mut WindowsStreamingWriter` is live, so without raw-ptr laundering
 // LLVM `noalias` lets release builds cache stale `is_done`/`parent` and