diff --git a/src/js/internal/fs/streams.ts b/src/js/internal/fs/streams.ts
index 44e8d040fd7b..57ee3e6ba4b3 100644
--- a/src/js/internal/fs/streams.ts
+++ b/src/js/internal/fs/streams.ts
@@ -527,7 +527,7 @@ function writeAll(data, size, pos, cb, retries = 0) {
 
     retries = bytesWritten ? 0 : retries + 1;
     size -= bytesWritten;
-    pos += bytesWritten;
+    if (pos !== undefined) pos += bytesWritten;
 
     // Try writing non-zero number of bytes up to 5 times.
     if (retries > 5) {
@@ -542,7 +542,7 @@ function writeAll(data, size, pos, cb, retries = 0) {
 }
 
 function writevAll(chunks, size, pos, cb, retries = 0) {
-  this[kFs].writev(this.fd, chunks, this.pos, (er, bytesWritten, buffers) => {
+  this[kFs].writev(this.fd, chunks, pos, (er, bytesWritten, buffers) => {
     // No data currently available and operation should be retried later.
     if (er?.code === "EAGAIN") {
       er = null;
@@ -557,7 +557,7 @@ function writevAll(chunks, size, pos, cb, retries = 0) {
 
     retries = bytesWritten ? 0 : retries + 1;
     size -= bytesWritten;
-    pos += bytesWritten;
+    if (pos !== undefined) pos += bytesWritten;
 
     // Try writing non-zero number of bytes up to 5 times.
     if (retries > 5) {
diff --git a/src/runtime/node/node_fs.rs b/src/runtime/node/node_fs.rs
index 32d9c8856a42..e005264052bd 100644
--- a/src/runtime/node/node_fs.rs
+++ b/src/runtime/node/node_fs.rs
@@ -3852,19 +3852,13 @@ pub mod args {
                         if !(current.is_number() || current.is_big_int()) {
                             break 'parse;
                         }
-                        let position = i52::from_js(current);
-                        if position >= 0 {
-                            args.position = Some(position);
-                        }
+                        args.position = write_position_from_js(current);
                         arguments.eat();
                     }
                     // fs.write(fd, string[, position[, encoding]], callback)
                     _ => {
                         if current.is_number() {
-                            let position = i52::from_js(current);
-                            if position >= 0 {
-                                args.position = Some(position);
-                            }
+                            args.position = write_position_from_js(current);
                         }
                         // Node consumes the position slot whatever its type
                         // (null, undefined, a non-number); the encoding is
@@ -10223,3 +10217,22 @@ impl i52 {
         (v.to_int64() << 12) >> 12
     }
 }
+
+/// Node's `GetOffset` for `fs.write` / `fs.writeSync` position: only a
+/// non-negative safe integer selects `pwrite`; NaN, ±Infinity, non-integers
+/// and negatives fall back to the current file offset (None).
+#[inline]
+fn write_position_from_js(v: JSValue) -> Option<i64> {
+    if let Some(num) = v.get_number() {
+        if num.is_finite()
+            && num.trunc() == num
+            && num >= 0.0
+            && num <= bun_jsc::MAX_SAFE_INTEGER as f64
+        {
+            return Some(num as i64);
+        }
+        return None;
+    }
+    let position = i52::from_js(v);
+    if position >= 0 { Some(position) } else { None }
+}
diff --git a/test/js/node/fs/fs.test.ts b/test/js/node/fs/fs.test.ts
index 16396f0dd211..4cac0ec0f01b 100644
--- a/test/js/node/fs/fs.test.ts
+++ b/test/js/node/fs/fs.test.ts
@@ -2024,6 +2024,64 @@ describe("writeSync", () => {
     closeSync(fd);
   });
 
+  // Node's binding maps any non-safe-integer position to -1 (current file
+  // offset). NaN previously coerced to 0, overwriting the start of the file.
+  it.each([
+    ["NaN", NaN],
+    ["Infinity", Infinity],
+    ["-Infinity", -Infinity],
+    ["-1", -1],
+    ["1.5", 1.5],
+  ])("treats position %s as the current file offset", async (_label, position) => {
+    using dir = tempDir("write-position-current", {});
+    const p = join(String(dir), "f");
+    writeFileSync(p, Buffer.alloc(10, "A"));
+    const seed = Buffer.alloc(5);
+    const expected = "AAAAAXXAAA";
+
+    // writeSync(fd, buffer, offset, length, position)
+    {
+      const fd = openSync(p, "r+");
+      try {
+        readSync(fd, seed, 0, 5, null);
+        expect(writeSync(fd, Buffer.from("XX"), 0, 2, position as number)).toBe(2);
+      } finally {
+        closeSync(fd);
+      }
+      expect(readFileSync(p, "utf8")).toBe(expected);
+    }
+
+    // fs.write(fd, buffer, offset, length, position, callback)
+    writeFileSync(p, Buffer.alloc(10, "A"));
+    {
+      const fd = openSync(p, "r+");
+      try {
+        readSync(fd, seed, 0, 5, null);
+        const { promise, resolve, reject } = Promise.withResolvers<number>();
+        fs.write(fd, Buffer.from("XX"), 0, 2, position as number, (err, written) =>
+          err ? reject(err) : resolve(written),
+        );
+        expect(await promise).toBe(2);
+      } finally {
+        closeSync(fd);
+      }
+      expect(readFileSync(p, "utf8")).toBe(expected);
+    }
+
+    // writeSync(fd, string, position)
+    writeFileSync(p, Buffer.alloc(10, "A"));
+    {
+      const fd = openSync(p, "r+");
+      try {
+        readSync(fd, seed, 0, 5, null);
+        expect(writeSync(fd, "XX", position as number)).toBe(2);
+      } finally {
+        closeSync(fd);
+      }
+      expect(readFileSync(p, "utf8")).toBe(expected);
+    }
+  });
+
   // writeSync(fd, string[, position[, encoding]]): the encoding used to be
   // parsed but never applied, so utf16le/hex/base64/latin1 all wrote raw UTF-8.
   it("honors the encoding argument for strings", () => {
@@ -3671,6 +3729,92 @@ describe("createWriteStream", () => {
       }
     });
   });
+
+  // With no `start` the stream passes `pos = undefined` to writeAll/writevAll.
+  // A partial write must retry at the current file offset (pos stays
+  // undefined), not at `undefined + bytesWritten` (NaN), which the native
+  // layer used to coerce to pwrite offset 0 and stamp the tail over the head.
+  it.each([
+    ["write", undefined],
+    ["write", 0],
+    ["writev", undefined],
+    ["writev", 0],
+  ] as const)("retries a partial %s at the correct offset with start %p", async (method, start) => {
+    using dir = tempDir("write-stream-partial", {});
+    const p = join(String(dir), "out");
+    const payload = Buffer.from("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
+    const positions: unknown[] = [];
+    let first = true;
+
+    const customFs: any = {
+      open: fs.open,
+      close: fs.close,
+      write(fd, buf, offset, length, position, cb) {
+        positions.push(position);
+        if (first) {
+          first = false;
+          const half = Math.floor(length / 2);
+          fs.write(fd, buf, offset, half, position, (err, written) => cb(err, written, buf));
+          return;
+        }
+        fs.write(fd, buf, offset, length, position, cb);
+      },
+      writev(fd, chunks, position, cb) {
+        positions.push(position);
+        if (first) {
+          first = false;
+          fs.writev(fd, [chunks[0]], position, (err, written) => cb(err, written, chunks));
+          return;
+        }
+        fs.writev(fd, chunks, position, cb);
+      },
+    };
+
+    const stream = createWriteStream(p, { fs: customFs, start } as any);
+    const { promise, resolve, reject } = Promise.withResolvers<void>();
+    stream.on("error", reject);
+    stream.on("finish", resolve);
+    if (method === "writev") {
+      stream.cork();
+      stream.write(payload.subarray(0, 10));
+      stream.write(payload.subarray(10));
+      stream.uncork();
+      stream.end();
+    } else {
+      stream.end(payload);
+    }
+    await promise;
+
+    expect(readFileSync(p)).toEqual(payload);
+    expect(positions.some(v => typeof v === "number" && Number.isNaN(v))).toBe(false);
+    if (start === 0) {
+      expect(positions).toEqual(method === "writev" ? [0, 10] : [0, 13]);
+    } else {
+      expect(positions).toEqual([undefined, undefined]);
+    }
+  });
+
+  // End-to-end: a kernel-enforced short write (RLIMIT_FSIZE) must surface as
+  // an EFBIG 'error' and leave the file a byte-exact prefix of the source.
+  it.skipIf(!isLinux)("surfaces EFBIG from a short write instead of overwriting the file head", async () => {
+    using dir = tempDir("write-stream-fsize", {});
+    const out = join(String(dir), "out");
+    const fixture = join(import.meta.dir, "write-stream-fsize-fixture.js");
+    await using proc = Bun.spawn({
+      cmd: ["/bin/sh", "-c", `ulimit -f 1024 && exec "$0" "$1" "$2"`, bunExe(), fixture, out],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+    expect(stderr).toBe("");
+    const result = JSON.parse(stdout.trim());
+    expect({ head: result.head, isPrefix: result.isPrefix }).toEqual({ head: "AAAAAAAA", isPrefix: true });
+    expect(result.events).toContain("error:EFBIG");
+    expect(result.events).not.toContain("finish");
+    expect(result.fileSize).toBeLessThanOrEqual(1 << 20);
+    expect(exitCode).toBe(0);
+  });
 });
 
 describe("fs/promises", () => {
diff --git a/test/js/node/fs/write-stream-fsize-fixture.js b/test/js/node/fs/write-stream-fsize-fixture.js
new file mode 100644
index 000000000000..998952f9f84b
--- /dev/null
+++ b/test/js/node/fs/write-stream-fsize-fixture.js
@@ -0,0 +1,35 @@
+// Run under `ulimit -f 1024` (RLIMIT_FSIZE = 1 MiB). Writes a single 4 MiB
+// chunk (1 MiB each of A/B/C/D) through createWriteStream and reports whether
+// the on-disk bytes are a byte-exact prefix of the source. A short write that
+// retries at offset 0 would stamp the tail block over the head.
+const fs = require("fs");
+const path = process.argv[2];
+
+const MiB = 1 << 20;
+const src = Buffer.concat([
+  Buffer.alloc(MiB, "A"),
+  Buffer.alloc(MiB, "B"),
+  Buffer.alloc(MiB, "C"),
+  Buffer.alloc(MiB, "D"),
+]);
+
+const events = [];
+const stream = fs.createWriteStream(path);
+stream.on("error", e => events.push("error:" + (e && e.code)));
+stream.on("finish", () => events.push("finish"));
+stream.on("close", () => {
+  const out = fs.existsSync(path) ? fs.readFileSync(path) : Buffer.alloc(0);
+  const head = out.subarray(0, 8).toString();
+  const isPrefix = src.subarray(0, out.length).equals(out);
+  process.stdout.write(
+    JSON.stringify({
+      events,
+      bytesWritten: stream.bytesWritten,
+      fileSize: out.length,
+      head,
+      isPrefix,
+    }) + "\n",
+  );
+});
+stream.write(src);
+stream.end();