diff --git a/.changeset/eager-newts-purr.md b/.changeset/eager-newts-purr.md
new file mode 100644
index 000000000..0021a5a35
--- /dev/null
+++ b/.changeset/eager-newts-purr.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1009
+---
+**`gsd-tools` no longer throws `EAGAIN` or truncates output under heavy load** — the CLI's stdout/stderr writes now retry the transient `EAGAIN`/`EINTR` errnos and handle short writes when the output stream is a full non-blocking pipe (e.g. the parallel test runner), instead of throwing or silently dropping bytes.
diff --git a/src/io.cts b/src/io.cts
index 97c7a43a0..33e1890d9 100644
--- a/src/io.cts
+++ b/src/io.cts
@@ -69,6 +69,63 @@ function reapStaleTempFiles(prefix = 'gsd-', { maxAgeMs = 5 * 60 * 1000, dirsOnl
 
 // ─── Output helpers ───────────────────────────────────────────────────────────
 
+/**
+ * Transient write errnos. When stdout/stderr is a NON-BLOCKING pipe — as it is
+ * under the parallel `node --test` runner on Linux CI — a full pipe buffer makes
+ * `fs.writeSync` throw EAGAIN, and a signal can interrupt it with EINTR. Both
+ * clear on retry once the reader drains. This is the same transient class the
+ * STATE.md lock path already retries (ACQUIRE_LOCK_RETRY_ERRNOS, #3776); #1008.
+ */
+const WRITE_RETRY_ERRNOS = new Set(['EAGAIN', 'EINTR']);
+
+// Bounded so a pathological never-draining fd cannot spin forever. Each retry
+// yields the thread for ~1ms via Atomics.wait (the project's sync-sleep idiom —
+// see clock.cts realClock.sleep), so the cap is ~1s of total back-pressure wait.
+const WRITE_MAX_RETRIES = 1000;
+const WRITE_RETRY_BACKOFF_MS = 1;
+
+// Sleep buffer is lazily allocated on the FIRST back-pressure retry (rare — only
+// when a non-blocking pipe is full) and then reused. Keeping it out of module
+// load costs nothing on the overwhelmingly common no-retry path and avoids
+// perturbing SharedArrayBuffer-allocation accounting in other modules (perf-316).
+let _writeSleepBuf: Int32Array | null = null;
+function backoffOnce(): void {
+  if (_writeSleepBuf === null) _writeSleepBuf = new Int32Array(new SharedArrayBuffer(4));
+  Atomics.wait(_writeSleepBuf, 0, 0, WRITE_RETRY_BACKOFF_MS);
+}
+
+/**
+ * Write the entire payload to `fd`, tolerating non-blocking-pipe back-pressure.
+ *
+ * `fs.writeSync` does NOT block on a non-blocking pipe: a full buffer throws
+ * EAGAIN, and a partially-drained buffer returns a SHORT count (fewer bytes than
+ * requested). The previous bare `fs.writeSync(fd, string)` call assumed it always
+ * blocked until the kernel accepted every byte — false under load, which both
+ * threw spurious errors and risked silently truncating output (#1008).
+ *
+ * This loops on short counts (advancing the offset) and retries EAGAIN/EINTR with
+ * a brief Atomics.wait backoff that yields the thread so the reader can drain.
+ * Non-transient errors (e.g. EPIPE) propagate unchanged.
+ */
+function writeAllSync(fd: number, data: string): void {
+  const buf = Buffer.from(data, 'utf8');
+  let offset = 0;
+  let retries = 0;
+  while (offset < buf.length) {
+    try {
+      offset += fs.writeSync(fd, buf, offset, buf.length - offset);
+    } catch (err) {
+      const code = (err as NodeJS.ErrnoException).code ?? '';
+      if (WRITE_RETRY_ERRNOS.has(code) && retries < WRITE_MAX_RETRIES) {
+        retries += 1;
+        backoffOnce();
+        continue;
+      }
+      throw err;
+    }
+  }
+}
+
 function output(result: unknown, raw: boolean, rawValue?: unknown): void {
   let data: string;
   if (raw && rawValue !== undefined) {
@@ -89,10 +146,10 @@ function output(result: unknown, raw: boolean, rawValue?: unknown): void {
     }
   }
   // process.stdout.write() is async when stdout is a pipe — process.exit()
-  // can tear down the process before the reader consumes the buffer.
-  // fs.writeSync(1, ...) blocks until the kernel accepts the bytes, and
-  // skipping process.exit() lets the event loop drain naturally.
-  fs.writeSync(1, data);
+  // can tear down the process before the reader consumes the buffer. writeAllSync
+  // pushes every byte synchronously (looping short counts, retrying EAGAIN/EINTR),
+  // and skipping process.exit() lets the event loop drain naturally.
+  writeAllSync(1, data);
 }
 
 /**
@@ -155,9 +212,9 @@ function getJsonErrorMode(): boolean { return _jsonErrorMode; }
 function error(message: string, reason: ErrorReasonValue = ERROR_REASON.UNKNOWN): never {
   if (_jsonErrorMode) {
     const payload = JSON.stringify({ ok: false, reason, message }) + '\n';
-    fs.writeSync(2, payload);
+    writeAllSync(2, payload);
   } else {
-    fs.writeSync(2, 'Error: ' + message + '\n');
+    writeAllSync(2, 'Error: ' + message + '\n');
   }
   process.exit(1);
 }
diff --git a/tests/io.test.cjs b/tests/io.test.cjs
index 3825be1c1..8663ac825 100644
--- a/tests/io.test.cjs
+++ b/tests/io.test.cjs
@@ -339,3 +339,113 @@ describe('core.cjs re-export shims', () => {
     assert.strictEqual(core.GSD_TEMP_DIR, io.GSD_TEMP_DIR);
   });
 });
+
+// ─── bug #1008: output()/error() tolerate a full / slow non-blocking pipe ─────
+//
+// The pre-fix bare `fs.writeSync(fd, data)` assumed it blocks until the kernel
+// accepts every byte — false when fd is a non-blocking pipe (the parallel
+// node:test runner on Linux): a full pipe throws EAGAIN and a partially-drained
+// pipe returns a SHORT count. These behavioral tests inject fs.writeSync via
+// mock.method (the approved fault-injection seam) and assert the observable
+// contract (no throw, full payload, real errors still surface). They are red
+// against the pre-fix io.cjs (throw / truncate).
+
+// Normalize either writeSync call form to the chunk it emits:
+//   buffer form:  writeSync(fd, buffer, offset, length)  ← the fixed writeAllSync loop
+//   string form:  writeSync(fd, string)                  ← the pre-fix bare call
+function bug1008ChunkOf(data, offset, length) {
+  if (Buffer.isBuffer(data)) {
+    const start = offset ?? 0;
+    const end = length === undefined ? data.length : start + length;
+    return data.subarray(start, end).toString('utf8');
+  }
+  return String(data);
+}
+
+function bug1008WriteError(code, errno) {
+  const e = new Error(`${code}: write`);
+  e.code = code;
+  e.errno = errno;
+  e.syscall = 'write';
+  return e;
+}
+
+describe('bug #1008: io.output() tolerates a full / slow non-blocking pipe', () => {
+  test('retries on EAGAIN and emits the full payload without throwing', (t) => {
+    const written = [];
+    let calls = 0;
+    t.mock.method(fs, 'writeSync', (fd, data, offset, length) => {
+      calls += 1;
+      if (calls === 1) throw bug1008WriteError('EAGAIN', -11); // pipe momentarily full
+      const chunk = bug1008ChunkOf(data, offset, length);
+      written.push(chunk);
+      return Buffer.byteLength(chunk, 'utf8');
+    });
+
+    const payload = { ok: true, n: 42 };
+    assert.doesNotThrow(() => io.output(payload, false));
+    assert.ok(calls >= 2, `expected a retry after EAGAIN, got ${calls} call(s)`);
+    assert.equal(written.join(''), JSON.stringify(payload, null, 2), 'full payload must reach the fd');
+  });
+
+  test('retries on EINTR (signal-interrupted write) too', (t) => {
+    const written = [];
+    let calls = 0;
+    t.mock.method(fs, 'writeSync', (fd, data, offset, length) => {
+      calls += 1;
+      if (calls === 1) throw bug1008WriteError('EINTR', -4);
+      const chunk = bug1008ChunkOf(data, offset, length);
+      written.push(chunk);
+      return Buffer.byteLength(chunk, 'utf8');
+    });
+
+    assert.doesNotThrow(() => io.output('plain', true, 'PLAIN-RAW'));
+    assert.equal(written.join(''), 'PLAIN-RAW');
+  });
+
+  test('handles short (partial) writes without truncating', (t) => {
+    const written = [];
+    const CAP = 3; // each writeSync accepts at most 3 bytes, like a draining pipe
+    t.mock.method(fs, 'writeSync', (fd, data, offset, length) => {
+      const chunk = bug1008ChunkOf(data, offset, length);
+      const part = chunk.slice(0, CAP);
+      written.push(part);
+      return Buffer.byteLength(part, 'utf8');
+    });
+
+    const payload = { message: 'a reasonably long ascii payload to force many short writes' };
+    io.output(payload, false);
+    assert.equal(written.join(''), JSON.stringify(payload, null, 2), 'no bytes may be dropped on short writes');
+  });
+
+  test('does NOT swallow a genuine, non-transient write error (EPIPE)', (t) => {
+    t.mock.method(fs, 'writeSync', () => { throw bug1008WriteError('EPIPE', -32); });
+    assert.throws(
+      () => io.output({ ok: true }, false),
+      (err) => err.code === 'EPIPE',
+      'real (non-transient) errors must still surface',
+    );
+  });
+});
+
+describe('bug #1008: io.error() tolerates a full non-blocking stderr pipe', () => {
+  test('retries on EAGAIN, emits the full message, and still exits', (t) => {
+    const written = [];
+    let calls = 0;
+    let exitCode = null;
+    t.mock.method(process, 'exit', (code) => { exitCode = code; }); // neutralize the hard exit
+    t.mock.method(fs, 'writeSync', (fd, data, offset, length) => {
+      calls += 1;
+      if (calls === 1) throw bug1008WriteError('EAGAIN', -11);
+      assert.equal(fd, 2, 'error() must write to stderr');
+      const chunk = bug1008ChunkOf(data, offset, length);
+      written.push(chunk);
+      return Buffer.byteLength(chunk, 'utf8');
+    });
+
+    assert.doesNotThrow(() => io.error('boom', io.ERROR_REASON.UNKNOWN));
+    assert.ok(calls >= 2, 'error() should retry after EAGAIN');
+    assert.equal(written.join(''), 'Error: boom\n');
+    assert.equal(exitCode, 1, 'error() must still exit(1) after a retried write');
+  });
+});