diff --git a/.changeset/1884-planning-lock-mkdir-failure.md b/.changeset/1884-planning-lock-mkdir-failure.md
new file mode 100644
index 000000000..a290595a9
--- /dev/null
+++ b/.changeset/1884-planning-lock-mkdir-failure.md
@@ -0,0 +1,6 @@
+---
+type: Fixed
+pr: 3472
+---
+
+**`withPlanningLock` no longer reports a phantom "held by a live process" timeout when `.planning/` cannot be created** — a best-effort `try { platformEnsureDir(...) } catch { /* ok */ }` swallowed the real mkdir failure (EACCES/ENOSPC/EROFS), so the subsequent lock write failed with ENOENT (parent missing), and because ENOENT is in the lock's retry set (added for a Docker overlay-fs race) the loop spun the full 10 s budget before throwing a misattributed contention error that pointed operators at a nonexistent lock-holder. The mkdir failure now propagates immediately with its real filesystem errno and message, so an unwritable or full disk is reported as itself, not as concurrent-writer contention. The Docker overlay-fs ENOENT *lock-write* race (directory present) is still retried as before, and every code path where `.planning/` already exists or can be created is unchanged. Part of epic #1879 (distinguish "absent" from "corrupt/permission-denied" across engine read paths). (#1884)
diff --git a/src/planning-workspace.cts b/src/planning-workspace.cts
index ef3502883..a921c1e3d 100644
--- a/src/planning-workspace.cts
+++ b/src/planning-workspace.cts
@@ -206,8 +206,15 @@ function withPlanningLock<T>(cwd: string, fn: () => T, clock?: Clock): T {
   const deadmanCeilingMs = 60000;
   const start = clock.now();
 
-  // Ensure .planning/ exists
-  try { platformEnsureDir(planningDir(cwd)); } catch { /* ok */ }
+  // Ensure .planning/ exists. A genuine failure here (EACCES/ENOSPC/EROFS/EMFILE)
+  // MUST surface immediately: the prior `catch { /* ok */ }` swallowed it, the lock
+  // write below then failed with ENOENT (parent dir missing), and ENOENT is retryable
+  // (PLANNING_LOCK_RETRY_ERRNOS — added for a Docker overlay-fs race), so the loop
+  // spun the full 10s budget and reported a PHANTOM "held by a live process"
+  // contention pointing at a nonexistent holder (epic #1879 / F16, #1884).
+  // `mkdirSync(recursive:true)` does not throw on an existing dir, so the normal
+  // path (dir already present) is unaffected; only real creation failures propagate.
+  platformEnsureDir(planningDir(cwd));
 
   function acquireLock(): void {
     // Atomic create — fails if file exists
diff --git a/tests/planning-lock-mkdir-failure-1884.test.cjs b/tests/planning-lock-mkdir-failure-1884.test.cjs
new file mode 100644
index 000000000..1edc64cf0
--- /dev/null
+++ b/tests/planning-lock-mkdir-failure-1884.test.cjs
@@ -0,0 +1,139 @@
+const { test, describe, beforeEach, afterEach } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+const { cleanup } = require('./helpers.cjs');
+const { makeFakeClock } = require('./helpers/clock.cjs');
+
+// Built lib is the test target (same surface the rest of the suite imports).
+const planningWorkspaceDirect = require('../gsd-core/bin/lib/planning-workspace.cjs');
+const { withPlanningLock } = planningWorkspaceDirect;
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Epic #1879 / F16 (#1884): withPlanningLock swallows platformEnsureDir failure.
+//
+// A mkdir failure (EACCES/ENOSPC/EROFS) creating `.planning/` was swallowed by a
+// `catch { /* ok */ }`; the subsequent lock write then threw ENOENT (parent
+// missing), and ENOENT is in PLANNING_LOCK_RETRY_ERRNOS (for the Docker
+// overlay-fs race), so the loop spun the full 10s budget and reported a PHANTOM
+// "held by a live process" contention pointing at a nonexistent holder.
+//
+// Fix: stop swallowing — surface the real FS errno immediately. These tests pin
+// the new contract: a corrupt/permission mkdir is surfaced fast, not folded into
+// a lock-contention retry.
+//
+// IO failure is forced via t.mock.method(fs, 'mkdirSync', ...), which auto-restores
+// per-test (never chmod 0o000 — root bypasses mode bits, leaking coverage). The fake
+// clock proves fail-fast with no wall-clock assertion.
+// ─────────────────────────────────────────────────────────────────────────────
+
+const realMkdirSync = fs.mkdirSync;
+
+describe('withPlanningLock surfaces mkdir failure fast (epic #1879 / F16, #1884)', () => {
+  let tmpDir;
+
+  beforeEach(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'gsd-planning-mkdir-fail-'));
+    // Deliberately do NOT pre-create tmpDir/.planning — the scenario under test
+    // is "the dir cannot be created."
+  });
+
+  afterEach(() => {
+    if (typeof planningWorkspaceDirect._resetLockProbes === 'function') {
+      planningWorkspaceDirect._resetLockProbes();
+    }
+    if (typeof planningWorkspaceDirect._resetPlanningLockTestHooks === 'function') {
+      planningWorkspaceDirect._resetPlanningLockTestHooks();
+    }
+    cleanup(tmpDir);
+  });
+
+  function failMkdirFor(t, targetDir, code) {
+    // Mock the shared fs.mkdirSync that platformEnsureDir calls dynamically.
+    // Re-throw the requested errno only for the planning dir; pass through everything
+    // else so unrelated test machinery keeps working. t.mock auto-restores after the test.
+    t.mock.method(fs, 'mkdirSync', (p, opts) => {
+      if (typeof p === 'string' && (p === targetDir || p.startsWith(targetDir + path.sep))) {
+        const err = new Error(code + ': permission denied, mkdir \'' + p + '\'');
+        err.code = code;
+        throw err;
+      }
+      return realMkdirSync.call(fs, p, opts);
+    });
+  }
+
+  test('EACCES creating .planning/ is surfaced immediately, not folded into a phantom lock timeout', (t) => {
+    const planningPath = path.join(tmpDir, '.planning');
+    failMkdirFor(t, planningPath, 'EACCES');
+
+    const clock = makeFakeClock(0);
+    let ran = false;
+    assert.throws(
+      () => withPlanningLock(tmpDir, () => { ran = true; return 'should-not-run'; }, clock),
+      (err) => err && err.code === 'EACCES',
+      'mkdir EACCES must surface as EACCES, not be swallowed into a phantom lock contention'
+    );
+    assert.strictEqual(ran, false, 'critical section must not run when the planning dir cannot be created');
+    assert.ok(clock.nowValue < 10000,
+      'must fail fast — clock must NOT spin to the 10s lockTimeout budget (nowValue=' + clock.nowValue + 'ms)');
+  });
+
+  test('ENOSPC creating .planning/ is surfaced immediately', (t) => {
+    failMkdirFor(t, path.join(tmpDir, '.planning'), 'ENOSPC');
+    const clock = makeFakeClock(0);
+    assert.throws(
+      () => withPlanningLock(tmpDir, () => 'x', clock),
+      (err) => err && err.code === 'ENOSPC',
+      'mkdir ENOSPC must surface as ENOSPC immediately'
+    );
+    assert.ok(clock.nowValue < 10000, 'must fail fast on ENOSPC (nowValue=' + clock.nowValue + 'ms)');
+  });
+
+  test('the thrown mkdir-failure error is NOT branded as a lock timeout (no phantom contention)', (t) => {
+    failMkdirFor(t, path.join(tmpDir, '.planning'), 'EACCES');
+    const clock = makeFakeClock(0);
+    let captured;
+    try {
+      withPlanningLock(tmpDir, () => 'x', clock);
+    } catch (err) {
+      captured = err;
+    }
+    assert.ok(captured, 'withPlanningLock must throw on mkdir EACCES');
+    assert.notStrictEqual(captured && captured.lockTimeout, true,
+      'a mkdir failure must NOT carry lockTimeout=true (that brands a phantom contention)');
+  });
+
+  test('normal path is unaffected: creatable .planning/ still runs the critical section', () => {
+    // No monkeypatch — .planning/ is creatable in the writable tmpDir.
+    const clock = makeFakeClock(0);
+    const result = withPlanningLock(tmpDir, () => 'ran-ok', clock);
+    assert.strictEqual(result, 'ran-ok', 'fn() must run and return when the planning dir is creatable');
+    assert.ok(fs.existsSync(path.join(tmpDir, '.planning', '.lock')) === false,
+      'lock is released after the critical section');
+  });
+
+  test('a transient ENOENT on the lock WRITE (not mkdir) is still retried, not surfaced immediately', (t) => {
+    // Distinguishes this fix's target (mkdir failure, surfaced immediately) from the
+    // Docker overlay-fs race PLANNING_LOCK_RETRY_ERRNOS exists for (a transient ENOENT
+    // on the lock FILE write, with .planning/ already present) — that race must still
+    // retry and eventually succeed, not regress into fail-fast.
+    let writeAttempts = 0;
+    const realWriteFileSync = fs.writeFileSync;
+    t.mock.method(fs, 'writeFileSync', (p, data, opts) => {
+      if (typeof p === 'string' && p.endsWith(path.join('.planning', '.lock')) && writeAttempts === 0) {
+        writeAttempts++;
+        const err = new Error('ENOENT: transient overlay-fs race, open \'' + p + '\'');
+        err.code = 'ENOENT';
+        throw err;
+      }
+      return realWriteFileSync.call(fs, p, data, opts);
+    });
+
+    const clock = makeFakeClock(0);
+    const result = withPlanningLock(tmpDir, () => 'survived-the-race', clock);
+    assert.strictEqual(result, 'survived-the-race', 'the critical section must still run after the transient race resolves');
+    assert.strictEqual(writeAttempts, 1, 'the race must have been hit exactly once (sanity check on the mock)');
+    assert.ok(clock.nowValue > 0, 'a genuine retry must have consumed at least one sleep cycle (nowValue=' + clock.nowValue + 'ms)');
+  });
+});