diff --git a/.gitignore b/.gitignore
index 2ec724c..97c5171 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,3 +14,6 @@ dist/
 .automaker/features/
 .automaker/checkpoints/
 .automaker/trajectory/
+
+# auto-mode runtime lock — never commit (tracking it conflicts every concurrent PR)
+.automaker-lock
diff --git a/lib/workspace-config.mjs b/lib/workspace-config.mjs
index e1aaca6..84c513a 100644
--- a/lib/workspace-config.mjs
+++ b/lib/workspace-config.mjs
@@ -166,6 +166,27 @@ export const WORKSPACE_STANDARD = /** @type {Rule[]} */ ([
     ok: (m) => !m.hasFile('.beads/beads.db'),
     fix: 'git rm --cached .beads/beads.db and add it to .gitignore',
   },
+  {
+    id: 'automaker-lock-gitignored',
+    description: '.automaker-lock is gitignored (auto-mode runtime lock — must never be committed)',
+    severity: 'error',
+    ok: (m) => resolveIgnored(m, '.automaker-lock', ['.automaker-lock']),
+    fix: 'Add `.automaker-lock` to .gitignore',
+  },
+  {
+    // The headline rule: a *tracked* .automaker-lock is what bites. auto-mode
+    // rewrites this lock on every run, so once it's committed every concurrent
+    // auto-mode PR mutates it and they all collide on it — the whole branch's
+    // PRs go CONFLICTING/DIRTY the moment one merges (observed fleet-wide:
+    // protoContent, protoMaker #4109/#4120). Gitignoring alone does NOT fix an
+    // already-tracked file — it must be untracked.
+    id: 'automaker-lock-not-committed',
+    description:
+      '.automaker-lock is NOT committed (a tracked runtime lock makes every concurrent auto-mode PR conflict on it)',
+    severity: 'error',
+    ok: (m) => !m.hasFile('.automaker-lock'),
+    fix: 'git rm --cached .automaker-lock and add it to .gitignore',
+  },
   {
     id: 'automaker-settings-committed',
     description: '.automaker/settings.json is committed (versioned per-repo agent baseline)',
@@ -322,6 +343,7 @@ export const REQUIRED_GITIGNORE = [
   '.automaker/features/',
   '.automaker/checkpoints/',
   '.automaker/trajectory/',
+  '.automaker-lock',
 ];
 
 /**
diff --git a/test/workspace-config.test.mjs b/test/workspace-config.test.mjs
index 94f1936..97d297c 100644
--- a/test/workspace-config.test.mjs
+++ b/test/workspace-config.test.mjs
@@ -33,6 +33,7 @@ function goodManifest() {
       '.automaker/features/',
       '.automaker/checkpoints/',
       '.automaker/trajectory/',
+      '.automaker-lock',
     ].join('\n')
   );
 }
@@ -134,7 +135,7 @@ test('missing automaker transient ignores is a WARN, not an error', () => {
   // Everything required present + ignored, but no transient-dir ignores.
   const m = manifest(
     ['.beads/issues.jsonl', '.automaker/settings.json'],
-    '.beads/beads.db\n.worktrees/'
+    '.beads/beads.db\n.worktrees/\n.automaker-lock'
   );
   const r = evaluateStandard(m);
   assert.equal(r.ok, true, 'warn-only violation should not fail the standard');
@@ -151,13 +152,15 @@ test('missing automaker transient ignores is a WARN, not an error', () => {
 
 test('a bare repo (nothing) reports all errors + the warn', () => {
   const r = evaluateStandard(manifest([], ''));
-  // 4 errors: issues.jsonl, beads-db-gitignored, automaker-settings, worktrees
-  // (beads-db-not-committed passes — it's absent — so it is NOT a violation)
-  assert.equal(r.errorCount, 4);
+  // 5 errors: issues.jsonl, beads-db-gitignored, automaker-settings, worktrees,
+  // automaker-lock-gitignored. (beads-db-not-committed AND automaker-lock-not-committed
+  // pass — both absent — so they are NOT violations.)
+  assert.equal(r.errorCount, 5);
   // 2 warns: automaker-transient-gitignored + workflow-security-lint
   assert.equal(r.warnCount, 2);
   assert.equal(r.ok, false);
   assert.ok(r.passed.includes('beads-db-not-committed'));
+  assert.ok(r.passed.includes('automaker-lock-not-committed'));
 });
 
 test('each rule carries a fix hint', () => {