diff --git a/docs/superpowers/plans/2026-05-31-discipline-guard-backlog.md b/docs/superpowers/plans/2026-05-31-discipline-guard-backlog.md
index aeb8427d..6ed0bd68 100644
--- a/docs/superpowers/plans/2026-05-31-discipline-guard-backlog.md
+++ b/docs/superpowers/plans/2026-05-31-discipline-guard-backlog.md
@@ -37,9 +37,34 @@ which stay hard-blacklisted because they can pull new/updated versions.
 
 `c:/` vs `/c/`, unexpanded `$env:` in gate messages. Polish only.
 
-### C. TDD-gate cross-actor
+### F. Parallel-session-lock false cross-worktree collision (2026-05-31, owner-raised)
 
-`enforce-tdd-gate` does not see test edits made by a subagent.
+Symptom: a session in worktree `discipline-guard` was blocked by
+`enforce-parallel-session-lock` (held by another session `7f6efd48`, pid changed
+12552→19044 across attempts → holder still active; pid is the transient hook-node pid,
+session_id is the stable identity).
+
+**Investigation (read-only):**
+- Lock keyed by `computeWorkspaceHash(process.cwd())` = md5(cwd).slice(0,12); file
+  `~/.claude/runtime/session-lock-<hash>.json`; release only on Stop; TTL 5 min.
+- 9 lock files accumulated → stale files leak when a session closes without a clean Stop.
+- `enforce-branch-switch` read branch "worktree-discipline-guard" via
+  `git branch --show-current` from `process.cwd()` → the hook's cwd IS the worktree →
+  **keying is already per-worktree** (NOT coarse main-dir). So the holder shared this
+  worktree's hash → genuine same-worktree concurrency, the lock working as designed —
+  NOT a false positive. Do NOT re-key (would weaken same-tree serialization).
+
+**Genuinely-fixable part (no weakening):** leaked lock on close-without-Stop blocks the next
+same-worktree session for up to TTL. Fix: release on SessionEnd (not only Stop) + prune
+stale lock files on acquire. Ground-truth the lock JSON before coding.
+
+### C. TDD-gate cross-actor — chosen: **Z** (full, 2026-05-31; on hold behind F)
+
+`enforce-tdd-gate` does not see test edits made by a subagent (scans only the controller's
+own turn; subagent test edit + RED live in `agent-<id>.jsonl`). **Z = Part 1 (close the
+projects/ Write hole — verified prerequisite) then Part 2 (read subagent transcript bound to
+a Task in this turn).** Condition 1 verified VIOLATED (no Write-tool gate covers
+`~/.claude/projects/`), so Variant 1 alone would weaken — safe only bundled with Part 1.
 
 ### D. Smoke 8 — live Workflow-gate F2 test
 
diff --git a/tools/enforce-parallel-session-lock.mjs b/tools/enforce-parallel-session-lock.mjs
index ccac04ff..1b930061 100644
--- a/tools/enforce-parallel-session-lock.mjs
+++ b/tools/enforce-parallel-session-lock.mjs
@@ -13,6 +13,7 @@
  */
 import { acquire, release, computeWorkspaceHash } from './parallel-session-lock.mjs';
 import { readFileSync, writeFileSync, unlinkSync, mkdirSync } from 'node:fs';
+import { execFileSync } from 'node:child_process';
 import { join, dirname } from 'node:path';
 import { readStdin, parseEventJson, exitDecision, runtimeDir } from './enforce-hook-helpers.mjs';
 
@@ -60,6 +61,38 @@ export function runReleaseAction({ event, cwd, readLock, deleteLock }) {
   return { released: true };
 }
 
+/**
+ * Resolve the stable work-tree root used as the lock key. Keys on the SESSION's
+ * cwd (`event.cwd`, stable across resume) resolved to the git work-tree root —
+ * NOT the hook's `process.cwd()`, which collapses to the main repo dir after a
+ * session resume and thereby false-blocks sessions in DIFFERENT worktrees.
+ * Pure (I/O injected): `runGitToplevel(dir)` returns the toplevel or '' on failure.
+ *
+ * @param {object} p
+ * @param {object} p.event
+ * @param {string} p.processCwd
+ * @param {(dir:string)=>string} p.runGitToplevel
+ * @returns {string}
+ */
+export function resolveWorkspacePath({ event, processCwd, runGitToplevel }) {
+  const dir = (event && typeof event.cwd === 'string' && event.cwd) ? event.cwd : processCwd;
+  try {
+    const top = runGitToplevel(dir);
+    if (top && typeof top === 'string') return top;
+  } catch { /* fall through to raw dir (fail-open) */ }
+  return dir;
+}
+
+function realGitToplevel(dir) {
+  try {
+    return execFileSync('git', ['-C', dir, 'rev-parse', '--show-toplevel'], {
+      encoding: 'utf-8',
+      timeout: 1000,
+      stdio: ['ignore', 'pipe', 'ignore'],
+    }).trim();
+  } catch { return ''; }
+}
+
 function lockPathFor(cwd) {
   return join(runtimeDir(), `session-lock-${computeWorkspaceHash(cwd)}.json`);
 }
@@ -82,7 +115,10 @@ async function main() {
   // a lock bug can NEVER wedge the user out of their own session.
   try {
     const event = parseEventJson(await readStdin());
-    const cwd = process.cwd();
+    // Key by the session's stable work-tree root (event.cwd → git toplevel),
+    // not the volatile hook process.cwd() (collapses to main on resume → false
+    // cross-worktree blocks). Fallback to process.cwd() keeps prior behavior.
+    const cwd = resolveWorkspacePath({ event, processCwd: process.cwd(), runGitToplevel: realGitToplevel });
     const p = lockPathFor(cwd);
 
     // Stop event carries no tool_name → release path.
diff --git a/tools/enforce-parallel-session-lock.test.mjs b/tools/enforce-parallel-session-lock.test.mjs
index e1d148c3..72fe7f49 100644
--- a/tools/enforce-parallel-session-lock.test.mjs
+++ b/tools/enforce-parallel-session-lock.test.mjs
@@ -131,3 +131,54 @@ describe('runReleaseAction — Stop release wiring', () => {
     expect(deleted).toBe(false);
   });
 });
+
+// Cross-worktree false-block fix (2026-05-31). The lock must key on the session's
+// stable work-tree root (from event.cwd → git toplevel), NOT the hook process.cwd()
+// — which collapses to the main repo dir after a session resume, making sessions in
+// DIFFERENT worktrees share one lock and block each other.
+import { resolveWorkspacePath } from './enforce-parallel-session-lock.mjs';
+
+describe('resolveWorkspacePath — stable worktree key', () => {
+  it('keys on event.cwd (the session worktree), not the hook process.cwd()', () => {
+    const r = resolveWorkspacePath({
+      event: { cwd: '/repo/.claude/worktrees/wt-A' },
+      processCwd: '/repo',
+      runGitToplevel: (dir) => dir,
+    });
+    expect(r).toBe('/repo/.claude/worktrees/wt-A');
+  });
+
+  it('gives different keys for two different worktrees (no cross-block)', () => {
+    const opts = { processCwd: '/repo', runGitToplevel: (dir) => dir };
+    const a = resolveWorkspacePath({ event: { cwd: '/repo/.claude/worktrees/wt-A' }, ...opts });
+    const b = resolveWorkspacePath({ event: { cwd: '/repo/.claude/worktrees/wt-B' }, ...opts });
+    expect(a).not.toBe(b);
+  });
+
+  it('resolves to the git work-tree root (collapses subdir variance)', () => {
+    const r = resolveWorkspacePath({
+      event: { cwd: '/repo/.claude/worktrees/wt-A/tools' },
+      processCwd: '/repo',
+      runGitToplevel: () => '/repo/.claude/worktrees/wt-A',
+    });
+    expect(r).toBe('/repo/.claude/worktrees/wt-A');
+  });
+
+  it('falls back to processCwd when event.cwd is absent', () => {
+    const r = resolveWorkspacePath({
+      event: { tool_name: 'Edit' },
+      processCwd: '/repo',
+      runGitToplevel: (dir) => dir,
+    });
+    expect(r).toBe('/repo');
+  });
+
+  it('falls back to the raw dir when git toplevel resolution fails (fail-open)', () => {
+    const r = resolveWorkspacePath({
+      event: { cwd: '/some/dir' },
+      processCwd: '/repo',
+      runGitToplevel: () => '',
+    });
+    expect(r).toBe('/some/dir');
+  });
+});