diff --git a/.changeset/plucky-cats-purr.md b/.changeset/plucky-cats-purr.md
new file mode 100644
index 000000000..10632e98a
--- /dev/null
+++ b/.changeset/plucky-cats-purr.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 579
+---
+Worktree executor agents no longer leak writes to the main checkout: a new PreToolUse hook (gsd-worktree-path-guard.js) hard-blocks Edit/Write/MultiEdit calls whose absolute path resolves outside the active worktree root.
diff --git a/bin/install.js b/bin/install.js
index 8f75f813f..5412a8ffc 100755
--- a/bin/install.js
+++ b/bin/install.js
@@ -9944,6 +9944,33 @@ function install(isGlobal, runtime = 'claude', options = {}) {
       console.warn(`  ${yellow}⚠${reset}  Skipped workflow guard hook — gsd-workflow-guard.js not found at target`);
     }
 
+    // Configure PreToolUse hook for worktree absolute-path safety (#260)
+    // Hard-blocks Edit/Write/MultiEdit tool calls with absolute paths that resolve
+    // outside the current worktree root. Prevents executor agents from
+    // accidentally writing to the main checkout when running in isolation="worktree".
+    const worktreePathGuardCommand = isGlobal
+      ? buildHookCommand(targetDir, 'gsd-worktree-path-guard.js', hookOpts)
+      : localCmd('gsd-worktree-path-guard.js');
+    const hasWorktreePathGuardHook = settings.hooks[preToolEvent].some(entry =>
+      entry.hooks && entry.hooks.some(h => h.command && h.command.includes('gsd-worktree-path-guard'))
+    );
+    const worktreePathGuardFile = path.join(targetDir, 'hooks', 'gsd-worktree-path-guard.js');
+    if (!hasWorktreePathGuardHook && fs.existsSync(worktreePathGuardFile) && worktreePathGuardCommand) {
+      settings.hooks[preToolEvent].push({
+        matcher: 'Write|Edit|MultiEdit',
+        hooks: [
+          {
+            type: 'command',
+            command: worktreePathGuardCommand,
+            timeout: 5
+          }
+        ]
+      });
+      console.log(`  ${green}✓${reset} Configured worktree path guard hook`);
+    } else if (!hasWorktreePathGuardHook && !fs.existsSync(worktreePathGuardFile)) {
+      console.warn(`  ${yellow}⚠${reset}  Skipped worktree path guard hook — gsd-worktree-path-guard.js not found at target`);
+    }
+
     // Configure commit validation hook (Conventional Commits enforcement, opt-in)
     const validateCommitCommand = isGlobal
       ? buildHookCommand(targetDir, 'gsd-validate-commit.sh', hookOpts)
diff --git a/docs/INVENTORY-MANIFEST.json b/docs/INVENTORY-MANIFEST.json
index 6f8bd3005..a7d20ea31 100644
--- a/docs/INVENTORY-MANIFEST.json
+++ b/docs/INVENTORY-MANIFEST.json
@@ -355,7 +355,8 @@
       "gsd-statusline.js",
       "gsd-update-banner.js",
       "gsd-validate-commit.sh",
-      "gsd-workflow-guard.js"
+      "gsd-workflow-guard.js",
+      "gsd-worktree-path-guard.js"
     ]
   }
 }
diff --git a/docs/INVENTORY.md b/docs/INVENTORY.md
index f46aea556..6b849034b 100644
--- a/docs/INVENTORY.md
+++ b/docs/INVENTORY.md
@@ -455,7 +455,7 @@ Full listing: `get-shit-done/bin/lib/*.cjs`.
 
 ---
 
-## Hooks (13 shipped)
+## Hooks (14 shipped)
 
 Full listing: `hooks/`.
 
@@ -470,6 +470,7 @@ Full listing: `hooks/`.
 | `gsd-workflow-guard.js` | `PreToolUse` | Detects file edits outside GSD workflow context (advisory, opt-in) |
 | `gsd-read-guard.js` | `PreToolUse` | Advisory guard preventing Edit/Write on unread files |
 | `gsd-read-injection-scanner.js` | `PostToolUse` | Scans tool Read results for prompt-injection patterns (v1.36+, PR #2201) |
+| `gsd-worktree-path-guard.js` | `PreToolUse` | Hard-blocks Edit/Write/MultiEdit with absolute paths outside the worktree root (PR #579, #260) |
 | `gsd-session-state.sh` | `PostToolUse` | Session-state tracking for shell-based runtimes |
 | `gsd-validate-commit.sh` | `PostToolUse` | Commit validation for conventional-commit enforcement |
 | `gsd-phase-boundary.sh` | `PostToolUse` | Phase-boundary detection for workflow transitions |
diff --git a/get-shit-done/bin/lib/installer-migration-report.cjs b/get-shit-done/bin/lib/installer-migration-report.cjs
index b7a4e894f..45d7624a6 100644
--- a/get-shit-done/bin/lib/installer-migration-report.cjs
+++ b/get-shit-done/bin/lib/installer-migration-report.cjs
@@ -122,6 +122,7 @@ const BUNDLED_GSD_HOOK_FILES = Object.freeze(new Set([
   'hooks/gsd-update-banner.js',
   'hooks/gsd-validate-commit.sh',
   'hooks/gsd-workflow-guard.js',
+  'hooks/gsd-worktree-path-guard.js',
 ]));
 
 // Classify a blocked prompt-user action into one of the safe-default
diff --git a/hooks/gsd-worktree-path-guard.js b/hooks/gsd-worktree-path-guard.js
new file mode 100644
index 000000000..2f8add19f
--- /dev/null
+++ b/hooks/gsd-worktree-path-guard.js
@@ -0,0 +1,169 @@
+#!/usr/bin/env node
+// gsd-hook-version: {{GSD_VERSION}}
+// GSD Worktree Path Guard — PreToolUse hook
+// Blocks Edit/Write/MultiEdit tool calls that target absolute paths outside the worktree root.
+//
+// Problem: gsd-executor agents spawned with isolation="worktree" sometimes issue
+// Edit/Write calls with absolute paths rooted at the MAIN repository instead of
+// the worktree (issue #260). The prose guard in agents/gsd-executor.md step 0b
+// is never enforced because the model under load skips it.
+//
+// This hook enforces the constraint at the tooling layer, making it HARD-BLOCKING.
+//
+// Triggers on: Edit, Write, and MultiEdit tool calls
+// Action: BLOCK (exit 2) if file_path is absolute and outside the worktree root
+// No-op: relative paths, non-worktree CWDs, hook errors (silent fail)
+
+const fs = require('fs');
+const path = require('path');
+const { spawnSync } = require('child_process');
+
+const SPAWNOPT = { encoding: 'utf8', stdio: ['ignore', 'pipe', 'ignore'], timeout: 2000 };
+
+function git(args, cwd) {
+  return spawnSync('git', args, { ...SPAWNOPT, cwd });
+}
+
+// Walk up from `start` to find the nearest existing directory.
+// Returns null if we reach the filesystem root without finding one.
+function nearestExistingDir(start) {
+  let dir = start;
+  let prev;
+  do {
+    prev = dir;
+    try { fs.accessSync(dir, fs.constants.F_OK); return dir; } catch { /* keep walking */ }
+    dir = path.dirname(dir);
+  } while (dir !== prev);
+  return null;
+}
+
+let input = '';
+const stdinTimeout = setTimeout(() => process.exit(0), 3000);
+process.stdin.setEncoding('utf8');
+process.stdin.on('data', chunk => input += chunk);
+process.stdin.on('end', () => {
+  clearTimeout(stdinTimeout);
+  try {
+    const data = JSON.parse(input);
+    const toolName = data.tool_name;
+
+    // Only guard Edit, Write, and MultiEdit tool calls
+    if (toolName !== 'Edit' && toolName !== 'Write' && toolName !== 'MultiEdit') {
+      process.exit(0);
+    }
+
+    const cwd = data.cwd || process.cwd();
+
+    // Detect whether CWD is inside a linked git worktree by inspecting
+    // the git-dir path. In a linked worktree, git rev-parse --git-dir
+    // returns a path containing .git/worktrees/ as a component.
+    // In the main repo or a submodule it returns .git (or a path without /worktrees/).
+    // This approach works even when cwd is a subdirectory of the worktree.
+    const gitDirResult = git(['rev-parse', '--git-dir'], cwd);
+    if (gitDirResult.status !== 0 || !gitDirResult.stdout) {
+      process.exit(0); // not a git repo — pass through
+    }
+
+    const gitDir = gitDirResult.stdout.trim();
+    // A linked worktree's --git-dir contains .git/worktrees/ as a path component
+    const isLinkedWorktree = /[/\\]\.git[/\\]worktrees[/\\]/.test(gitDir);
+    if (!isLinkedWorktree) {
+      process.exit(0); // main repo, submodule, or separate-git-dir — no-op
+    }
+
+    // Get the raw --show-toplevel output for the worktree (cwd).
+    // We keep it raw (not path.resolve'd) to compare directly with the
+    // file's toplevel — same git binary, same format, no normalization needed.
+    const wtTopResult = git(['rev-parse', '--show-toplevel'], cwd);
+    if (wtTopResult.status !== 0 || !wtTopResult.stdout) {
+      process.exit(0); // can't determine root — fail open
+    }
+    const wtTopRaw = wtTopResult.stdout.trim();
+
+    const rawFilePath = data.tool_input?.file_path || '';
+    if (!rawFilePath) {
+      process.exit(0);
+    }
+
+    // Relative paths are always safe — they resolve relative to CWD inside the worktree
+    if (!path.isAbsolute(rawFilePath)) {
+      process.exit(0);
+    }
+
+    // Normalise .. traversal so /worktree/src/../../../main/file
+    // resolves to its true location before we check containment.
+    const filePath = path.resolve(rawFilePath);
+
+    // Find the nearest existing ancestor of filePath so we can ask git
+    // for its toplevel. The file itself may not exist yet (Write creates
+    // new files), but at least one ancestor directory must exist.
+    // We check the file itself first in case it already exists.
+    const checkDir = nearestExistingDir(
+      (() => {
+        try {
+          return fs.statSync(filePath).isDirectory() ? filePath : path.dirname(filePath);
+        } catch {
+          return path.dirname(filePath);
+        }
+      })()
+    );
+
+    if (!checkDir) {
+      // Walked to root without finding any directory — path is synthetic.
+      // Block conservatively.
+      const output = {
+        decision: 'block',
+        reason:
+          `Worktree path guard: '${filePath}' has no existing ancestor directory — ` +
+          `cannot verify it is inside the worktree '${wtTopRaw}'. Use a relative path instead.`,
+      };
+      process.stdout.write(JSON.stringify(output));
+      process.exit(2);
+    }
+
+    // Ask git for the toplevel of the file's location.
+    // Comparing two raw git --show-toplevel outputs avoids every
+    // platform-specific path normalisation pitfall (Windows 8.3 short names,
+    // case differences between realpathSync and path.resolve, forward- vs
+    // back-slash inconsistencies) — both values come from the same git binary
+    // in the same format by definition.
+    const fileTopResult = git(['rev-parse', '--show-toplevel'], checkDir);
+
+    if (fileTopResult.status !== 0 || !fileTopResult.stdout) {
+      // checkDir is not inside any git repo → cannot be inside the worktree.
+      const output = {
+        decision: 'block',
+        reason:
+          `Worktree path guard: '${filePath}' is not inside any git repository — ` +
+          `it cannot be inside the worktree at '${wtTopRaw}'. Use a relative path instead.`,
+      };
+      process.stdout.write(JSON.stringify(output));
+      process.exit(2);
+    }
+
+    const fileTopRaw = fileTopResult.stdout.trim();
+
+    // Same git toplevel → file is inside the worktree → allow
+    if (fileTopRaw === wtTopRaw) {
+      process.exit(0);
+    }
+
+    // BLOCK: file resolves to a different git root than the active worktree
+    const output = {
+      decision: 'block',
+      reason:
+        `Worktree path guard: '${filePath}' resolves to git root '${fileTopRaw}' which ` +
+        `differs from the active worktree root '${wtTopRaw}'. This likely means an ` +
+        `absolute path was derived from the orchestrator's main repository instead of ` +
+        `the active worktree. To fix: use a relative path, or re-derive the base ` +
+        `directory with \`git rev-parse --show-toplevel\` from within the worktree ` +
+        `(hook cwd: '${cwd}').`,
+    };
+
+    process.stdout.write(JSON.stringify(output));
+    process.exit(2);
+  } catch {
+    // Silent fail — never block valid tool calls due to hook errors
+    process.exit(0);
+  }
+});
diff --git a/hooks/managed-hooks-registry.cjs b/hooks/managed-hooks-registry.cjs
index bdb0dac78..e29c7bb3c 100644
--- a/hooks/managed-hooks-registry.cjs
+++ b/hooks/managed-hooks-registry.cjs
@@ -29,6 +29,7 @@ const MANAGED_HOOKS = [
   'gsd-update-banner.js',
   'gsd-validate-commit.sh',
   'gsd-workflow-guard.js',
+  'gsd-worktree-path-guard.js',
 ];
 
 module.exports = { MANAGED_HOOKS };
diff --git a/scripts/build-hooks.js b/scripts/build-hooks.js
index b8182fea2..30b322ad2 100644
--- a/scripts/build-hooks.js
+++ b/scripts/build-hooks.js
@@ -33,6 +33,7 @@ const HOOKS_TO_COPY = [
   'gsd-statusline.js',
   'gsd-update-banner.js',
   'gsd-workflow-guard.js',
+  'gsd-worktree-path-guard.js',
   // Community hooks (bash, opt-in via .planning/config.json hooks.community)
   'gsd-session-state.sh',
   'gsd-validate-commit.sh',
diff --git a/tests/bug-1754-js-hook-guard.test.cjs b/tests/bug-1754-js-hook-guard.test.cjs
index 3910195b8..b403db4fb 100644
--- a/tests/bug-1754-js-hook-guard.test.cjs
+++ b/tests/bug-1754-js-hook-guard.test.cjs
@@ -27,6 +27,7 @@ const JS_HOOKS = [
   { name: 'gsd-prompt-guard.js',      registrationAnchor: 'hasPromptGuardHook' },
   { name: 'gsd-read-guard.js',        registrationAnchor: 'hasReadGuardHook' },
   { name: 'gsd-workflow-guard.js',    registrationAnchor: 'hasWorkflowGuardHook' },
+  { name: 'gsd-worktree-path-guard.js', registrationAnchor: 'hasWorktreePathGuardHook' },
 ];
 
 describe('bug #1754: .js hook registration guards', () => {
diff --git a/tests/bug-260-worktree-path-guard.test.cjs b/tests/bug-260-worktree-path-guard.test.cjs
new file mode 100644
index 000000000..4de81ee09
--- /dev/null
+++ b/tests/bug-260-worktree-path-guard.test.cjs
@@ -0,0 +1,423 @@
+/**
+ * Regression tests for bug #260 — gsd-worktree-path-guard.js
+ *
+ * Executor agents spawned with isolation="worktree" sometimes issue Edit/Write
+ * calls with absolute paths rooted at the MAIN repository instead of the
+ * worktree. The prose guard in gsd-executor.md step 0b is skipped under load,
+ * so we enforce the constraint at the tooling layer with a PreToolUse hook.
+ *
+ * This file verifies all guard behaviours:
+ *   1. No-op in the main repo (.git is a directory)
+ *   2. Relative path always passes
+ *   3. Non-Edit/Write tools always pass
+ *   4. Absolute path inside worktree root passes
+ *   5. Absolute path outside worktree root is BLOCKED (exit 2)
+ *   6. Sibling path that merely shares a prefix is BLOCKED (/ boundary check)
+ *   7. install.js has an fs.existsSync guard for gsd-worktree-path-guard.js
+ */
+
+'use strict';
+
+const { describe, test, before, after } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { spawnSync, execFileSync } = require('node:child_process');
+
+const HOOK_PATH = path.join(__dirname, '..', 'hooks', 'gsd-worktree-path-guard.js');
+const INSTALL_SRC = path.join(__dirname, '..', 'bin', 'install.js');
+
+/**
+ * Resolve symlinks in a path so that we compare the same canonical form
+ * that `git rev-parse --show-toplevel` returns. On macOS /tmp is a symlink
+ * to /private/tmp, which causes path prefix checks to fail without this.
+ */
+function realp(p) {
+  try { return fs.realpathSync(p); } catch { return p; }
+}
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+function git(cwd, args) {
+  return execFileSync('git', args, { cwd, encoding: 'utf8', stdio: ['ignore', 'pipe', 'pipe'] });
+}
+
+/**
+ * Create a plain git repo (main repo — .git is a directory).
+ */
+function makeMainRepo() {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'gsd-260-main-'));
+  git(dir, ['init', '-q']);
+  git(dir, ['config', 'user.email', 'test@example.com']);
+  git(dir, ['config', 'user.name', 'Test User']);
+  git(dir, ['config', 'commit.gpgsign', 'false']);
+  fs.writeFileSync(path.join(dir, 'README.md'), '# test\n');
+  git(dir, ['add', 'README.md']);
+  git(dir, ['commit', '-q', '-m', 'chore: init']);
+  return dir;
+}
+
+/**
+ * Create a worktree off mainRepo and return its path.
+ * In the worktree, .git is a FILE (the gitdir pointer).
+ */
+function makeWorktree(mainRepo) {
+  const wtDir = fs.mkdtempSync(path.join(os.tmpdir(), 'gsd-260-wt-'));
+  fs.rmdirSync(wtDir); // git worktree add creates the dir itself
+  git(mainRepo, ['worktree', 'add', '-q', '-b', 'wt-test-branch', wtDir]);
+  return wtDir;
+}
+
+/**
+ * Run the hook with a given payload, returning the spawnSync result.
+ */
+function runHook(cwd, payload) {
+  return spawnSync(process.execPath, [HOOK_PATH], {
+    cwd,
+    input: JSON.stringify(payload),
+    encoding: 'utf8',
+  });
+}
+
+// ---------------------------------------------------------------------------
+// Fixture lifecycle
+// ---------------------------------------------------------------------------
+
+let mainRepo;
+let worktreeDir;
+
+before(() => {
+  mainRepo = realp(makeMainRepo());
+  worktreeDir = realp(makeWorktree(mainRepo));
+});
+
+after(() => {
+  // Remove worktree registration before deleting the directory
+  try { git(mainRepo, ['worktree', 'remove', '--force', worktreeDir]); } catch { /* ignore */ }
+  try { fs.rmSync(mainRepo, { recursive: true, force: true }); } catch { /* ignore */ }
+  try { fs.rmSync(worktreeDir, { recursive: true, force: true }); } catch { /* ignore */ }
+});
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+describe('bug #260: gsd-worktree-path-guard.js', () => {
+
+  // 1. No-op in main repo
+  describe('no-op in main repo', () => {
+    test('Edit call in main repo (.git is a directory) exits 0', () => {
+      const payload = {
+        cwd: mainRepo,
+        tool_name: 'Edit',
+        tool_input: { file_path: path.join(mainRepo, 'src', 'foo.ts') },
+      };
+      const result = runHook(mainRepo, payload);
+      assert.strictEqual(result.status, 0, `Expected exit 0 in main repo, got ${result.status}. stderr: ${result.stderr}`);
+      assert.strictEqual(result.stdout, '', 'Expected no stdout in main repo no-op');
+    });
+
+    test('Write call in main repo exits 0', () => {
+      const payload = {
+        cwd: mainRepo,
+        tool_name: 'Write',
+        tool_input: { file_path: path.join(mainRepo, 'out.txt') },
+      };
+      const result = runHook(mainRepo, payload);
+      assert.strictEqual(result.status, 0);
+      assert.strictEqual(result.stdout, '');
+    });
+  });
+
+  // 2. Relative path always passes
+  describe('relative path', () => {
+    test('Edit with relative file_path exits 0 even in worktree', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Edit',
+        tool_input: { file_path: 'src/foo.ts' },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 0, `Relative path should always pass. stderr: ${result.stderr}`);
+      assert.strictEqual(result.stdout, '');
+    });
+
+    test('Write with relative file_path exits 0 in worktree', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Write',
+        tool_input: { file_path: 'dist/bundle.js' },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 0);
+      assert.strictEqual(result.stdout, '');
+    });
+  });
+
+  // 3. Non-Edit/Write tools always pass
+  describe('non-Edit/Write tools', () => {
+    test('Bash tool exits 0', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Bash',
+        tool_input: { command: 'ls' },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 0);
+    });
+
+    test('Read tool exits 0', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Read',
+        tool_input: { file_path: path.join(mainRepo, 'README.md') },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 0);
+    });
+
+    test('Grep tool exits 0', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Grep',
+        tool_input: { pattern: 'foo', path: mainRepo },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 0);
+    });
+  });
+
+  // 4. Absolute path inside worktree passes
+  describe('path inside worktree', () => {
+    test('Edit with absolute path inside worktree root exits 0', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Edit',
+        tool_input: { file_path: path.join(worktreeDir, 'src', 'foo.ts') },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 0, `Path inside worktree should pass. stderr: ${result.stderr}`);
+      assert.strictEqual(result.stdout, '');
+    });
+
+    test('Edit targeting exactly the worktree root exits 0', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Edit',
+        tool_input: { file_path: worktreeDir },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 0);
+    });
+  });
+
+  // 5. Absolute path outside worktree is BLOCKED
+  describe('path outside worktree is blocked', () => {
+    test('Edit targeting main repo root exits 2 with block decision', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Edit',
+        tool_input: { file_path: path.join(mainRepo, 'src', 'index.ts') },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 2, `Expected exit 2 (block), got ${result.status}. stderr: ${result.stderr}`);
+      let parsed;
+      assert.doesNotThrow(() => { parsed = JSON.parse(result.stdout); }, 'stdout must be valid JSON');
+      assert.strictEqual(parsed.decision, 'block', 'Expected decision:"block" in output');
+    });
+
+    test('Write targeting main repo root exits 2 with block decision', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Write',
+        tool_input: { file_path: path.join(mainRepo, 'out.txt') },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 2);
+      const parsed = JSON.parse(result.stdout);
+      assert.strictEqual(parsed.decision, 'block');
+    });
+
+    test('block output includes the offending path in reason', () => {
+      const offendingPath = path.join(mainRepo, 'src', 'leak.ts');
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Edit',
+        tool_input: { file_path: offendingPath },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 2);
+      const parsed = JSON.parse(result.stdout);
+      assert.ok(
+        parsed.reason && parsed.reason.includes(offendingPath),
+        `block reason should include the offending path. Got: ${parsed.reason}`
+      );
+    });
+  });
+
+  // 6. Sibling directory path is BLOCKED (validates the '/' boundary check)
+  describe('sibling path is blocked', () => {
+    test('path that shares prefix with worktree root but is a sibling exits 2', () => {
+      // e.g. worktreeDir = /tmp/gsd-260-wt-XXXXX
+      // sibling       = /tmp/gsd-260-wt-XXXXXsibling/file.ts
+      // This would pass a naive startsWith(wtRoot) check without the '/' suffix.
+      const siblingPath = worktreeDir + '-sibling/file.ts';
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Edit',
+        tool_input: { file_path: siblingPath },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 2,
+        `Sibling path "${siblingPath}" must be blocked (exit 2), got ${result.status}. ` +
+        `This validates the '/' boundary check in startsWith(wtRoot + '/'). stderr: ${result.stderr}`
+      );
+      const parsed = JSON.parse(result.stdout);
+      assert.strictEqual(parsed.decision, 'block');
+    });
+  });
+
+  // 7. Adversarial: subdirectory cwd still guards correctly (Codex finding #2)
+  describe('subdirectory cwd', () => {
+    test('hook fires when cwd is a subdirectory of the worktree, not just its root', () => {
+      // The orchestrator may set cwd to a subdirectory. The hook must still
+      // detect the worktree context via git rev-parse --git-dir and block.
+      const subDir = path.join(worktreeDir, 'src');
+      fs.mkdirSync(subDir, { recursive: true });
+      const payload = {
+        cwd: subDir,
+        tool_name: 'Edit',
+        tool_input: { file_path: path.join(mainRepo, 'src', 'index.ts') },
+      };
+      const result = runHook(subDir, payload);
+      assert.strictEqual(result.status, 2,
+        `Hook must block even when cwd is a subdirectory of the worktree. ` +
+        `Got exit ${result.status}. stderr: ${result.stderr}`
+      );
+      const parsed = JSON.parse(result.stdout);
+      assert.strictEqual(parsed.decision, 'block');
+    });
+
+    test('path inside worktree passes even when cwd is a subdirectory', () => {
+      const subDir = path.join(worktreeDir, 'src');
+      fs.mkdirSync(subDir, { recursive: true });
+      const payload = {
+        cwd: subDir,
+        tool_name: 'Edit',
+        tool_input: { file_path: path.join(worktreeDir, 'src', 'foo.ts') },
+      };
+      const result = runHook(subDir, payload);
+      assert.strictEqual(result.status, 0,
+        `Absolute path inside worktree should pass regardless of cwd. ` +
+        `Got exit ${result.status}. stderr: ${result.stderr}`
+      );
+    });
+  });
+
+  // 8. Adversarial: `..` traversal is normalised before the containment check (Codex finding #1)
+  describe('dot-dot traversal is blocked', () => {
+    test('path with .. that escapes the worktree is blocked', () => {
+      // /worktree/src/../../../main-repo/file.ts resolves outside the worktree
+      const traversalPath = path.join(worktreeDir, 'src', '..', '..', '..', mainRepo.replace(/^\//, ''), 'file.ts');
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'Edit',
+        tool_input: { file_path: traversalPath },
+      };
+      const result = runHook(worktreeDir, payload);
+      // After path.resolve, the path should equal something outside the worktree
+      const resolved = path.resolve(traversalPath);
+      if (resolved.startsWith(worktreeDir + path.sep) || resolved === worktreeDir) {
+        // The traversal happened to stay inside — skip this assertion
+        assert.ok(true, 'traversal resolved inside worktree (environment-dependent)');
+      } else {
+        assert.strictEqual(result.status, 2,
+          `Traversal path "${traversalPath}" resolves to "${resolved}" which is outside the worktree. ` +
+          `Must be blocked. Got exit ${result.status}. stderr: ${result.stderr}`
+        );
+        const parsed = JSON.parse(result.stdout);
+        assert.strictEqual(parsed.decision, 'block');
+      }
+    });
+  });
+
+  // 9. MultiEdit is also guarded (Codex finding #5)
+  describe('MultiEdit tool is guarded', () => {
+    test('MultiEdit with outside absolute path is blocked', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'MultiEdit',
+        tool_input: { file_path: path.join(mainRepo, 'src', 'index.ts') },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 2,
+        `MultiEdit targeting outside path must be blocked. Got ${result.status}. stderr: ${result.stderr}`
+      );
+      const parsed = JSON.parse(result.stdout);
+      assert.strictEqual(parsed.decision, 'block');
+    });
+
+    test('MultiEdit with inside absolute path passes', () => {
+      const payload = {
+        cwd: worktreeDir,
+        tool_name: 'MultiEdit',
+        tool_input: { file_path: path.join(worktreeDir, 'src', 'foo.ts') },
+      };
+      const result = runHook(worktreeDir, payload);
+      assert.strictEqual(result.status, 0,
+        `MultiEdit inside worktree should pass. Got ${result.status}. stderr: ${result.stderr}`
+      );
+    });
+  });
+
+});
+
+// ---------------------------------------------------------------------------
+// Static analysis: install.js guard
+// ---------------------------------------------------------------------------
+
+describe('install.js guard for gsd-worktree-path-guard.js', () => {
+  let src;
+
+  before(() => {
+    src = fs.readFileSync(INSTALL_SRC, 'utf-8');
+  });
+
+  test('install.js has hasWorktreePathGuardHook variable', () => {
+    assert.ok(
+      src.includes('hasWorktreePathGuardHook'),
+      'hasWorktreePathGuardHook variable not found in install.js'
+    );
+  });
+
+  test('install.js checks fs.existsSync before registering gsd-worktree-path-guard.js', () => {
+    const anchorIdx = src.indexOf('hasWorktreePathGuardHook');
+    assert.ok(anchorIdx !== -1, 'hasWorktreePathGuardHook not found in install.js');
+
+    const blockStart = anchorIdx;
+    const blockEnd = Math.min(src.length, anchorIdx + 1200);
+    const block = src.slice(blockStart, blockEnd);
+
+    assert.ok(
+      block.includes('fs.existsSync') || block.includes('existsSync'),
+      'install.js must call fs.existsSync on the target path before registering ' +
+      'gsd-worktree-path-guard.js in settings.json. Without this guard, the hook ' +
+      'is registered even when the .js file was never copied (root cause of #1754).'
+    );
+  });
+
+  test('install.js emits a skip warning when gsd-worktree-path-guard.js is missing', () => {
+    const anchorIdx = src.indexOf('hasWorktreePathGuardHook');
+    assert.ok(anchorIdx !== -1, 'hasWorktreePathGuardHook not found in install.js');
+
+    const block = src.slice(anchorIdx, Math.min(src.length, anchorIdx + 1200));
+
+    assert.ok(
+      block.includes('Skipped') && block.includes('gsd-worktree-path-guard'),
+      'install.js must emit a skip warning mentioning gsd-worktree-path-guard when the file is not found'
+    );
+  });
+});