diff --git a/docs/TEST_INTEGRITY.md b/docs/TEST_INTEGRITY.md
index 9ec2e52..cba341f 100644
--- a/docs/TEST_INTEGRITY.md
+++ b/docs/TEST_INTEGRITY.md
@@ -3454,3 +3454,34 @@ test_integrity_note:
     did_relax_any_assertion: false
     if_true_explain_spec_basis: ""
 ```
+
+# Release brief resolver tolerates unrelated vanished Git sibling
+
+```yaml
+test_integrity_note:
+  change_type: ["new_tests"]
+  scenarios:
+    - id: SCN-setup-unrelated-vanished-sibling-tolerated
+      given: "A real installed Git fixture has a selected registered worktree and a separate Git-registered sibling whose directory is renamed away but whose fixture data and registration remain"
+      when: "The installed-project resolver reads the selected worktree, and strict refusal cases exercise missing selected root, foreign/unregistered checkout, stale or absent installation identity, registered common-Git mismatch, and registered branch mismatch"
+      then: "The resolver returns the canonical root while local-state inputs, Git worktree-admin bytes and refs, install manifest, and moved sibling bytes remain unchanged; every strict refusal retains its existing error code"
+  observable_outcomes:
+    - "The selected same-installation registered branch resolves to its canonical project root despite an unrelated missing Git sibling"
+    - "The sibling remains absent; the full relevant Git worktree-admin entry and exact ref listing remain present and byte-identical"
+    - "Every file, directory inventory, and mode under declared local project state remains identical, including state.db and WAL/SHM files when present"
+    - "The moved sibling fixture, including its .git administration file, remains byte-identical"
+    - "The install manifest remains byte-identical; the resolver performs no prune, unregister, or fixture cleanup"
+    - "A missing selected root retains ENOENT; foreign/unregistered, stale or absent install identity, registered common-Git mismatch, and registered branch mismatch retain UNREGISTERED_PROJECT_WORKTREE"
+  determinism_controls:
+    - "Fixed fixture inputs and branch names, real local Git worktrees, temporary project and state roots, and no network or provider calls"
+    - "A filesystem rename makes one registered sibling path absent without pruning or changing Git's registration; deterministic recursive SHA-256/mode/inventory snapshots cover local state, Git worktree administration, and moved fixture data"
+  anti_cheat_rationale:
+    prevents:
+      - "Treating an unregistered or copied checkout as canonical after a lookup error"
+      - "Swallowing current-root or non-absence filesystem failures"
+      - "Pruning, unregistering, or rewriting the missing sibling to make resolution pass"
+      - "Relying only on porcelain output while silently changing SQLite, Git admin/ref, or moved-sibling bytes"
+  relaxation:
+    did_relax_any_assertion: false
+    if_true_explain_spec_basis: ""
+```
diff --git a/src/kernel/project-root.mjs b/src/kernel/project-root.mjs
index eeea65b..ee7a6b5 100644
--- a/src/kernel/project-root.mjs
+++ b/src/kernel/project-root.mjs
@@ -28,7 +28,17 @@ export function resolveInstalledProjectRoot(root, env = process.env) {
   const current = read(join(canonical, '.torch', 'install-manifest.json'));
   const registered = current.external?.find(entry => entry.type === 'worktree' && existsSync(entry.path) && realpathSync(entry.path) === root);
   const common = directory => realpathSync(resolve(directory, git(directory, ['rev-parse', '--git-common-dir'])));
-  const members = git(canonical, ['worktree', 'list', '--porcelain']).split('\n').filter(line => line.startsWith('worktree ')).map(line => realpathSync(line.slice(9)));
+  const members = [];
+  for (const line of git(canonical, ['worktree', 'list', '--porcelain']).split('\n').filter(value => value.startsWith('worktree '))) {
+    const memberPath = line.slice(9);
+    try {
+      members.push(realpathSync(memberPath));
+    } catch (error) {
+      // A vanished unrelated worktree must not block this checkout's lookup.
+      // Keep errors for the selected root and non-absence failures fail-closed.
+      if (resolve(memberPath) === root || !['ENOENT', 'ENOTDIR'].includes(error.code)) throw error;
+    }
+  }
   if (current.projectId !== manifest.projectId || current.installationId !== manifest.installationId
       || !registered || common(root) !== common(canonical) || !members.includes(root)
       || git(root, ['branch', '--show-current']) !== registered.branch) {
diff --git a/test/fleet/setup.test.mjs b/test/fleet/setup.test.mjs
index b051f36..d7d6ad2 100644
--- a/test/fleet/setup.test.mjs
+++ b/test/fleet/setup.test.mjs
@@ -1,8 +1,9 @@
 import assert from 'node:assert/strict';
+import { createHash } from 'node:crypto';
 import { execFileSync } from 'node:child_process';
-import { mkdtempSync, readFileSync, writeFileSync, cpSync } from 'node:fs';
+import { existsSync, lstatSync, mkdtempSync, readFileSync, readdirSync, readlinkSync, renameSync, writeFileSync, cpSync } from 'node:fs';
 import { tmpdir } from 'node:os';
-import { join } from 'node:path';
+import { join, resolve } from 'node:path';
 import test from 'node:test';
 import { analyzeRepository } from '../../src/kernel/analyze.mjs';
 import { proposeDomains } from '../../src/kernel/domains.mjs';
@@ -30,6 +31,33 @@ function fixture() {
   installProject({ repository, proposal, env });
   return { root, env, parentOverride: join(root, 'worktrees') };
 }
+function snapshotTree(root) {
+  const records = [];
+  const visit = (path, relativePath) => {
+    const stat = lstatSync(path);
+    const record = { path: relativePath || '.', mode: stat.mode & 0o7777 };
+    if (stat.isDirectory()) {
+      record.type = 'directory';
+      records.push(record);
+      for (const name of readdirSync(path).sort()) visit(join(path, name), relativePath ? join(relativePath, name) : name);
+    } else if (stat.isSymbolicLink()) {
+      record.type = 'symlink';
+      record.target = readlinkSync(path);
+      records.push(record);
+    } else if (stat.isFile()) {
+      const bytes = readFileSync(path);
+      record.type = 'file';
+      record.size = bytes.length;
+      record.sha256 = createHash('sha256').update(bytes).digest('hex');
+      records.push(record);
+    } else {
+      record.type = 'other';
+      records.push(record);
+    }
+  };
+  visit(root, '');
+  return records;
+}
 
 test('SCN-guided-setup: approved setup checkpoints owned files and provisions without agents', () => {
   const f = fixture();
@@ -79,6 +107,69 @@ test('SCN-worktree-canonical-brief: current instructions win and unregistered or
   assert.throws(() => resolveInstalledProjectRoot(entry.path, f.env), { code: 'UNREGISTERED_PROJECT_WORKTREE' });
 });
 
+test('SCN-setup-unrelated-vanished-sibling-tolerated: registered checkout resolves while an absent Git sibling remains registered', () => {
+  const f = fixture();
+  setupProject({ ...f, repository: inspectRepository(f.root), authorized: true });
+  const entry = readInstallManifest(f.root).external.find(item => item.type === 'worktree');
+  const missingSibling = join(f.parentOverride, 'vanished-sibling');
+  const movedSibling = join(f.parentOverride, 'moved-sibling-fixture');
+  git(f.root, ['worktree', 'add', '-b', 'vanished-sibling', missingSibling]);
+  const siblingAdminPath = resolve(missingSibling, git(missingSibling, ['rev-parse', '--git-dir']));
+  renameSync(missingSibling, movedSibling);
+  assert.equal(existsSync(missingSibling), false);
+
+  const manifestPath = join(f.root, '.torch/install-manifest.json');
+  const stateRoot = readInstallManifest(f.root).external.find(item => item.type === 'local-state').path;
+  const manifestBefore = readFileSync(manifestPath, 'utf8');
+  const stateBefore = snapshotTree(stateRoot);
+  const siblingAdminBefore = snapshotTree(siblingAdminPath);
+  const movedSiblingBefore = snapshotTree(movedSibling);
+  const worktreesBefore = git(f.root, ['worktree', 'list', '--porcelain']);
+  const refsBefore = git(f.root, ['for-each-ref', '--format=%(refname) %(objectname)']);
+  assert.ok(worktreesBefore.includes(`worktree ${missingSibling}`));
+  assert.ok(refsBefore.split('\n').includes(`refs/heads/vanished-sibling ${git(f.root, ['rev-parse', 'refs/heads/vanished-sibling'])}`));
+
+  assert.equal(resolveInstalledProjectRoot(entry.path, f.env), f.root);
+
+  assert.equal(existsSync(missingSibling), false);
+  assert.equal(readFileSync(manifestPath, 'utf8'), manifestBefore);
+  assert.deepEqual(snapshotTree(stateRoot), stateBefore);
+  assert.deepEqual(snapshotTree(siblingAdminPath), siblingAdminBefore);
+  assert.deepEqual(snapshotTree(movedSibling), movedSiblingBefore);
+  assert.equal(git(f.root, ['worktree', 'list', '--porcelain']), worktreesBefore);
+  assert.equal(git(f.root, ['for-each-ref', '--format=%(refname) %(objectname)']), refsBefore);
+
+  assert.throws(() => resolveInstalledProjectRoot(join(f.parentOverride, 'missing-selected-root'), f.env), { code: 'ENOENT' });
+
+  const foreignRoot = mkdtempSync(join(tmpdir(), 'torch-foreign-worktree-'));
+  cpSync(join(entry.path, '.torch'), join(foreignRoot, '.torch'), { recursive: true });
+  git(foreignRoot, ['init', '-b', entry.branch]);
+  const foreignManifestPath = join(foreignRoot, '.torch/install-manifest.json');
+  const canonicalManifest = JSON.parse(manifestBefore);
+  const foreignManifest = JSON.parse(readFileSync(foreignManifestPath, 'utf8'));
+  foreignManifest.installationId = 'stale-installation';
+  writeFileSync(foreignManifestPath, JSON.stringify(foreignManifest));
+  assert.throws(() => resolveInstalledProjectRoot(foreignRoot, f.env), { code: 'UNREGISTERED_PROJECT_WORKTREE' });
+  delete foreignManifest.installationId;
+  writeFileSync(foreignManifestPath, JSON.stringify(foreignManifest));
+  assert.throws(() => resolveInstalledProjectRoot(foreignRoot, f.env), { code: 'UNREGISTERED_PROJECT_WORKTREE' });
+  foreignManifest.installationId = canonicalManifest.installationId;
+  writeFileSync(foreignManifestPath, JSON.stringify(foreignManifest));
+  assert.throws(() => resolveInstalledProjectRoot(foreignRoot, f.env), { code: 'UNREGISTERED_PROJECT_WORKTREE' });
+
+  const foreignRegistration = { ...entry, path: foreignRoot };
+  canonicalManifest.external.push(foreignRegistration);
+  writeFileSync(manifestPath, JSON.stringify(canonicalManifest));
+  try {
+    assert.throws(() => resolveInstalledProjectRoot(foreignRoot, f.env), { code: 'UNREGISTERED_PROJECT_WORKTREE' });
+  } finally {
+    writeFileSync(manifestPath, manifestBefore);
+  }
+
+  git(entry.path, ['switch', '-c', 'registered-branch-mismatch']);
+  assert.throws(() => resolveInstalledProjectRoot(entry.path, f.env), { code: 'UNREGISTERED_PROJECT_WORKTREE' });
+});
+
 test('SCN-install-restore: detach and explicit restore preserve configuration and worktree history without starting sessions', () => {
   const f = fixture();
   setupProject({ ...f, repository: inspectRepository(f.root), authorized: true });