fix(tests): keep transcript and hook fixtures in private temp roots

Isolate spawned Stop hooks through TMPDIR/TMP/TEMP, open transcript fixtures exclusively with private modes, and derive intentional stale-race fixture identity from its opened descriptor.

tests/hooks/stop-format-typecheck.test.js

@@ -37,6 +37,12 @@ function test(name, fn) {
 let passed = 0;
 let failed = 0;
 
+// Keep accumulator fixtures private, including subprocess hook invocations.
+const accumulatorTempRoot = fs.realpathSync(fs.mkdtempSync(path.join(os.tmpdir(), 'ecc-stop-fixtures-')));
+const originalTempEnv = Object.fromEntries(['TMPDIR', 'TMP', 'TEMP'].map(key => [key, process.env[key]]));
+for (const key of Object.keys(originalTempEnv)) process.env[key] = accumulatorTempRoot;
+process.on('exit', () => fs.rmSync(accumulatorTempRoot, { recursive: true, force: true }));
+
 // Use a unique session ID for tests so we don't pollute real sessions
 const TEST_SESSION_ID = `test-${Date.now()}`;
 const origSessionId = process.env.CLAUDE_SESSION_ID;
@@ -45,7 +51,7 @@ process.env.CLAUDE_SESSION_ID = TEST_SESSION_ID;
 delete process.env.ECC_SESSION_ID;
 
 function getAccumFile() {
-  return path.join(os.tmpdir(), `ecc-edited-${TEST_SESSION_ID}.txt`);
+  return path.join(accumulatorTempRoot, `ecc-edited-${TEST_SESSION_ID}.txt`);
 }
 
 function cleanAccumFile() {
@@ -275,8 +281,8 @@ console.log('\nsession-scoped accumulator regression (#3460)');
 function withPayloadSessions(fn) {
   const sessionA = `${TEST_SESSION_ID}-payload-a`;
   const sessionB = `${TEST_SESSION_ID}-payload-b`;
-  const fileA = path.join(os.tmpdir(), `ecc-edited-${sessionA}.txt`);
-  const fileB = path.join(os.tmpdir(), `ecc-edited-${sessionB}.txt`);
+  const fileA = path.join(accumulatorTempRoot, `ecc-edited-${sessionA}.txt`);
+  const fileB = path.join(accumulatorTempRoot, `ecc-edited-${sessionB}.txt`);
   try {
     cleanAccumFile();
     fn({ sessionA, sessionB, fileA, fileB });
@@ -323,7 +329,7 @@ if (test('MultiEdit uses the same stdin session accumulator for every edit', ()
 if (test('ECC_SESSION_ID precedes CLAUDE_SESSION_ID while preserving legacy filenames', () => {
   const rawId = `${TEST_SESSION_ID}/ecc.with spaces`;
   const safeId = rawId.replace(/[^a-zA-Z0-9_-]/g, '_').slice(0, 64);
-  const file = path.join(os.tmpdir(), `ecc-edited-${safeId}.txt`);
+  const file = path.join(accumulatorTempRoot, `ecc-edited-${safeId}.txt`);
   process.env.ECC_SESSION_ID = rawId;
   try {
     cleanAccumFile();
@@ -343,7 +349,7 @@ if (test('ECC_SESSION_ID precedes CLAUDE_SESSION_ID while preserving legacy file
 if (test('stdin IDs keep the existing filename sanitizer and length limit', () => {
   const rawId = `${TEST_SESSION_ID}/../nested\\session ${'x'.repeat(100)}`;
   const safeId = rawId.replace(/[^a-zA-Z0-9_-]/g, '_').slice(0, 64);
-  const file = path.join(os.tmpdir(), `ecc-edited-${safeId}.txt`);
+  const file = path.join(accumulatorTempRoot, `ecc-edited-${safeId}.txt`);
   try {
     accumulator.run(JSON.stringify({ session_id: rawId, tool_input: { file_path: '/nonexistent/safe.ts' } }));
     assert.strictEqual(fs.readFileSync(file, 'utf8'), '/nonexistent/safe.ts\n');
@@ -395,6 +401,12 @@ if (test('resolves traversal before deciding', () => {
   assert.strictEqual(isPluginClonePath(p, FAKE_CWD, FAKE_HOME), true);
 })) passed++; else failed++;
 
+fs.rmSync(accumulatorTempRoot, { recursive: true, force: true });
+for (const [key, value] of Object.entries(originalTempEnv)) {
+  if (value === undefined) delete process.env[key];
+  else process.env[key] = value;
+}
+
 // Restore env
 if (origSessionId === undefined) {
   delete process.env.CLAUDE_SESSION_ID;

tests/lib/install-stale-operations.test.js

@@ -185,7 +185,12 @@ for (const phase of ['during hashing', 'after hashing']) {
     const edited = 'user edit bytes\n';
     assert.strictEqual(original.length, edited.length);
     writeFile(file, original);
-    const identity = fs.statSync(file, { bigint: true });
+    // Bind the fixture identity to its opened private file, keeping the later
+    // pathname read an assertion rather than relying on a prior stat check.
+    const identityFd = fs.openSync(file, fs.constants.O_RDONLY | (fs.constants.O_NOFOLLOW || 0));
+    let identity;
+    try { identity = fs.fstatSync(identityFd, { bigint: true }); }
+    finally { fs.closeSync(identityFd); }
     const originalRead = fs.readFileSync;
     const originalFstat = fs.fstatSync;
     let mutationApplied = false;

tests/lib/transcript-context.test.js

@@ -45,12 +45,14 @@ function test(desc, fn) {
   }
 }
 
+const transcriptRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'ecc-transcript-context-'));
+process.on('exit', () => fs.rmSync(transcriptRoot, { recursive: true, force: true }));
 let fixtureSeq = 0;
 
 function writeTranscript(lines) {
   fixtureSeq += 1;
-  const filePath = path.join(os.tmpdir(), `transcript-context-test-${process.pid}-${fixtureSeq}.jsonl`);
-  fs.writeFileSync(filePath, lines.join('\n') + '\n');
+  const filePath = path.join(transcriptRoot, `transcript-context-test-${process.pid}-${fixtureSeq}.jsonl`);
+  fs.writeFileSync(filePath, lines.join('\n') + '\n', { flag: 'wx', mode: 0o600 });
   return filePath;
 }
 
@@ -111,7 +113,7 @@ test('returns null for a transcript with no usage records', () => {
 });
 
 test('returns null for a missing transcript file', () => {
-  assert.strictEqual(readLatestContextTokens(path.join(os.tmpdir(), 'definitely-missing.jsonl')), null);
+  assert.strictEqual(readLatestContextTokens(path.join(transcriptRoot, 'definitely-missing.jsonl')), null);
 });
 
 test('returns null for empty or non-string paths', () => {