diff --git a/lib/agent/driver.js b/lib/agent/driver.js
index 8ebe3e8..54e6e16 100644
--- a/lib/agent/driver.js
+++ b/lib/agent/driver.js
@@ -171,7 +171,8 @@ export function createDriver({ profile, ports, transcripts, env = process.env, l
         const at = { sessionId, projectDir: profile.payload.projectDir(payload, context) };
         const waitLeft = (budgetOf = 'prompt') => waitBudgetOf(profile.budgets, budgetOf, { lifetime, startedAt });
         return await HANDLERS[event]({ profile, ports, hostEvent, payload, context, at, waitLeft, startedAt, options });
-      } catch {
+      } catch (error) {
+        options.onError?.(error);
         return profile.reply.empty(hostEvent, payload);
       }
     },
diff --git a/lib/channels/hook/runner.js b/lib/channels/hook/runner.js
index a591080..fca96d4 100644
--- a/lib/channels/hook/runner.js
+++ b/lib/channels/hook/runner.js
@@ -13,7 +13,7 @@ import { readPayload } from './stdin.js';
  * One command-hook invocation: the host's payload on stdin, the agent's answer as JSON on stdout, exit 0. Inside
  * this plugin's own headless distillation the hook answers the agent's empty reply and does nothing else.
  */
-export async function runHookProcess(agent, hostEvent, { stdin = process.stdin, stdout = process.stdout, env = process.env, exit = process.exit, profileOf = profileNamed } = {}) {
+export async function runHookProcess(agent, hostEvent, { stdin = process.stdin, stdout = process.stdout, stderr = process.stderr, env = process.env, exit = process.exit, profileOf = profileNamed } = {}) {
   const startedAt = Date.now();
   const write = (answer) => stdout.write(JSON.stringify(answer ?? {}), () => exit(0));
   const profile = await profileOf(agent).catch(() => null);
@@ -22,5 +22,14 @@ export async function runHookProcess(agent, hostEvent, { stdin = process.stdin,
   if (isHeadlessRun(env)) return write(profile.reply.empty(hostEvent));
   const { ports } = environmentFor({ profile, env, lifetime: EPHEMERAL, runBackground: runBackgroundTasks });
   const driver = createDriver({ profile, ports, transcripts, env, lifetime: EPHEMERAL, startedAt });
-  return write(await driver.handle(hostEvent, payload));
+  let diagnostic;
+  const answer = await driver.handle(hostEvent, payload, {
+    onError(error) {
+      if (['EINVALIDSTATE', 'ESTATEIO', 'ELOCKED'].includes(error?.code)) {
+        diagnostic = `Evolver hook skipped a state operation (${error.code}). Check the plugin state directory and retry.\n`;
+      }
+    },
+  });
+  if (diagnostic) return stderr.write(diagnostic, () => write(answer));
+  return write(answer);
 }
diff --git a/lib/runtime/process/owner.js b/lib/runtime/process/owner.js
index 8d39208..5ea48fa 100644
--- a/lib/runtime/process/owner.js
+++ b/lib/runtime/process/owner.js
@@ -6,8 +6,32 @@ import { readlinkSync } from 'node:fs';
 import path from 'node:path';
 
 const SHELLS = new Set(['sh', 'bash', 'zsh', 'dash', 'ksh', 'fish', 'cmd.exe', 'powershell', 'pwsh']);
-const PS_TIMEOUT_MS = 1000;
+export const PROCESS_START_TIMEOUT_MS = 1_000;
+// POSIX ps reports start times to the nearest second.
+export const PROCESS_START_TOLERANCE_MS = 2_000;
 const MAX_ANCESTORS = 16;
+const ownStartTime = Math.round(Date.now() - process.uptime() * 1000);
+
+/** A bounded process start-time probe for lock recovery. The hot path needs no subprocess. */
+export function processStartTime(pid, timeout = PROCESS_START_TIMEOUT_MS) {
+  if (pid === process.pid) return ownStartTime;
+  if (!Number.isSafeInteger(pid) || pid <= 0 || timeout <= 0) return null;
+  const windows = process.platform === 'win32';
+  const systemRoot = process.env.SystemRoot ?? 'C:\\Windows';
+  if (windows && !path.isAbsolute(systemRoot)) return null;
+  const executable = windows ? path.join(systemRoot, 'System32', 'WindowsPowerShell', 'v1.0', 'powershell.exe') : '/bin/ps';
+  const args = windows
+    ? ['-NoProfile', '-NonInteractive', '-Command', `([DateTimeOffset] (Get-Process -Id ${pid} -ErrorAction Stop).StartTime).ToUnixTimeMilliseconds()`]
+    : ['-p', String(pid), '-o', 'lstart='];
+  const env = windows ? process.env : { ...process.env, LC_ALL: 'C', TZ: 'UTC' };
+  const result = spawnSync(executable, args, {
+    env, encoding: 'utf8', windowsHide: true, timeout, killSignal: 'SIGKILL',
+  });
+  const started = result.status === 0 ? result.stdout.trim() : '';
+  if (!started) return null;
+  const time = windows ? Number(started) : Date.parse(started + ' GMT');
+  return Number.isSafeInteger(time) && time > 0 ? time : null;
+}
 
 /**
  * Whether the process that wrote a state entry has exited. Signal 0 only probes: ESRCH means no such process;
@@ -35,7 +59,7 @@ function executablePathOf(pid, command) {
 }
 
 export function processInfo(pid) {
-  const probe = spawnSync('ps', ['-o', 'ppid=,comm=', '-p', String(pid)], { encoding: 'utf8', timeout: PS_TIMEOUT_MS });
+  const probe = spawnSync('ps', ['-o', 'ppid=,comm=', '-p', String(pid)], { encoding: 'utf8', timeout: PROCESS_START_TIMEOUT_MS });
   const match = /^\s*(\d+)\s+(.+?)\s*$/.exec(probe.stdout ?? '');
   if (!match) return null;
   const command = match[2].replace(/^-/, '');
diff --git a/lib/runtime/store/admission.js b/lib/runtime/store/admission.js
new file mode 100644
index 0000000..b416d0b
--- /dev/null
+++ b/lib/runtime/store/admission.js
@@ -0,0 +1,137 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 EvoMap
+
+import crypto from 'node:crypto';
+import fs from 'node:fs';
+import path from 'node:path';
+
+import { ownerIsGone, processStartTime, PROCESS_START_TIMEOUT_MS, PROCESS_START_TOLERANCE_MS } from '../process/owner.js';
+
+export const LOCK_WAIT_MS = 2_000;
+export const LOCK_RETRY_MS = 20;
+export const LOCK_ACCESS_ERRORS = new Set(['EPERM', 'EBUSY', 'EACCES']);
+const ADMISSION_IDENTITY_CHECK_MS = 10_000;
+const ABANDONED_PREFIX = 'abandoned-';
+const abandonedAdmissions = new Map();
+
+export function pause(ms) {
+  Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, ms);
+}
+
+function reclaimAdmission(admission, deadline, starts) {
+  try {
+    const info = fs.lstatSync(admission);
+    if (!info.isDirectory()) return false;
+    const entries = fs.readdirSync(admission);
+    if (entries.length > 0) {
+      const marker = entries.find((name) => name.startsWith(ABANDONED_PREFIX));
+      const ownerName = marker ? marker.slice(ABANDONED_PREFIX.length) : entries[0];
+      if (entries.some((name) => name !== ownerName && name !== marker)) return false;
+      const owner = /^owner-(\d+)-[a-f0-9]+(?:-([a-f0-9]{24}|[0-9]{13}))?$/.exec(ownerName);
+      if (!owner) return false;
+      const pid = Number(owner[1]);
+      if (!Number.isSafeInteger(pid) || pid <= 0) return false;
+      const abandonedHere = abandonedAdmissions.get(admission) === ownerName;
+      const markerPath = marker && path.join(admission, marker);
+      if (markerPath && !fs.lstatSync(markerPath).isFile()) return false;
+      if (!markerPath && !abandonedHere && !ownerIsGone(pid)) {
+        const birth = Number(owner[2]);
+        const remaining = deadline - Date.now();
+        if (owner[2]?.length !== 13 || !Number.isSafeInteger(birth) || Date.now() - info.mtimeMs < ADMISSION_IDENTITY_CHECK_MS || remaining <= 0) return false;
+        if (!starts.has(pid)) starts.set(pid, processStartTime(pid, Math.min(remaining, PROCESS_START_TIMEOUT_MS)));
+        const actualBirth = starts.get(pid);
+        // Only a clearly newer process proves reuse. ps timestamps round to whole seconds.
+        if (!actualBirth || actualBirth <= birth + PROCESS_START_TOLERANCE_MS) return false;
+      }
+      if (entries.includes(ownerName)) fs.unlinkSync(path.join(admission, ownerName));
+      if (markerPath) fs.unlinkSync(markerPath);
+      if (abandonedHere) abandonedAdmissions.delete(admission);
+    }
+    // Only the inspected unique owner is unlinked. A fresh guard is already nonempty, so rmdir cannot
+    // remove it even if another reclaimer replaced the directory after our read.
+    fs.rmdirSync(admission);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+function enterAdmission(lockDir, deadline, starts) {
+  const admission = `${lockDir}.admission`;
+  const owner = `owner-${process.pid}-${crypto.randomBytes(12).toString('hex')}-${processStartTime(process.pid)}`;
+  const staging = `${admission}.${owner}.claim`;
+  fs.mkdirSync(path.dirname(admission), { recursive: true, mode: 0o700 });
+  fs.mkdirSync(staging, { mode: 0o700 });
+  let published = false;
+  try {
+    fs.writeFileSync(path.join(staging, owner), '', { mode: 0o600, flag: 'wx' });
+    // Publishing a prepopulated directory never exposes an empty fresh guard to a stale reclaimer.
+    for (let attempt = 0; attempt < 2; attempt += 1) {
+      try {
+        fs.renameSync(staging, admission);
+        published = true;
+        abandonedAdmissions.delete(admission);
+        return { admission, owner };
+      } catch {
+        if (attempt > 0 || !reclaimAdmission(admission, deadline, starts)) return null;
+      }
+    }
+    return null;
+  } finally {
+    if (!published) fs.rmSync(staging, { recursive: true, force: true });
+  }
+}
+
+export function retryLockAccess(access) {
+  for (let attempt = 0; ; attempt += 1) {
+    try {
+      return access();
+    } catch (error) {
+      if (attempt === 2 || !LOCK_ACCESS_ERRORS.has(error?.code)) throw error;
+      pause(LOCK_RETRY_MS);
+    }
+  }
+}
+
+function releaseAdmission({ admission, owner }) {
+  try {
+    retryLockAccess(() => fs.unlinkSync(path.join(admission, owner)));
+  } catch (cause) {
+    if (cause?.code !== 'ENOENT') {
+      throw Object.assign(new Error(`Could not release admission ${admission}`, { cause }), { code: 'ELOCKED' });
+    }
+  }
+  // The guard is released when its unique owner is gone. An empty directory can be reclaimed.
+  try { retryLockAccess(() => fs.rmdirSync(admission)); } catch {}
+}
+
+function abandonAdmission({ admission, owner }) {
+  // A separate marker can be written even when Windows denies access to the owner file.
+  try {
+    retryLockAccess(() => fs.writeFileSync(path.join(admission, ABANDONED_PREFIX + owner), '', { mode: 0o600, flag: 'wx' }));
+  } catch {
+    abandonedAdmissions.set(admission, owner);
+  }
+}
+
+// Every directory mutation uses this short, nonblocking admission. No work or await belongs inside it.
+export function admitted(lockDir, mutate, rollback, deadline = Date.now() + LOCK_WAIT_MS, starts = new Map()) {
+  const admission = enterAdmission(lockDir, deadline, starts);
+  if (!admission) return null;
+  let result;
+  try {
+    result = mutate();
+  } catch (error) {
+    try { releaseAdmission(admission); } catch { abandonAdmission(admission); }
+    throw error;
+  }
+  try {
+    releaseAdmission(admission);
+  } catch (error) {
+    // Publish only after rollback, so another process cannot enter while it runs.
+    try { rollback?.(); } catch (rollbackError) { error.rollbackError = rollbackError; }
+    abandonAdmission(admission);
+    if (rollback) throw error;
+  }
+  return result;
+}
diff --git a/lib/runtime/store/file-lock.js b/lib/runtime/store/file-lock.js
index f730098..e1d0888 100644
--- a/lib/runtime/store/file-lock.js
+++ b/lib/runtime/store/file-lock.js
@@ -5,32 +5,54 @@ import crypto from 'node:crypto';
 import fs from 'node:fs';
 import path from 'node:path';
 
-import { ownerIsGone } from '../process/owner.js';
+import { ownerIsGone, processStartTime, PROCESS_START_TIMEOUT_MS, PROCESS_START_TOLERANCE_MS } from '../process/owner.js';
+import { admitted, LOCK_ACCESS_ERRORS, LOCK_WAIT_MS, LOCK_RETRY_MS, pause, retryLockAccess } from './admission.js';
 
-const LOCK_WAIT_MS = 2_000;
-const LOCK_RETRY_MS = 20;
 const WORK_LOCK_STALE_MS = 2 * 60 * 1000;
+const WORK_OWNER_PROBE_MS = process.platform === 'win32' ? 5_000 : PROCESS_START_TIMEOUT_MS;
+const WORK_OWNER_WITH_START = /^\d+:[a-f0-9]+:([1-9]\d{12})$/;
 const CLAIM_STALE_MS = 60 * 1000;
+const abandonedWork = new Map();
+const abandonedState = new Map();
 
 export function readState(target, fallback) {
   try {
     const parsed = JSON.parse(fs.readFileSync(target, 'utf8'));
-    return parsed && typeof parsed === 'object' && Array.isArray(parsed) === Array.isArray(fallback) ? parsed : fallback;
-  } catch {
-    return fallback;
+    if (!parsed || typeof parsed !== 'object' || Array.isArray(parsed) !== Array.isArray(fallback)) throw Object.assign(new Error(`Invalid state shape in ${target}; repair it before retrying.`), { code: 'EINVALIDSTATE' });
+    return parsed;
+  } catch (error) {
+    if (error?.code === 'ENOENT') return fallback;
+    if (error instanceof SyntaxError) throw Object.assign(new Error(`Invalid JSON in ${target}; repair it before retrying.`), { code: 'EINVALIDSTATE', cause: error });
+    if (error?.code === 'EINVALIDSTATE') throw error;
+    throw Object.assign(new Error(`Could not read state in ${target}; check access and retry.`), { code: 'ESTATEIO', cause: error });
   }
 }
 
 export function writeState(target, value) {
   fs.mkdirSync(path.dirname(target), { recursive: true, mode: 0o700 });
   const temporary = `${target}.${process.pid}.${crypto.randomBytes(6).toString('hex')}.tmp`;
-  fs.writeFileSync(temporary, JSON.stringify(value), { mode: 0o600, flag: 'wx' });
+  const deadline = Date.now() + LOCK_WAIT_MS;
+  let descriptor;
   try {
-    fs.renameSync(temporary, target);
-  } catch (error) {
-    if (!['EEXIST', 'EPERM'].includes(error?.code)) throw error;
-    fs.rmSync(target, { force: true });
-    fs.renameSync(temporary, target);
+    descriptor = fs.openSync(temporary, 'wx', 0o600);
+    try {
+      fs.writeFileSync(descriptor, JSON.stringify(value));
+    } catch (error) {
+      try { fs.closeSync(descriptor); } catch {}
+      throw error;
+    }
+    fs.closeSync(descriptor);
+    for (;;) {
+      try {
+        fs.renameSync(temporary, target);
+        return;
+      } catch (error) {
+        if (!['EEXIST', 'EACCES', 'EPERM', 'EBUSY'].includes(error?.code) || Date.now() >= deadline) throw error;
+        pause(LOCK_RETRY_MS);
+      }
+    }
+  } finally {
+    if (descriptor !== undefined) removeState(temporary);
   }
 }
 
@@ -41,74 +63,81 @@ export function removeState(target) {
   }
 }
 
-// Atomics.wait is the only sleep that holds a synchronous read-modify-write together.
-function pause(ms) {
-  Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, ms);
-}
-
-function contentOf(file) {
+function stateOwnerOf(file) {
   try {
-    return fs.readFileSync(file, 'utf8');
-  } catch {
-    return null;
+    return retryLockAccess(() => fs.readFileSync(file, 'utf8'));
+  } catch (error) {
+    if (error?.code === 'ENOENT') return null;
+    throw error;
   }
 }
 
 const ownerPidOf = (token) => Number(token.split(':')[0]);
 
-// Another process may reclaim the same dead lock and a third take a fresh one before this removal happens, so the
-// lock is first moved aside, and only what was inspected as dead is deleted; anything else is put back.
-function reclaim(lockPath, deadToken) {
-  const aside = `${lockPath}.${crypto.randomBytes(6).toString('hex')}.reclaimed`;
-  try {
-    fs.renameSync(lockPath, aside);
-  } catch {
-    return;
-  }
-  if (contentOf(aside) !== deadToken) {
-    try {
-      fs.linkSync(aside, lockPath);
-    } catch {
-    }
-  }
-  removeState(aside);
-}
-
 function acquire(lockPath, deadline) {
   const token = `${process.pid}:${crypto.randomBytes(8).toString('hex')}`;
-  for (;;) {
+  const starts = new Map();
+  while (Date.now() < deadline) {
+    let acquired = false;
     try {
-      fs.mkdirSync(path.dirname(lockPath), { recursive: true, mode: 0o700 });
-      fs.writeFileSync(lockPath, token, { mode: 0o600, flag: 'wx' });
-      return token;
+      const result = admitted(lockPath, () => {
+        try {
+          fs.writeFileSync(lockPath, token, { mode: 0o600, flag: 'wx' });
+          acquired = true;
+          return token;
+        } catch (error) {
+          if (error?.code !== 'EEXIST') throw error;
+          // Admission serializes inspection and removal with every other acquisition.
+          const held = stateOwnerOf(lockPath);
+          if (held !== null && (abandonedState.get(lockPath) === held || ownerIsGone(ownerPidOf(held)))) {
+            fs.unlinkSync(lockPath);
+            abandonedState.delete(lockPath);
+          }
+          return null;
+        }
+      }, () => {
+        if (!acquired) return;
+        abandonedState.set(lockPath, token);
+        if (stateOwnerOf(lockPath) === token) retryLockAccess(() => fs.unlinkSync(lockPath));
+        abandonedState.delete(lockPath);
+      }, deadline, starts);
+      if (result) {
+        abandonedState.delete(lockPath);
+        return result;
+      }
     } catch (error) {
-      if (error?.code !== 'EEXIST') return null;
-      const held = contentOf(lockPath);
-      if (held !== null && ownerIsGone(ownerPidOf(held))) reclaim(lockPath, held);
-      else if (Date.now() >= deadline) return null;
-      else pause(LOCK_RETRY_MS);
+      if (!LOCK_ACCESS_ERRORS.has(error?.code)) throw error;
     }
+    if (Date.now() < deadline) pause(LOCK_RETRY_MS);
   }
+  return null;
 }
 
 function release(lockPath, token) {
-  if (contentOf(lockPath) === token) removeState(lockPath);
+  try {
+    if (stateOwnerOf(lockPath) === token) retryLockAccess(() => fs.unlinkSync(lockPath));
+  } catch {
+    // The state write is already settled. Keep the exact token for the next admitted acquire.
+    abandonedState.set(lockPath, token);
+  }
 }
 
 /**
  * Applies `mutate` to a state file under an exclusive lock, so hook processes sharing it cannot overwrite each
- * other's entries. A dead owner's lock is taken over; one that cannot be had within LOCK_WAIT_MS is given up on
- * and the update runs unguarded, which never blocks a session. `mutate` returns undefined to leave the file.
+ * other's entries. A dead owner's lock is taken over; failed admission never permits an unlocked write.
+ * `mutate` returns undefined to leave the file.
  */
 export function updateState(target, fallback, mutate) {
   const lockPath = `${target}.lock`;
-  const token = acquire(lockPath, Date.now() + LOCK_WAIT_MS);
+  let token, cause;
+  try { token = acquire(lockPath, Date.now() + LOCK_WAIT_MS); } catch (error) { cause = error; }
+  if (!token) throw Object.assign(new Error('State is busy or unavailable; no update was made', { cause }), { code: 'ELOCKED' });
   try {
     const next = mutate(readState(target, fallback));
     if (next !== undefined) writeState(target, next);
     return next;
   } finally {
-    if (token) release(lockPath, token);
+    release(lockPath, token);
   }
 }
 
@@ -180,38 +209,107 @@ function isStale(lockDir, now) {
   }
 }
 
+function workOwnerOf(lockDir) {
+  try {
+    return retryLockAccess(() => fs.readFileSync(path.join(lockDir, 'owner'), 'utf8'));
+  } catch (cause) {
+    if (cause?.code === 'ENOENT') return null;
+    throw Object.assign(new Error(`Could not read lock owner ${lockDir}`, { cause }), { code: 'ELOCKED' });
+  }
+}
+
 function ownsLock(lockDir, token) {
-  return contentOf(path.join(lockDir, 'owner')) === token;
+  return workOwnerOf(lockDir) === token;
 }
 
-function tryLock(lockDir, now) {
-  fs.mkdirSync(path.dirname(lockDir), { recursive: true, mode: 0o700 });
-  if (isStale(lockDir, now)) fs.rmSync(lockDir, { recursive: true, force: true });
-  try {
-    fs.mkdirSync(lockDir);
-  } catch {
-    return null;
-  }
-  const token = crypto.randomBytes(12).toString('hex');
-  fs.writeFileSync(path.join(lockDir, 'owner'), token);
-  return {
-    refresh: () => {
-      if (ownsLock(lockDir, token)) fs.utimesSync(lockDir, new Date(), new Date());
-    },
-    release: () => {
+async function releaseWorkLock(lockDir, token) {
+  const deadline = Date.now() + LOCK_WAIT_MS;
+  const starts = new Map();
+  for (;;) {
+    if (admitted(lockDir, () => {
       if (ownsLock(lockDir, token)) fs.rmSync(lockDir, { recursive: true, force: true });
-    },
-  };
+      return true;
+    }, null, deadline, starts)) return;
+    if (Date.now() >= deadline) throw Object.assign(new Error(`Could not release lock ${lockDir}`), { code: 'ELOCKED' });
+    await new Promise((resolve) => setTimeout(resolve, LOCK_RETRY_MS));
+  }
 }
 
-/** Runs slow `work` while holding a lock directory no other process gets meanwhile; false when it is held. */
+function tryLock(lockDir, now) {
+  let token, suspect, reusedToken;
+  const take = () => admitted(lockDir, () => {
+    const stale = isStale(lockDir, now);
+    if (abandonedWork.has(lockDir) || stale) {
+      let held;
+      try { held = workOwnerOf(lockDir); } catch { return null; }
+      if (held !== abandonedWork.get(lockDir)) {
+        if (!stale) return null;
+        if (held !== reusedToken && /^\d+:/.test(held ?? '')) {
+          const pid = ownerPidOf(held);
+          if (!ownerIsGone(pid)) {
+            const birth = WORK_OWNER_WITH_START.exec(held)?.[1];
+            if (birth) suspect = { token: held, pid, birth: Number(birth) };
+            return null;
+          }
+        }
+      }
+      fs.rmSync(lockDir, { recursive: true, force: true });
+      abandonedWork.delete(lockDir);
+    }
+    try {
+      fs.mkdirSync(lockDir);
+    } catch {
+      return null;
+    }
+    token = `${process.pid}:${crypto.randomBytes(12).toString('hex')}:${processStartTime(process.pid)}`;
+    fs.writeFileSync(path.join(lockDir, 'owner'), token);
+    return {
+      refresh: () => {
+        try {
+          return admitted(lockDir, () => {
+            if (ownsLock(lockDir, token)) fs.utimesSync(lockDir, new Date(), new Date());
+          });
+        } catch {
+          // Timer-driven refresh is best effort; a live pid stays protected and final release still surfaces errors.
+          return null;
+        }
+      },
+      release: async () => {
+        try { await releaseWorkLock(lockDir, token); }
+        catch (error) { abandonedWork.set(lockDir, token); throw error; }
+      },
+    };
+  }, () => {
+    if (!token) return;
+    abandonedWork.set(lockDir, token);
+    if (ownsLock(lockDir, token)) fs.rmSync(lockDir, { recursive: true, force: true });
+    abandonedWork.delete(lockDir);
+  });
+  const lock = take();
+  if (lock || !suspect) return lock;
+  // A foreign lookup can be slow. Release admission, then recheck staleness and the exact token.
+  const actualBirth = processStartTime(suspect.pid, WORK_OWNER_PROBE_MS);
+  if (!actualBirth || actualBirth <= suspect.birth + PROCESS_START_TOLERANCE_MS) return null;
+  reusedToken = suspect.token;
+  return take();
+}
+
+/** Runs slow `work` under one owner's lock; dead or legacy directories recover after their lease expires. */
 export async function withLock(lockDir, work, now = Date.now()) {
   const lock = tryLock(lockDir, now);
   if (!lock) return false;
+  let workFailed = false;
   try {
     await work(lock);
     return true;
+  } catch (error) {
+    workFailed = true;
+    throw error;
   } finally {
-    lock.release();
+    try {
+      await lock.release();
+    } catch (error) {
+      if (!workFailed) throw error;
+    }
   }
 }
diff --git a/test/channels/hook.test.js b/test/channels/hook.test.js
index ac20184..29cf3b2 100644
--- a/test/channels/hook.test.js
+++ b/test/channels/hook.test.js
@@ -1,4 +1,7 @@
 import assert from 'node:assert/strict';
+import { createHash } from 'node:crypto';
+import { mkdirSync, readFileSync, writeFileSync } from 'node:fs';
+import { join } from 'node:path';
 import { Readable } from 'node:stream';
 import { after, before, test } from 'node:test';
 
@@ -19,12 +22,12 @@ let proxy;
 before(async () => { proxy = await fakeProxy(); });
 after(() => proxy.close());
 
-function invoke(hostEvent, payload, { env = proxy.env, raw } = {}) {
+function invoke(hostEvent, payload, { env = proxy.env, raw, stderr } = {}) {
   return new Promise((resolve) => {
     let written = '';
     const stdout = { write: (text, done) => { written += text; done(); } };
     const stdin = Readable.from([raw ?? JSON.stringify(payload)]);
-    runHookProcess('testhook', hostEvent, { stdin, stdout, env, exit: () => resolve(JSON.parse(written)), profileOf: async () => profile });
+    runHookProcess('testhook', hostEvent, { stdin, stdout, stderr, env, exit: () => resolve(JSON.parse(written)), profileOf: async () => profile });
   });
 }
 
@@ -44,3 +47,18 @@ test('inside the plugin\'s own headless run, a hook answers the empty reply and
 test('a payload that is not JSON still gets an answer', async () => {
   assert.deepEqual(await invoke('UserPromptSubmit', null, { raw: '{not json' }), { empty: 'UserPromptSubmit' });
 });
+
+test('corrupt hook state stays intact and emits a diagnostic alongside the neutral reply', async () => {
+  const dir = profile.stateDir(proxy.env);
+  const key = createHash('sha256').update('s-corrupt').digest('hex').slice(0, 16);
+  const file = join(dir, `testhook-session-${key}.json`);
+  mkdirSync(dir, { recursive: true });
+  writeFileSync(file, '{"private text":');
+  let diagnostic = '';
+  const stderr = { write: (text, done) => { diagnostic += text; done(); } };
+
+  assert.deepEqual(await invoke('UserPromptSubmit', { session_id: 's-corrupt', cwd: '/proj', prompt: PROMPT }, { stderr }), { empty: 'UserPromptSubmit' });
+  assert.match(diagnostic, /EINVALIDSTATE/);
+  assert.doesNotMatch(diagnostic, /private text/);
+  assert.equal(readFileSync(file, 'utf8'), '{"private text":');
+});
diff --git a/test/helpers/lock-contenders.js b/test/helpers/lock-contenders.js
new file mode 100644
index 0000000..1e412a7
--- /dev/null
+++ b/test/helpers/lock-contenders.js
@@ -0,0 +1,39 @@
+import { spawn } from 'node:child_process';
+import { rmSync, writeFileSync } from 'node:fs';
+import { join } from 'node:path';
+
+export async function until(check) {
+  const deadline = Date.now() + 10_000;
+  while (!check()) {
+    if (Date.now() >= deadline) throw new Error('timed out waiting for a lock contender');
+    await new Promise((resolve) => setTimeout(resolve, 10));
+  }
+}
+
+export function lockContenders(t, dir, script, args, cleanupMarkers) {
+  const children = [];
+  const results = () => Promise.all(children);
+  t.after(async () => {
+    try {
+      for (const name of cleanupMarkers) writeFileSync(join(dir, name), '');
+    } finally {
+      await results();
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+  return {
+    run(role) {
+      const child = spawn(process.execPath, ['--input-type=module', '-e', script, ...args, role], { windowsHide: true });
+      let stderr = '';
+      const watchdog = setTimeout(() => child.kill('SIGKILL'), 15_000);
+      child.stderr.on('data', (data) => { stderr += data; });
+      child.on('error', (error) => { stderr += String(error); });
+      children.push(new Promise((resolve) => child.on('close', (code) => {
+        clearTimeout(watchdog);
+        resolve({ code, stderr });
+      })));
+      return child;
+    },
+    results,
+  };
+}
diff --git a/test/runtime/file-lock.test.js b/test/runtime/file-lock.test.js
new file mode 100644
index 0000000..0e40c47
--- /dev/null
+++ b/test/runtime/file-lock.test.js
@@ -0,0 +1,621 @@
+import assert from 'node:assert/strict';
+import childProcess, { spawnSync } from 'node:child_process';
+import { syncBuiltinESMExports } from 'node:module';
+import fs from 'node:fs';
+import { existsSync, mkdirSync, mkdtempSync, readFileSync, readdirSync, rmSync, rmdirSync, statSync, unlinkSync, utimesSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { dirname, join } from 'node:path';
+import { test } from 'node:test';
+
+import { processStartTime } from '../../lib/runtime/process/owner.js';
+import { withLock } from '../../lib/runtime/store/file-lock.js';
+import { lockContenders, until } from '../helpers/lock-contenders.js';
+
+const RUNTIME = new URL('../../lib/runtime/store/file-lock.js', import.meta.url).href;
+const STALE = new Date(0);
+
+function scratch(t, token = null) {
+  const dir = mkdtempSync(join(tmpdir(), 'evolver-work-lock-'));
+  const lock = join(dir, 'work.lock');
+  if (token !== null) {
+    mkdirSync(lock);
+    writeFileSync(join(lock, 'owner'), token);
+  }
+  t.after(() => rmSync(dir, { recursive: true, force: true }));
+  return { dir, lock };
+}
+
+function holdAdmission(lock, pid = process.pid) {
+  const admission = `${lock}.admission`;
+  const owner = join(admission, `owner-${pid}-deadbeef`);
+  mkdirSync(admission);
+  writeFileSync(owner, 'fixture owner');
+  return { admission, owner, release: () => { unlinkSync(owner); rmdirSync(admission); } };
+}
+
+test('taking an uncontended lock does not launch a process lookup', async (t) => {
+  const { lock } = scratch(t);
+  let launches = 0;
+  const spawn = childProcess.spawnSync;
+  childProcess.spawnSync = (...args) => { launches += 1; return spawn(...args); };
+  syncBuiltinESMExports();
+  try {
+    assert.equal(await withLock(lock, () => {}), true);
+  } finally {
+    childProcess.spawnSync = spawn;
+    syncBuiltinESMExports();
+  }
+  assert.equal(launches, 0);
+});
+
+test('stale legacy and dead-owner directories recover, but a fresh legacy lock stays held', async (t) => {
+  const { lock } = scratch(t, 'legacy token');
+  let entered = 0;
+  assert.equal(await withLock(lock, async () => { entered += 1; }), false);
+  utimesSync(lock, STALE, STALE);
+  assert.equal(await withLock(lock, async () => { entered += 1; }), true);
+  const exited = spawnSync(process.execPath, ['-e', '']);
+  assert.equal(exited.status, 0);
+  mkdirSync(lock);
+  writeFileSync(join(lock, 'owner'), `${exited.pid}:deadbeef`);
+  holdAdmission(lock, exited.pid);
+  utimesSync(lock, STALE, STALE);
+  assert.equal(await withLock(lock, async () => { entered += 1; }), true);
+  assert.equal(entered, 2);
+  assert.equal(existsSync(lock), false);
+  assert.equal(existsSync(`${lock}.admission`), false);
+});
+
+test('an old mtime never evicts a live owner, which can refresh and release its lock', async (t) => {
+  const { dir, lock } = scratch(t);
+  assert.equal(await withLock(lock, async (held) => {
+    utimesSync(lock, STALE, STALE);
+    let entered = false;
+    assert.equal(await withLock(lock, async () => { entered = true; }), false);
+    assert.equal(entered, false);
+    held.refresh();
+    assert.ok(statSync(lock).mtimeMs > STALE.getTime());
+  }), true);
+  assert.equal(existsSync(lock), false);
+  assert.deepEqual(readdirSync(dir), []);
+});
+
+test('only a stale worker lock with proof of PID reuse can displace a live PID', async (t) => {
+  const birth = processStartTime(process.pid);
+  const prefix = `${process.pid}:deadbeef`;
+  for (const token of [prefix, `${prefix}:${birth}`, `${prefix}:${birth - 1_000}`, `${prefix}:invalid`]) {
+    const { lock } = scratch(t, token);
+    utimesSync(lock, STALE, STALE);
+    assert.equal(await withLock(lock, () => assert.fail('owner identity is not disproved')), false);
+    assert.equal(readFileSync(join(lock, 'owner'), 'utf8'), token);
+  }
+
+  const { dir, lock } = scratch(t, `${prefix}:${birth - 10_000}`);
+  assert.equal(await withLock(lock, () => assert.fail('the lease is still fresh')), false);
+  utimesSync(lock, STALE, STALE);
+  assert.equal(await withLock(lock, () => {
+    assert.match(readFileSync(join(lock, 'owner'), 'utf8'), new RegExp(`^${process.pid}:[a-f0-9]+:${birth}$`));
+  }), true);
+  assert.deepEqual(readdirSync(dir), []);
+});
+
+test('a foreign PID probe leaves admission free and cannot remove a different worker token', async (t) => {
+  const pid = process.ppid;
+  const birth = Date.now();
+  const old = `${pid}:aaaa:${birth - 10_000}`;
+  const replacement = `${pid}:bbbb:${birth - 10_000}`;
+  const { lock } = scratch(t, old);
+  utimesSync(lock, STALE, STALE);
+  const spawn = childProcess.spawnSync;
+  let probes = 0;
+  childProcess.spawnSync = () => {
+    probes += 1;
+    assert.equal(existsSync(`${lock}.admission`), false);
+    writeFileSync(join(lock, 'owner'), replacement);
+    const stdout = process.platform === 'win32' ? String(birth) : new Date(birth).toUTCString().replace(/ GMT$/, '');
+    return { status: 0, stdout };
+  };
+  syncBuiltinESMExports();
+  try {
+    assert.equal(await withLock(lock, () => assert.fail('the observed token was replaced')), false);
+  } finally {
+    childProcess.spawnSync = spawn;
+    syncBuiltinESMExports();
+  }
+  assert.equal(probes, 1);
+  assert.equal(readFileSync(join(lock, 'owner'), 'utf8'), replacement);
+});
+
+test('a transient owner read error never turns a stale live lock into a legacy lock', async (t) => {
+  for (const code of ['EBUSY', 'EPERM', 'EACCES', 'EIO']) {
+    const { dir, lock } = scratch(t);
+    assert.equal(await withLock(lock, async () => {
+      const owner = join(lock, 'owner');
+      const token = readFileSync(owner, 'utf8');
+      utimesSync(lock, STALE, STALE);
+      const read = fs.readFileSync;
+      let refused = false;
+      let entered = false;
+      fs.readFileSync = (target, ...args) => {
+        if (target === owner && !refused) {
+          refused = true;
+          throw Object.assign(new Error('transient owner read refusal'), { code });
+        }
+        return read(target, ...args);
+      };
+      try {
+        assert.equal(await withLock(lock, async () => { entered = true; }), false, code);
+      } finally {
+        fs.readFileSync = read;
+      }
+      assert.equal(refused, true);
+      assert.equal(entered, false, code);
+      assert.equal(readFileSync(owner, 'utf8'), token, 'the live owner stays intact');
+    }), true);
+    assert.deepEqual(readdirSync(dir), [], 'the original owner still releases normally');
+  }
+});
+
+test('a stale directory left before its owner was written is recoverable', async (t) => {
+  const { dir, lock } = scratch(t);
+  mkdirSync(lock);
+  utimesSync(lock, STALE, STALE);
+  let entered = false;
+  assert.equal(await withLock(lock, async () => { entered = true; }), true);
+  assert.equal(entered, true);
+  assert.deepEqual(readdirSync(dir), []);
+});
+
+test('release retries a transient owner read failure and really removes its lock', async (t) => {
+  const { dir, lock } = scratch(t);
+  const owner = join(lock, 'owner');
+  const read = fs.readFileSync;
+  let refused = false;
+  try {
+    assert.equal(await withLock(lock, async () => {
+      fs.readFileSync = (target, ...args) => {
+        if (target === owner && !refused) {
+          refused = true;
+          throw Object.assign(new Error('transient read refusal'), { code: 'EBUSY' });
+        }
+        return read(target, ...args);
+      };
+    }), true);
+  } finally {
+    fs.readFileSync = read;
+  }
+  assert.equal(refused, true);
+  assert.deepEqual(readdirSync(dir), []);
+});
+
+test('release refuses an unreadable owner explicitly without claiming cleanup', async (t) => {
+  for (const code of ['EBUSY', 'EPERM', 'EIO']) {
+    const { lock } = scratch(t);
+    const owner = join(lock, 'owner');
+    const read = fs.readFileSync;
+    let token;
+    try {
+      await assert.rejects(withLock(lock, async () => {
+        token = read(owner, 'utf8');
+        fs.readFileSync = (target, ...args) => {
+          if (target === owner) throw Object.assign(new Error('persistent read refusal'), { code });
+          return read(target, ...args);
+        };
+      }), { code: 'ELOCKED' });
+    } finally {
+      fs.readFileSync = read;
+    }
+    assert.equal(readFileSync(owner, 'utf8'), token, code);
+    assert.equal(await withLock(lock, () => {}), true, 'known abandoned work is recoverable after access returns');
+  }
+});
+
+test('a stale foreign admission is probed once within the release deadline', async (t) => {
+  const { lock } = scratch(t);
+  const spawn = childProcess.spawnSync;
+  const timeouts = [];
+  try {
+    await assert.rejects(withLock(lock, () => {
+      const guard = holdAdmission(lock, process.ppid);
+      unlinkSync(guard.owner);
+      writeFileSync(join(guard.admission, `owner-${process.ppid}-a1-${Date.now() - 30_000}`), '');
+      utimesSync(guard.admission, STALE, STALE);
+      childProcess.spawnSync = (_command, _args, { timeout }) => {
+        timeouts.push(timeout);
+        return { status: null, stdout: '' };
+      };
+      syncBuiltinESMExports();
+    }), { code: 'ELOCKED' });
+  } finally {
+    childProcess.spawnSync = spawn;
+    syncBuiltinESMExports();
+  }
+  assert.equal(timeouts.length, 1);
+  assert.ok(timeouts[0] > 0 && timeouts[0] <= 1_000);
+  assert.equal(existsSync(lock), true, 'an inconclusive identity probe does not reclaim a live guard');
+});
+
+test('a release failure preserves the original work error', async (t) => {
+  const { lock } = scratch(t);
+  const read = fs.readFileSync;
+  const workError = new Error('work failed');
+  try {
+    await assert.rejects(withLock(lock, async () => {
+      fs.readFileSync = (file, ...args) => {
+        if (String(file) === join(lock, 'owner')) throw Object.assign(new Error('unreadable owner'), { code: 'EIO' });
+        return read(file, ...args);
+      };
+      throw workError;
+    }), (error) => error === workError);
+  } finally {
+    fs.readFileSync = read;
+  }
+});
+
+test('refresh leaves an unreadable live lock untouched without crashing its timer caller', async (t) => {
+  const { dir, lock } = scratch(t);
+  assert.equal(await withLock(lock, async (held) => {
+    const owner = join(lock, 'owner');
+    utimesSync(lock, STALE, STALE);
+    const read = fs.readFileSync;
+    fs.readFileSync = (target, ...args) => {
+      if (target === owner) throw Object.assign(new Error('unreadable owner'), { code: 'EPERM' });
+      return read(target, ...args);
+    };
+    try {
+      assert.doesNotThrow(() => held.refresh());
+      assert.equal(statSync(lock).mtimeMs, STALE.getTime());
+    } finally {
+      fs.readFileSync = read;
+    }
+  }), true);
+  assert.deepEqual(readdirSync(dir), []);
+});
+
+test('refresh tolerates file access refusals and the next refresh and release still work', async (t) => {
+  for (const method of ['utimesSync', 'mkdirSync', 'readFileSync']) {
+    const { dir, lock } = scratch(t);
+    assert.equal(await withLock(lock, async (held) => {
+      utimesSync(lock, STALE, STALE);
+      const access = fs[method];
+      let refused = false;
+      fs[method] = (target, ...args) => {
+        const matches = method === 'mkdirSync' ? String(target).startsWith(`${lock}.admission.`)
+          : target === (method === 'readFileSync' ? join(lock, 'owner') : lock);
+        if (matches && !refused) {
+          refused = true;
+          throw Object.assign(new Error('transient access refusal'), { code: 'EPERM' });
+        }
+        return access(target, ...args);
+      };
+      try {
+        assert.doesNotThrow(() => held.refresh(), method);
+        if (method !== 'readFileSync') assert.equal(statSync(lock).mtimeMs, STALE.getTime());
+      } finally {
+        fs[method] = access;
+      }
+      assert.equal(refused, true, method);
+      held.refresh();
+      assert.ok(statSync(lock).mtimeMs > STALE.getTime());
+    }), true);
+    assert.deepEqual(readdirSync(dir), []);
+  }
+});
+
+test('release does not delete a missing or different owner', async (t) => {
+  for (const replacement of [null, `${process.pid}:another-owner`]) {
+    const { lock } = scratch(t);
+    assert.equal(await withLock(lock, async () => {
+      if (replacement === null) unlinkSync(join(lock, 'owner'));
+      else writeFileSync(join(lock, 'owner'), replacement);
+    }), true);
+    assert.equal(existsSync(lock), true);
+    assert.equal(existsSync(join(lock, 'owner')), replacement !== null);
+    if (replacement !== null) assert.equal(readFileSync(join(lock, 'owner'), 'utf8'), replacement);
+  }
+});
+
+test('release waits asynchronously for transient admission contention', async (t) => {
+  const { lock } = scratch(t);
+  let guard;
+  let finished = false;
+  let refreshedAt;
+  const running = withLock(lock, async (held) => {
+    guard = holdAdmission(lock);
+    utimesSync(lock, STALE, STALE);
+    held.refresh();
+    refreshedAt = statSync(lock).mtimeMs;
+  }).then(() => { finished = true; });
+  try {
+    await new Promise((resolve) => setImmediate(resolve));
+    assert.equal(refreshedAt, STALE.getTime(), 'refresh also respects admission');
+    assert.equal(finished, false);
+    assert.equal(existsSync(lock), true);
+  } finally {
+    guard.release();
+    await running;
+  }
+  assert.equal(existsSync(lock), false);
+});
+
+test('release refuses bounded admission contention without deleting the held directory', async (t) => {
+  const { lock } = scratch(t);
+  let guard;
+  const started = Date.now();
+  await assert.rejects(withLock(lock, async () => {
+    guard = holdAdmission(lock);
+  }), { code: 'ELOCKED' });
+  assert.ok(Date.now() - started < 5_000, 'cleanup cannot wait forever');
+  assert.equal(existsSync(lock), true);
+  assert.equal(readFileSync(guard.owner, 'utf8'), 'fixture owner');
+});
+
+test('an admission cleanup failure is surfaced instead of silently orphaning a live guard', async (t) => {
+  const { lock } = scratch(t);
+  const admission = `${lock}.admission`;
+  const unlink = fs.unlinkSync;
+  fs.unlinkSync = (target) => {
+    if (dirname(target) === admission) throw Object.assign(new Error('refused by fixture'), { code: 'EPERM' });
+    return unlink(target);
+  };
+  let entered = false;
+  try {
+    await assert.rejects(withLock(lock, async () => { entered = true; }), { code: 'ELOCKED' });
+  } finally {
+    fs.unlinkSync = unlink;
+  }
+  assert.equal(entered, false);
+  assert.equal(existsSync(lock), false, 'the failed acquire must not leave a work lock');
+  assert.ok(readdirSync(admission).some((name) => name.startsWith(`owner-${process.pid}-`)));
+  assert.equal(await withLock(lock, async () => { entered = true; }), true);
+  assert.equal(entered, true);
+});
+
+test('another process reclaims a failed admission release while the original owner is alive', async (t) => {
+  const { dir, lock } = scratch(t);
+  const admission = `${lock}.admission`;
+  const unlink = fs.unlinkSync;
+  try {
+    assert.equal(await withLock(lock, () => {
+      fs.unlinkSync = (target) => {
+        if (dirname(target) === admission) throw Object.assign(new Error('fixture cleanup refusal'), { code: 'EPERM' });
+        return unlink(target);
+      };
+    }), true);
+  } finally {
+    fs.unlinkSync = unlink;
+  }
+  assert.equal(existsSync(lock), false);
+  assert.ok(readdirSync(admission).some((name) => name.startsWith(`owner-${process.pid}-`)));
+
+  const other = spawnSync(process.execPath, ['--input-type=module', '-e', `
+    import assert from 'node:assert/strict';
+    const [runtime, lock, owner] = process.argv.slice(1);
+    const { withLock } = await import(runtime);
+    process.kill(Number(owner), 0);
+    assert.equal(await withLock(lock, () => {}), true, 'the stranded admission can be reclaimed');
+  `, RUNTIME, lock, String(process.pid)], { encoding: 'utf8', timeout: 10_000, windowsHide: true });
+  assert.ifError(other.error);
+  assert.equal(other.status, 0, other.stderr);
+  assert.deepEqual(readdirSync(dir), []);
+});
+
+test('admission recovery resumes after the abandoned owner has already been removed', async (t) => {
+  const { dir, lock } = scratch(t);
+  const guard = holdAdmission(lock);
+  const marker = join(guard.admission, `abandoned-owner-${process.pid}-abcd`);
+  writeFileSync(marker, '');
+  assert.equal(await withLock(lock, () => assert.fail('a different owner is still active')), false);
+  assert.equal(existsSync(guard.owner), true);
+
+  unlinkSync(guard.owner);
+  assert.equal(await withLock(lock, () => {}), true);
+  assert.deepEqual(readdirSync(dir), []);
+});
+
+test('a transient admission cleanup refusal retries and leaves later work unblocked', async (t) => {
+  const { dir, lock } = scratch(t);
+  const unlink = fs.unlinkSync;
+  let refused = false;
+  fs.unlinkSync = (target) => {
+    if (dirname(target) === `${lock}.admission` && !refused) {
+      refused = true;
+      throw Object.assign(new Error('temporarily refused by fixture'), { code: 'EPERM' });
+    }
+    return unlink(target);
+  };
+  let entered = 0;
+  try {
+    assert.equal(await withLock(lock, async () => { entered += 1; }), true);
+  } finally {
+    fs.unlinkSync = unlink;
+  }
+  assert.equal(refused, true);
+  assert.equal(entered, 1);
+  assert.deepEqual(readdirSync(dir), []);
+  assert.equal(await withLock(lock, async () => { entered += 1; }), true);
+  assert.equal(entered, 2);
+  assert.deepEqual(readdirSync(dir), []);
+});
+
+test('an old admission stays held by the same process, but a reused PID can be recovered', async (t) => {
+  const { lock } = scratch(t);
+  const admission = `${lock}.admission`;
+  const unlink = fs.unlinkSync;
+  fs.unlinkSync = (target) => {
+    if (dirname(target) === admission) throw Object.assign(new Error('refused by fixture'), { code: 'EPERM' });
+    return unlink(target);
+  };
+  try {
+    await assert.rejects(withLock(lock, async () => assert.fail()), { code: 'ELOCKED' });
+  } finally {
+    fs.unlinkSync = unlink;
+  }
+  const original = readdirSync(admission).find((name) => name.startsWith(`owner-${process.pid}-`));
+  const birth = original.split('-')[3];
+  assert.match(birth ?? '', /^\d{13}$/);
+  const replaceOwner = (name) => {
+    for (const name of readdirSync(admission)) unlinkSync(join(admission, name));
+    writeFileSync(join(admission, name), '');
+    utimesSync(admission, STALE, STALE);
+  };
+  replaceOwner(`owner-${process.pid}-a1-${birth}`);
+  assert.equal(await withLock(lock, async () => assert.fail('same process still owns the guard')), false);
+  const differentBirth = Number(birth) - 10_000;
+  replaceOwner(`owner-${process.pid}-a2-${differentBirth}`);
+  assert.equal(await withLock(lock, async () => {}), true);
+});
+
+test('busy and unrecognised admission guards stay intact and leave no staging directories', async (t) => {
+  const { dir, lock } = scratch(t);
+  const guard = holdAdmission(lock);
+  assert.equal(await withLock(lock, async () => assert.fail('a live guard is held')), false);
+  assert.deepEqual(readdirSync(dir), ['work.lock.admission']);
+  writeFileSync(join(guard.admission, 'unexpected'), 'keep');
+  assert.equal(await withLock(lock, async () => assert.fail('multiple owners are unknown')), false);
+  assert.equal(readdirSync(guard.admission).length, 2);
+  unlinkSync(guard.owner);
+  assert.equal(await withLock(lock, async () => assert.fail('an unknown owner is held')), false);
+  assert.equal(readFileSync(join(guard.admission, 'unexpected'), 'utf8'), 'keep');
+  assert.deepEqual(readdirSync(dir), ['work.lock.admission']);
+});
+
+test('two contenders cannot replace the fresh lock while another reclaims a stale directory', async (t) => {
+  const dir = mkdtempSync(join(tmpdir(), 'evolver-work-lock-'));
+  const lock = join(dir, 'work.lock');
+  mkdirSync(lock);
+  writeFileSync(join(lock, 'owner'), 'legacy token');
+  utimesSync(lock, STALE, STALE);
+  const marker = (name) => join(dir, name);
+  const script = `
+    import fs from 'node:fs';
+    import path from 'node:path';
+    import { withLock } from ${JSON.stringify(RUNTIME)};
+    const [dir, lock, role] = process.argv.slice(1);
+    const marker = (name) => path.join(dir, name);
+    const wait = async (name) => {
+      const deadline = Date.now() + 10000;
+      while (!fs.existsSync(marker(name))) {
+        if (Date.now() > deadline) throw new Error('timed out waiting for ' + name);
+        await new Promise((resolve) => setTimeout(resolve, 10));
+      }
+    };
+    if (role === 'A') {
+      const stat = fs.statSync;
+      let paused = false;
+      fs.statSync = (file, ...args) => {
+        const result = stat(file, ...args);
+        if (file === lock && !paused) {
+          paused = true;
+          fs.writeFileSync(marker('stale-read'), '');
+          const deadline = Date.now() + 10000;
+          while (!fs.existsSync(marker('resume'))) {
+            if (Date.now() > deadline) throw new Error('timed out waiting to resume');
+            Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, 10);
+          }
+        }
+        return result;
+      };
+    }
+    const result = await withLock(lock, async () => {
+      fs.writeFileSync(marker(role + '-entered'), '');
+      await wait('release');
+    });
+    fs.writeFileSync(marker(role + '-result'), JSON.stringify(result));
+  `;
+  const { run, results } = lockContenders(t, dir, script, [dir, lock], ['resume', 'release']);
+  run('A');
+  await until(() => existsSync(marker('stale-read')));
+  run('B');
+  run('C');
+  await until(() => ['B', 'C'].every((role) => existsSync(marker(role + '-entered')) || existsSync(marker(role + '-result'))));
+  writeFileSync(marker('resume'), '');
+  await until(() => existsSync(marker('A-entered')));
+  const entered = ['A', 'B', 'C'].filter((role) => existsSync(marker(role + '-entered')));
+  writeFileSync(marker('release'), '');
+  const outcomes = await results();
+  assert.ok(outcomes.every(({ code }) => code === 0), JSON.stringify(outcomes));
+  assert.deepEqual(entered, ['A'], 'only the process reclaiming the inspected stale directory may enter');
+  assert.deepEqual(['A', 'B', 'C'].map((role) => JSON.parse(readFileSync(marker(role + '-result'), 'utf8'))), [true, false, false]);
+});
+
+test('a delayed dead-admission reclaimer cannot unlink the fresh owner another contender published', async (t) => {
+  const dir = mkdtempSync(join(tmpdir(), 'evolver-admission-race-'));
+  const lock = join(dir, 'work.lock');
+  mkdirSync(lock);
+  writeFileSync(join(lock, 'owner'), 'legacy token');
+  utimesSync(lock, STALE, STALE);
+  const exited = spawnSync(process.execPath, ['-e', '']);
+  assert.equal(exited.status, 0);
+  holdAdmission(lock, exited.pid);
+  const marker = (name) => join(dir, name);
+  const script = `
+    import fs from 'node:fs';
+    import path from 'node:path';
+    import { withLock } from ${JSON.stringify(RUNTIME)};
+    const [dir, lock, role] = process.argv.slice(1);
+    const marker = (name) => path.join(dir, name);
+    const waitSync = (name) => {
+      const deadline = Date.now() + 10000;
+      while (!fs.existsSync(marker(name))) {
+        if (Date.now() > deadline) throw new Error('timed out waiting for ' + name);
+        Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, 10);
+      }
+    };
+    if (role === 'A') {
+      const list = fs.readdirSync;
+      let paused = false;
+      fs.readdirSync = (file, ...args) => {
+        const result = list(file, ...args);
+        if (file === lock + '.admission' && !paused) {
+          paused = true;
+          fs.writeFileSync(marker('dead-inspected'), '');
+          waitSync('A-resume');
+        }
+        return result;
+      };
+    }
+    if (role === 'B') {
+      const rename = fs.renameSync;
+      let paused = false;
+      fs.renameSync = (source, target) => {
+        const result = rename(source, target);
+        if (target === lock + '.admission' && !paused) {
+          paused = true;
+          fs.writeFileSync(marker('fresh-held'), '');
+          waitSync('B-resume');
+        }
+        return result;
+      };
+    }
+    const result = await withLock(lock, async () => {
+      fs.writeFileSync(marker(role + '-entered'), '');
+      const deadline = Date.now() + 10000;
+      while (!fs.existsSync(marker('release'))) {
+        if (Date.now() > deadline) throw new Error('timed out waiting to release');
+        await new Promise((resolve) => setTimeout(resolve, 10));
+      }
+    });
+    fs.writeFileSync(marker(role + '-result'), JSON.stringify(result));
+  `;
+  const { run, results } = lockContenders(t, dir, script, [dir, lock], ['A-resume', 'B-resume', 'release']);
+  run('A');
+  await until(() => existsSync(marker('dead-inspected')));
+  const fresh = run('B');
+  await until(() => existsSync(marker('fresh-held')));
+  run('C');
+  await until(() => existsSync(marker('C-result')) || existsSync(marker('C-entered')));
+  assert.equal(existsSync(marker('C-entered')), false, 'publishing a fresh guard never exposes an empty directory');
+  writeFileSync(marker('A-resume'), '');
+  await until(() => existsSync(marker('A-result')) || existsSync(marker('A-entered')));
+  assert.equal(existsSync(marker('A-entered')), false, 'the delayed reclaimer cannot displace the fresh guard');
+  assert.equal(readdirSync(`${lock}.admission`).length, 1);
+  assert.match(readdirSync(`${lock}.admission`)[0], new RegExp(`^owner-${fresh.pid}-`));
+  writeFileSync(marker('B-resume'), '');
+  await until(() => existsSync(marker('B-entered')));
+  assert.deepEqual(['A', 'B', 'C'].filter((role) => existsSync(marker(role + '-entered'))), ['B']);
+  writeFileSync(marker('release'), '');
+  const outcomes = await results();
+  assert.ok(outcomes.every(({ code }) => code === 0), JSON.stringify(outcomes));
+  assert.deepEqual(['A', 'B', 'C'].map((role) => JSON.parse(readFileSync(marker(role + '-result'), 'utf8'))), [false, true, false]);
+});
diff --git a/test/runtime/file-stores.test.js b/test/runtime/file-stores.test.js
index 7f52193..0f56c33 100644
--- a/test/runtime/file-stores.test.js
+++ b/test/runtime/file-stores.test.js
@@ -1,11 +1,13 @@
 import assert from 'node:assert/strict';
 import { spawn } from 'node:child_process';
-import { mkdtempSync, readdirSync, rmSync, writeFileSync } from 'node:fs';
+import crypto from 'node:crypto';
+import { existsSync, mkdtempSync, readdirSync, rmSync, writeFileSync } from 'node:fs';
+import fs from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import { after, test } from 'node:test';
 
-import { readState } from '../../lib/runtime/store/file-lock.js';
+import { readState, updateState } from '../../lib/runtime/store/file-lock.js';
 import { fileStoresOf } from '../../lib/runtime/store/file-stores.js';
 
 const dirs = [];
@@ -42,6 +44,232 @@ test('the outbox takes queued reports and keeps what each delivery attempt settl
   assert.equal(seen[0].sentAt, 5);
 });
 
+test('an unreadable or invalid draft table is preserved and never passed to an update as empty', async t => {
+  for (const failure of ['EBUSY', 'EACCES', 'invalid-json', 'wrong-shape']) {
+    const { dir, stores: cursor } = stores();
+    const target = join(dir, 'cursor-distill-drafts.json');
+    const original = failure === 'invalid-json' ? '[unfinished' : failure === 'wrong-shape' ? '{}' : '[{"title":"keep this draft"}]';
+    writeFileSync(target, original);
+    const originalRead = fs.readFileSync;
+    t.mock.method(fs, 'readFileSync', (file, ...args) => {
+      if (file === target && ['EBUSY', 'EACCES'].includes(failure)) throw Object.assign(new Error('fixture read denial'), { code: failure });
+      return originalRead(file, ...args);
+    });
+    let mutated = false;
+    await assert.rejects(cursor.drafts.update(drafts => { mutated = true; return [...drafts, { title: 'new draft' }]; }), failure);
+    assert.equal(mutated, false, failure);
+    t.mock.restoreAll();
+    assert.equal(fs.readFileSync(target, 'utf8'), original, failure);
+    assert.equal(existsSync(`${target}.lock`), false, failure);
+  }
+  const { stores: cursor } = stores();
+  assert.deepEqual(await cursor.drafts.read(), [], 'an absent table is still a fresh store');
+  await cursor.drafts.update(() => [{ title: 'first draft' }]);
+  assert.deepEqual(await cursor.drafts.read(), [{ title: 'first draft' }]);
+});
+
+test('a corrupt draft file is preserved for every packaged host', async () => {
+  for (const agent of readdirSync(new URL('../../hosts/', import.meta.url))) {
+    if (!existsSync(new URL(`../../hosts/${agent}/profile.js`, import.meta.url))) continue;
+    const { dir, stores: host } = stores(agent);
+    const target = join(dir, `${agent}-distill-drafts.json`);
+    writeFileSync(target, '[unfinished');
+    await assert.rejects(host.drafts.update(() => assert.fail('mutator must not run')), { code: 'EINVALIDSTATE' });
+    assert.equal(fs.readFileSync(target, 'utf8'), '[unfinished', agent);
+  }
+});
+
+test('a refused state lock preserves drafts and never runs their mutator', async t => {
+  const { dir, stores: cursor } = stores();
+  const target = join(dir, 'cursor-distill-drafts.json');
+  const lock = `${target}.lock`;
+  const original = '[{"title":"keep this draft"}]';
+  writeFileSync(target, original);
+  const write = fs.writeFileSync;
+  t.mock.method(fs, 'writeFileSync', (file, ...args) => {
+    if (file === lock) throw Object.assign(new Error('fixture lock creation denial'), { code: 'EROFS' });
+    return write(file, ...args);
+  });
+  let mutations = 0;
+  try {
+    await assert.rejects(cursor.drafts.update(drafts => { mutations += 1; return [...drafts, { title: 'new draft' }]; }), { code: 'ELOCKED' });
+  } finally {
+    t.mock.restoreAll();
+  }
+  assert.equal(mutations, 0);
+  assert.equal(fs.readFileSync(target, 'utf8'), original);
+  assert.deepEqual(readdirSync(dir), ['cursor-distill-drafts.json']);
+});
+
+test('a held live state lock preserves drafts and never runs their mutator after waiting', async () => {
+  const { dir, stores: cursor } = stores();
+  const target = join(dir, 'cursor-distill-drafts.json');
+  const lock = `${target}.lock`;
+  const original = '[{"title":"keep this draft"}]';
+  const held = `${process.pid}:fixture-live-owner`;
+  writeFileSync(target, original);
+  writeFileSync(lock, held);
+  let mutations = 0;
+  await assert.rejects(cursor.drafts.update(drafts => { mutations += 1; return [...drafts, { title: 'new draft' }]; }), { code: 'ELOCKED' });
+  assert.equal(mutations, 0);
+  assert.equal(fs.readFileSync(target, 'utf8'), original);
+  assert.equal(fs.readFileSync(lock, 'utf8'), held);
+});
+
+for (const closeFails of [false, true]) test(`a partial temporary state write preserves drafts and cleans up before a retry${closeFails ? ' even when close also fails' : ''}`, async t => {
+  const { dir, stores: cursor } = stores();
+  const target = join(dir, 'cursor-distill-drafts.json');
+  const previous = [{ title: 'keep this draft' }];
+  await cursor.drafts.update(() => previous);
+  const original = fs.readFileSync(target, 'utf8');
+  const temporary = file => typeof file === 'string' && file.startsWith(target + '.') && file.endsWith('.tmp');
+  const descriptors = new Set();
+  const open = fs.openSync, write = fs.writeFileSync, close = fs.closeSync;
+  t.mock.method(fs, 'openSync', (file, ...args) => {
+    const descriptor = open(file, ...args);
+    if (temporary(file)) descriptors.add(descriptor);
+    return descriptor;
+  });
+  let failed = false;
+  let closeFailed = false;
+  const writeFailure = Object.assign(new Error('fixture storage exhausted after a partial write'), { code: 'ENOSPC' });
+  if (closeFails) t.mock.method(fs, 'closeSync', descriptor => {
+    const owned = descriptors.delete(descriptor);
+    const result = close(descriptor);
+    if (owned) {
+      closeFailed = true;
+      throw Object.assign(new Error('fixture close failure after closing the descriptor'), { code: 'EIO' });
+    }
+    return result;
+  });
+  t.mock.method(fs, 'writeFileSync', (file, ...args) => {
+    if (!failed && (temporary(file) || descriptors.has(file))) {
+      failed = true;
+      write(file, '[{"partial":', ...args.slice(1));
+      throw writeFailure;
+    }
+    return write(file, ...args);
+  });
+  try {
+    await assert.rejects(cursor.drafts.update(drafts => [...drafts, { title: 'new draft' }]), error => {
+      assert.equal(error, writeFailure);
+      assert.equal(error.code, 'ENOSPC');
+      return true;
+    });
+  } finally {
+    t.mock.restoreAll();
+  }
+  assert.equal(failed, true);
+  assert.equal(closeFailed, closeFails);
+  assert.equal(fs.readFileSync(target, 'utf8'), original);
+  assert.deepEqual(readdirSync(dir), ['cursor-distill-drafts.json']);
+  await cursor.drafts.update(drafts => [...drafts, { title: 'retry succeeded' }]);
+  assert.deepEqual(await cursor.drafts.read(), [...previous, { title: 'retry succeeded' }]);
+});
+
+test('a temporary descriptor close failure preserves drafts without replacing the target and permits a retry', async t => {
+  const { dir, stores: cursor } = stores();
+  const target = join(dir, 'cursor-distill-drafts.json');
+  const previous = [{ title: 'keep this draft' }];
+  await cursor.drafts.update(() => previous);
+  const original = fs.readFileSync(target, 'utf8');
+  const descriptors = new Set();
+  const open = fs.openSync, close = fs.closeSync, rename = fs.renameSync;
+  t.mock.method(fs, 'openSync', (file, ...args) => {
+    const descriptor = open(file, ...args);
+    if (typeof file === 'string' && file.startsWith(target + '.') && file.endsWith('.tmp')) descriptors.add(descriptor);
+    return descriptor;
+  });
+  let failed = false;
+  t.mock.method(fs, 'closeSync', descriptor => {
+    const owned = descriptors.delete(descriptor);
+    const result = close(descriptor);
+    if (owned) {
+      failed = true;
+      throw Object.assign(new Error('fixture close failure after closing the descriptor'), { code: 'EIO' });
+    }
+    return result;
+  });
+  let replacements = 0;
+  t.mock.method(fs, 'renameSync', (from, to) => {
+    if (to === target) replacements += 1;
+    return rename(from, to);
+  });
+  try {
+    await assert.rejects(cursor.drafts.update(drafts => [...drafts, { title: 'new draft' }]), { code: 'EIO' });
+  } finally {
+    t.mock.restoreAll();
+  }
+  assert.equal(failed, true);
+  assert.equal(replacements, 0);
+  assert.equal(fs.readFileSync(target, 'utf8'), original);
+  assert.deepEqual(readdirSync(dir), ['cursor-distill-drafts.json']);
+  await cursor.drafts.update(drafts => [...drafts, { title: 'retry succeeded' }]);
+  assert.deepEqual(await cursor.drafts.read(), [...previous, { title: 'retry succeeded' }]);
+});
+
+test('a temporary file collision preserves the existing file and releases the state lock', async t => {
+  const { dir, stores: cursor } = stores();
+  const target = join(dir, 'cursor-distill-drafts.json');
+  await cursor.drafts.update(() => [{ title: 'keep this draft' }]);
+  const original = fs.readFileSync(target, 'utf8');
+  const suffix = '010203040506';
+  const collision = `${target}.${process.pid}.${suffix}.tmp`;
+  const collisionContents = 'another invocation owns these bytes';
+  writeFileSync(collision, collisionContents);
+  const random = crypto.randomBytes;
+  t.mock.method(crypto, 'randomBytes', (size, ...args) => size === 6 ? Buffer.from(suffix, 'hex') : random(size, ...args));
+  try {
+    await assert.rejects(cursor.drafts.update(drafts => [...drafts, { title: 'new draft' }]), { code: 'EEXIST' });
+  } finally {
+    t.mock.restoreAll();
+  }
+  assert.equal(fs.readFileSync(target, 'utf8'), original);
+  assert.equal(fs.readFileSync(collision, 'utf8'), collisionContents);
+  assert.equal(existsSync(`${target}.lock`), false);
+  await cursor.drafts.update(drafts => [...drafts, { title: 'retry succeeded' }]);
+  assert.deepEqual(await cursor.drafts.read(), [{ title: 'keep this draft' }, { title: 'retry succeeded' }]);
+  assert.equal(fs.readFileSync(collision, 'utf8'), collisionContents);
+});
+
+test('a failed draft replacement preserves the previous file, releases its lock and removes its temporary file', async t => {
+  const { dir, stores: cursor } = stores();
+  const target = join(dir, 'cursor-distill-drafts.json');
+  await cursor.drafts.update(() => [{ title: 'keep this draft' }]);
+  const before = fs.readFileSync(target, 'utf8');
+  const rename = fs.renameSync;
+  t.mock.method(fs, 'renameSync', (from, to) => {
+    if (to === target) throw Object.assign(new Error('fixture replacement denial'), { code: 'EPERM' });
+    return rename(from, to);
+  });
+  await assert.rejects(cursor.drafts.update(drafts => [...drafts, { title: 'new draft' }]), { code: 'EPERM' });
+  t.mock.restoreAll();
+  assert.equal(fs.readFileSync(target, 'utf8'), before);
+  assert.deepEqual(readdirSync(dir), ['cursor-distill-drafts.json']);
+  await cursor.drafts.update(drafts => [...drafts, { title: 'retry succeeded' }]);
+  assert.deepEqual(await cursor.drafts.read(), [{ title: 'keep this draft' }, { title: 'retry succeeded' }]);
+});
+
+test('a transient replacement denial retries while readers can still see the previous drafts', async t => {
+  const { dir, stores: cursor } = stores();
+  const target = join(dir, 'cursor-distill-drafts.json');
+  const previous = [{ title: 'keep this draft' }];
+  await cursor.drafts.update(() => previous);
+  const rename = fs.renameSync;
+  let attempts = 0;
+  t.mock.method(fs, 'renameSync', (from, to) => {
+    if (to === target) {
+      assert.deepEqual(JSON.parse(fs.readFileSync(target, 'utf8')), previous);
+      if (++attempts === 1) throw Object.assign(new Error('fixture sharing denial'), { code: 'EBUSY' });
+    }
+    return rename(from, to);
+  });
+  await cursor.drafts.update(drafts => [...drafts, { title: 'new draft' }]);
+  assert.equal(attempts, 2);
+  assert.deepEqual(await cursor.drafts.read(), [...previous, { title: 'new draft' }]);
+  assert.deepEqual(readdirSync(dir), ['cursor-distill-drafts.json']);
+});
+
 test('a report appended after an interrupted queue write survives draining and commit', async () => {
   const { dir, stores: cursor } = stores();
   const earlier = { report: { task_id: 'earlier', asset_id: 'a' }, sessionId: 's1' };
@@ -74,11 +302,36 @@ test('parallel processes updating one state file through the lock lose no update
   const lock = new URL('../../lib/runtime/store/file-lock.js', import.meta.url).href;
   const script = `import { updateState } from ${JSON.stringify(lock)};
 for (let i = 0; i < 25; i += 1) updateState(${JSON.stringify(target)}, {}, (state) => ({ count: (state.count ?? 0) + 1 }));`;
-  const run = () => new Promise((resolve) => spawn(process.execPath, ['--input-type=module', '-e', script]).on('close', resolve));
-  await Promise.all([run(), run(), run(), run()]);
+  const run = () => new Promise((resolve) => {
+    const child = spawn(process.execPath, ['--input-type=module', '-e', script]);
+    let stderr = '';
+    child.stderr.on('data', chunk => { stderr += chunk; });
+    child.on('close', code => resolve({ code, stderr }));
+  });
+  const results = await Promise.all([run(), run(), run(), run()]);
+  assert.deepEqual(results.map(result => result.code), [0, 0, 0, 0], results.map(result => result.stderr).join('\n'));
   assert.equal(readState(target, {}).count, 100);
 });
 
+test('transient sharing denial while releasing a state lock leaves no orphaned lock', t => {
+  for (const operation of ['readFileSync', 'unlinkSync']) {
+    const { dir } = stores();
+    const target = join(dir, 'counter.json'), lock = `${target}.lock`;
+    const original = fs[operation];
+    let denied = false;
+    t.mock.method(fs, operation, (file, ...args) => {
+      if (file === lock && !denied) { denied = true; throw Object.assign(new Error('sharing violation'), { code: 'EPERM' }); }
+      return original(file, ...args);
+    });
+    updateState(target, {}, state => ({ count: (state.count ?? 0) + 1 }));
+    assert.equal(denied, true);
+    assert.equal(existsSync(lock), false, operation);
+    updateState(target, {}, state => ({ count: state.count + 1 }));
+    assert.equal(readState(target, {}).count, 2);
+    t.mock.restoreAll();
+  }
+});
+
 test('a session file outlives the outbox: a session resumed within a month still counts its turns on', async () => {
   const dir = mkdtempSync(join(tmpdir(), 'evolver-prune-'));
   dirs.push(dir);
diff --git a/test/runtime/state-lock.test.js b/test/runtime/state-lock.test.js
new file mode 100644
index 0000000..b1f39f3
--- /dev/null
+++ b/test/runtime/state-lock.test.js
@@ -0,0 +1,323 @@
+import assert from 'node:assert/strict';
+import { spawn, spawnSync } from 'node:child_process';
+import fs from 'node:fs';
+import { existsSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { dirname, join } from 'node:path';
+import { test } from 'node:test';
+
+import { updateState } from '../../lib/runtime/store/file-lock.js';
+import { lockContenders, until } from '../helpers/lock-contenders.js';
+
+const RUNTIME = new URL('../../lib/runtime/store/file-lock.js', import.meta.url).href;
+const BUSY = { code: 'ELOCKED', message: 'State is busy or unavailable; no update was made' };
+
+function scratch(t) {
+  const dir = mkdtempSync(join(tmpdir(), 'evolver-state-lock-'));
+  const target = join(dir, 'state.json');
+  writeFileSync(target, '{"count":0}');
+  t.after(() => rmSync(dir, { recursive: true, force: true }));
+  return { dir, target, lock: `${target}.lock` };
+}
+
+test('a failed admission release after acquiring a state token lets the next update proceed', (t) => {
+  const { target, lock } = scratch(t);
+  const unlink = fs.unlinkSync;
+  fs.unlinkSync = (file) => {
+    if (typeof file === 'string' && dirname(file) === `${lock}.admission`) {
+      throw Object.assign(new Error('refused by fixture'), { code: 'EPERM' });
+    }
+    return unlink(file);
+  };
+  try {
+    assert.throws(() => updateState(target, {}, () => assert.fail('mutator must not run')), BUSY);
+  } finally {
+    fs.unlinkSync = unlink;
+  }
+  assert.equal(existsSync(lock), false);
+  assert.deepEqual(updateState(target, {}, (state) => ({ count: state.count + 1 })), { count: 1 });
+});
+
+test('a failed acquire rollback keeps its own token recoverable', (t) => {
+  const { target, lock } = scratch(t);
+  const unlink = fs.unlinkSync;
+  fs.unlinkSync = (file) => {
+    if (String(file) === lock || dirname(String(file)) === `${lock}.admission`) {
+      throw Object.assign(new Error('refused by fixture'), { code: 'EPERM' });
+    }
+    return unlink(file);
+  };
+  try {
+    assert.throws(() => updateState(target, {}, () => assert.fail('mutator must not run')), BUSY);
+  } finally {
+    fs.unlinkSync = unlink;
+  }
+  assert.equal(existsSync(lock), true);
+  assert.deepEqual(updateState(target, {}, (state) => ({ count: state.count + 1 })), { count: 1 });
+  assert.equal(existsSync(lock), false);
+});
+
+test('a settled state write stays successful and its stranded token can be reclaimed', (t) => {
+  const { target, lock } = scratch(t);
+  const unlink = fs.unlinkSync;
+  try {
+    assert.deepEqual(updateState(target, {}, (state) => {
+      fs.unlinkSync = (file) => {
+        if (String(file) === lock) throw Object.assign(new Error('refused by fixture'), { code: 'EPERM' });
+        return unlink(file);
+      };
+      return { count: state.count + 1 };
+    }), { count: 1 });
+  } finally {
+    fs.unlinkSync = unlink;
+  }
+  assert.equal(existsSync(lock), true);
+  assert.deepEqual(updateState(target, {}, (state) => ({ count: state.count + 1 })), { count: 2 });
+  assert.equal(existsSync(lock), false);
+});
+
+for (const stage of ['mutator', 'read', 'replacement']) {
+  test(`an original ${stage} failure survives a release denial and its stranded state token is reclaimed on retry`, t => {
+    const { dir, target, lock } = scratch(t);
+    const original = readFileSync(target, 'utf8');
+    const failure = Object.assign(new Error(`fixture ${stage} failure`), { code: stage === 'read' ? 'EACCES' : 'EIO' });
+    const read = fs.readFileSync, rename = fs.renameSync, unlink = fs.unlinkSync;
+    if (stage === 'read') t.mock.method(fs, 'readFileSync', (file, ...args) => {
+      if (file === target) throw failure;
+      return read(file, ...args);
+    });
+    if (stage === 'replacement') t.mock.method(fs, 'renameSync', (from, to) => {
+      if (to === target) throw failure;
+      return rename(from, to);
+    });
+    let releaseDenials = 0;
+    t.mock.method(fs, 'unlinkSync', (file, ...args) => {
+      if (file === lock) {
+        releaseDenials += 1;
+        throw Object.assign(new Error('fixture state token release denial'), { code: 'EPERM' });
+      }
+      return unlink(file, ...args);
+    });
+    let mutations = 0;
+    try {
+      assert.throws(() => updateState(target, {}, state => {
+        mutations += 1;
+        assert.deepEqual(state, { count: 0 });
+        if (stage === 'mutator') throw failure;
+        return { count: state.count + 1 };
+      }), error => {
+        if (stage === 'read') {
+          assert.equal(error.code, 'ESTATEIO');
+          assert.equal(error.cause, failure);
+        } else assert.equal(error, failure);
+        return true;
+      });
+    } finally {
+      t.mock.restoreAll();
+    }
+    assert.equal(mutations, stage === 'read' ? 0 : 1);
+    assert.ok(releaseDenials > 0, 'the state token release was actually refused');
+    assert.equal(readFileSync(target, 'utf8'), original);
+    assert.equal(existsSync(lock), true);
+    assert.match(readFileSync(lock, 'utf8'), new RegExp(`^${process.pid}:`));
+    assert.deepEqual(fs.readdirSync(dir).sort(), ['state.json', 'state.json.lock']);
+    let retries = 0;
+    assert.deepEqual(updateState(target, {}, state => { retries += 1; return { count: state.count + 1 }; }), { count: 1 });
+    assert.equal(retries, 1);
+    assert.deepEqual(JSON.parse(readFileSync(target, 'utf8')), { count: 1 });
+    assert.deepEqual(fs.readdirSync(dir), ['state.json']);
+  });
+}
+
+test('a held state lock never runs an unguarded mutator after its wait expires', (t) => {
+  const { target, lock } = scratch(t);
+  const held = `${process.pid}:alive`;
+  writeFileSync(lock, held);
+  let called = false;
+  const started = Date.now();
+  assert.throws(() => updateState(target, {}, () => { called = true; return { count: 1 }; }), BUSY);
+  assert.ok(Date.now() - started < 5_000);
+  assert.equal(called, false);
+  assert.equal(readFileSync(target, 'utf8'), '{"count":0}');
+  assert.equal(readFileSync(lock, 'utf8'), held);
+});
+
+test('a legacy dead state owner is recovered, and file permission errors stop before mutation', (t) => {
+  const { target, lock } = scratch(t);
+  const exited = spawnSync(process.execPath, ['-e', '']);
+  assert.equal(exited.status, 0);
+  writeFileSync(lock, `${exited.pid}:deadbeef`);
+  assert.deepEqual(updateState(target, {}, (stored) => ({ count: stored.count + 1 })), { count: 1 });
+  assert.equal(existsSync(lock), false);
+  const write = fs.writeFileSync;
+  fs.writeFileSync = (file, ...args) => {
+    if (file === lock) throw Object.assign(new Error('refused by fixture'), { code: 'EROFS' });
+    return write(file, ...args);
+  };
+  let called = false;
+  try {
+    assert.throws(() => updateState(target, {}, () => { called = true; return {}; }), BUSY);
+  } finally {
+    fs.writeFileSync = write;
+  }
+  assert.equal(called, false);
+  assert.deepEqual(JSON.parse(readFileSync(target, 'utf8')), { count: 1 });
+  assert.deepEqual(updateState(target, {}, (stored) => ({ count: stored.count + 1 })), { count: 2 });
+});
+
+test('a transient sharing denial retries outside admission and runs the mutator only once', (t) => {
+  const { target, lock } = scratch(t);
+  const write = fs.writeFileSync;
+  let attempts = 0;
+  fs.writeFileSync = (file, ...args) => {
+    if (file === lock && ++attempts === 1) throw Object.assign(new Error('temporarily refused by fixture'), { code: 'EPERM' });
+    return write(file, ...args);
+  };
+  let called = 0;
+  try {
+    assert.deepEqual(updateState(target, {}, (stored) => { called += 1; return { count: stored.count + 1 }; }), { count: 1 });
+  } finally {
+    fs.writeFileSync = write;
+  }
+  assert.equal(attempts, 2);
+  assert.equal(called, 1);
+  assert.equal(existsSync(lock), false);
+});
+
+test('an admission setup refusal leaves state unchanged and never calls the mutator', (t) => {
+  const { target, lock } = scratch(t);
+  const mkdir = fs.mkdirSync;
+  fs.mkdirSync = (file, ...args) => {
+    if (String(file).startsWith(`${lock}.admission.`)) throw Object.assign(new Error('read-only fixture'), { code: 'EROFS' });
+    return mkdir(file, ...args);
+  };
+  let called = false;
+  try {
+    assert.throws(() => updateState(target, {}, () => { called = true; return {}; }), BUSY);
+  } finally {
+    fs.mkdirSync = mkdir;
+  }
+  assert.equal(called, false);
+  assert.equal(readFileSync(target, 'utf8'), '{"count":0}');
+  assert.deepEqual(updateState(target, {}, (stored) => ({ count: stored.count + 1 })), { count: 1 });
+});
+
+test('a refused dead-owner reclaim obeys the same bounded wait without calling the mutator', async (t) => {
+  const { target, lock } = scratch(t);
+  const exited = spawnSync(process.execPath, ['-e', '']);
+  assert.equal(exited.status, 0);
+  const held = `${exited.pid}:deadbeef`;
+  writeFileSync(lock, held);
+  const script = `
+    import fs from 'node:fs';
+    import { updateState } from ${JSON.stringify(RUNTIME)};
+    const [target, lock] = process.argv.slice(1);
+    const unlink = fs.unlinkSync;
+    let reclaims = 0;
+    fs.unlinkSync = (file) => {
+      if (String(file) === lock) {
+        reclaims += 1;
+        throw Object.assign(new Error('refused by fixture'), { code: 'EPERM' });
+      }
+      return unlink(file);
+    };
+    let called = false;
+    try { updateState(target, {}, () => { called = true; return {}; }); }
+    catch (error) { process.stdout.write(JSON.stringify({ code: error.code, called, reclaims })); }
+  `;
+  const child = spawn(process.execPath, ['--input-type=module', '-e', script, target, lock]);
+  let stdout = '';
+  let stderr = '';
+  child.stdout.on('data', (data) => { stdout += data; });
+  child.stderr.on('data', (data) => { stderr += data; });
+  const watchdog = setTimeout(() => child.kill('SIGKILL'), 5_000);
+  const code = await new Promise((resolve) => child.on('close', resolve));
+  clearTimeout(watchdog);
+  assert.equal(code, 0, stderr || 'a failed reclaim must not spin forever');
+  const result = JSON.parse(stdout);
+  assert.equal(result.code, 'ELOCKED');
+  assert.equal(result.called, false);
+  assert.ok(result.reclaims > 0);
+  assert.equal(readFileSync(target, 'utf8'), '{"count":0}');
+  assert.equal(readFileSync(lock, 'utf8'), held);
+  assert.deepEqual(updateState(target, {}, (stored) => ({ count: stored.count + 1 })), { count: 1 });
+});
+
+test('three state updaters preserve every increment while recovering one dead file lock', async (t) => {
+  const dir = mkdtempSync(join(tmpdir(), 'evolver-state-lock-race-'));
+  const target = join(dir, 'state.json');
+  const lock = `${target}.lock`;
+  writeFileSync(target, '{"count":0}');
+  const exited = spawnSync(process.execPath, ['-e', '']);
+  assert.equal(exited.status, 0);
+  const dead = `${exited.pid}:deadbeef`;
+  writeFileSync(lock, dead);
+  const marker = (name) => join(dir, name);
+  const script = `
+    import fs from 'node:fs';
+    import path from 'node:path';
+    import { updateState } from ${JSON.stringify(RUNTIME)};
+    const [dir, target, lock, role] = process.argv.slice(1);
+    const marker = (name) => path.join(dir, name);
+    const wait = (name) => {
+      const deadline = Date.now() + 10000;
+      while (!fs.existsSync(marker(name))) {
+        if (Date.now() > deadline) throw new Error('timed out waiting for ' + name);
+        Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, 10);
+      }
+    };
+    // Start all contenders before orchestrating the lock interleaving.
+    updateState(marker(role + '-startup.json'), {}, () => undefined);
+    fs.writeFileSync(marker(role + '-ready'), '');
+    wait(role + '-start');
+    const read = fs.readFileSync;
+    let paused = false;
+    fs.readFileSync = (file, ...args) => {
+      const result = read(file, ...args);
+      if (role === 'A' && file === lock && !paused) {
+        paused = true;
+        fs.writeFileSync(marker('A-read-dead'), '');
+        wait('A-resume');
+      }
+      return result;
+    };
+    const rename = fs.renameSync;
+    fs.renameSync = (source, destination) => {
+      try { return rename(source, destination); }
+      catch (error) {
+        if (destination === lock + '.admission') fs.writeFileSync(marker(role + '-guard-busy'), '');
+        throw error;
+      }
+    };
+    const write = fs.writeFileSync;
+    fs.writeFileSync = (file, ...args) => {
+      if (file === lock) write(marker(role + '-attempted'), '');
+      return write(file, ...args);
+    };
+    updateState(target, {}, (stored) => {
+      try { fs.writeFileSync(marker('active'), role, { flag: 'wx' }); }
+      catch { fs.writeFileSync(marker('overlap'), ''); }
+      fs.writeFileSync(marker(role + '-entered'), '');
+      wait('release');
+      if (fs.readFileSync(marker('active'), 'utf8') === role) fs.unlinkSync(marker('active'));
+      return { count: stored.count + 1 };
+    });
+  `;
+  const { run, results } = lockContenders(t, dir, script, [dir, target, lock], ['A-start', 'B-start', 'C-start', 'A-resume', 'release']);
+  for (const role of ['A', 'B', 'C']) run(role);
+  await until(() => ['A', 'B', 'C'].every((role) => existsSync(marker(role + '-ready'))));
+  writeFileSync(marker('A-start'), '');
+  await until(() => existsSync(marker('A-read-dead')));
+  writeFileSync(marker('B-start'), '');
+  await until(() => existsSync(marker('B-entered')) || existsSync(marker('B-guard-busy')));
+  assert.equal(existsSync(marker('B-entered')), false, 'the competing process waits for admission');
+  writeFileSync(marker('A-resume'), '');
+  await until(() => ['A', 'B'].some((role) => existsSync(marker(role + '-entered'))));
+  writeFileSync(marker('C-start'), '');
+  await until(() => existsSync(marker('C-attempted')) || existsSync(marker('C-guard-busy')));
+  writeFileSync(marker('release'), '');
+  const outcomes = await results();
+  assert.ok(outcomes.every(({ code }) => code === 0), JSON.stringify(outcomes));
+  assert.equal(existsSync(marker('overlap')), false, 'state mutators never overlap during dead-owner recovery');
+  assert.deepEqual(JSON.parse(readFileSync(target, 'utf8')), { count: 3 });
+});