diff --git a/src/utils/lock.ts b/src/utils/lock.ts
index 8caf7f2..bc1ec9c 100644
--- a/src/utils/lock.ts
+++ b/src/utils/lock.ts
@@ -91,7 +91,13 @@ function writeLockOwnerSync(lockPath: string): void {
 }
 
 async function removeStaleLock(lockPath: string): Promise<boolean> {
-  const owner = await readLockOwnerDuringAcquisitionGrace(lockPath)
+  let owner: { pid: number } | undefined
+  try {
+    owner = await readLockOwnerDuringAcquisitionGrace(lockPath)
+  } catch {
+    // Unreadable or ambiguous owner metadata — never delete (fail closed vs mutual exclusion loss).
+    return false
+  }
 
   if (owner && isProcessAlive(owner.pid)) return false
 
@@ -117,12 +123,14 @@ async function readLockOwnerDuringAcquisitionGrace(lockPath: string): Promise<{
 
 async function readLockOwner(lockPath: string): Promise<{ pid: number } | undefined> {
   try {
-    const owner = JSON.parse(await readFile(join(lockPath, 'owner.json'), 'utf8')) as { pid?: unknown }
+    const raw = await readFile(join(lockPath, 'owner.json'), 'utf8')
+    const owner = JSON.parse(raw) as { pid?: unknown }
     return typeof owner.pid === 'number' && Number.isInteger(owner.pid) && owner.pid > 0
       ? { pid: owner.pid }
       : undefined
-  } catch {
-    return undefined
+  } catch (error) {
+    if (isMissingPathError(error)) return undefined
+    throw error
   }
 }
 
@@ -142,6 +150,12 @@ function isFileExistsError(error: unknown): boolean {
   )
 }
 
+function isMissingPathError(error: unknown): boolean {
+  const code =
+    typeof error === 'object' && error !== null && 'code' in error ? (error as { code?: unknown }).code : undefined
+  return code === 'ENOENT' || code === 'ENOTDIR'
+}
+
 export async function withResourceLock<T>(options: ResourceLockOptions, run: () => Promise<T>): Promise<T> {
   const release = await acquireResourceLock(options)
 
diff --git a/test/utils/lock.test.ts b/test/utils/lock.test.ts
index a75630f..09a4503 100644
--- a/test/utils/lock.test.ts
+++ b/test/utils/lock.test.ts
@@ -1,4 +1,4 @@
-import { existsSync, mkdirSync, rmSync, writeFileSync } from 'node:fs'
+import { chmodSync, existsSync, mkdirSync, rmSync, writeFileSync } from 'node:fs'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { afterAll, afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
@@ -70,6 +70,51 @@ describe('resource locks', () => {
     await release()
   })
 
+  it('does not remove the lock directory when owner.json is unreadable', async () => {
+    if (process.platform === 'win32') return
+
+    const lockPath = getResourceLockPath(['unreadable-owner'])
+    mkdirSync(lockPath, { recursive: true })
+    const ownerPath = join(lockPath, 'owner.json')
+    writeFileSync(ownerPath, `${JSON.stringify({ pid: process.pid })}\n`, 'utf8')
+    chmodSync(ownerPath, 0o000)
+
+    try {
+      await expect(
+        acquireResourceLock({
+          resource: 'agent lifecycle',
+          scope: ['unreadable-owner'],
+        }),
+      ).rejects.toMatchObject({
+        name: 'ResourceLockError',
+      })
+      expect(existsSync(lockPath)).toBe(true)
+    } finally {
+      try {
+        chmodSync(ownerPath, 0o644)
+      } catch {
+        // ignore cleanup chmod failures
+      }
+    }
+  })
+
+  it('does not remove the lock directory when owner.json is not valid JSON', async () => {
+    const lockPath = getResourceLockPath(['invalid-json-owner'])
+    mkdirSync(lockPath, { recursive: true })
+    writeFileSync(join(lockPath, 'owner.json'), '{broken\n', 'utf8')
+
+    await expect(
+      acquireResourceLock({
+        resource: 'agent lifecycle',
+        scope: ['invalid-json-owner'],
+      }),
+    ).rejects.toMatchObject({
+      name: 'ResourceLockError',
+    })
+
+    expect(existsSync(lockPath)).toBe(true)
+  })
+
   it('does not delete the lock directory when owner.json appears during the acquisition grace window', async () => {
     const lockPath = getResourceLockPath(['grace-window'])
     mkdirSync(lockPath, { recursive: true })