diff --git a/src/plugins/codex.ts b/src/plugins/codex.ts
index 04348c3..49d33af 100644
--- a/src/plugins/codex.ts
+++ b/src/plugins/codex.ts
@@ -1,12 +1,17 @@
+import { randomUUID } from 'node:crypto';
 import fsp from 'node:fs/promises';
+import type { FileHandle } from 'node:fs/promises';
 import os from 'node:os';
 import path from 'node:path';
 import type { AgentPlugin, InstallConfig, PluginStatus } from './base.js';
 import { SUPPORTED_AGENTS } from './registry.js';
 
 const AGENT_KEY = 'codex';
+const OWNED_DESCRIPTION = 'Elydora audit and freeze enforcement';
 const GUARD_STATUS = 'Checking Elydora agent state';
 const AUDIT_STATUS = 'Recording Elydora tool use';
+const GUARD_SCRIPT = 'guard.js';
+const AUDIT_SCRIPT = 'hook.js';
 const entry = SUPPORTED_AGENTS.get(AGENT_KEY)!;
 
 type JsonObject = Record<string, unknown>;
@@ -15,6 +20,18 @@ function isObject(value: unknown): value is JsonObject {
   return typeof value === 'object' && value !== null && !Array.isArray(value);
 }
 
+function errorMessage(error: unknown): string {
+  return error instanceof Error ? error.message : String(error);
+}
+
+function hasErrorCode(error: unknown, code: string): boolean {
+  return isObject(error) && error.code === code;
+}
+
+function asError(error: unknown): Error {
+  return error instanceof Error ? error : new Error(String(error));
+}
+
 function resolveConfigPath(): string {
   const configDir = entry.configDir.replace(/^~/, os.homedir());
   return path.join(configDir, entry.configFile);
@@ -47,11 +64,22 @@ function eventGroups(hooks: JsonObject, event: string): JsonObject[] {
   return value;
 }
 
+function isManagedCommand(command: unknown, scriptName: string, agentId?: string): boolean {
+  if (typeof command !== 'string') return false;
+  const normalized = command.toLowerCase();
+  if (!normalized.includes('.elydora') || !normalized.includes(scriptName)) return false;
+  return !agentId || command.includes(agentId);
+}
+
 function isElydoraHandler(handler: JsonObject, agentId?: string): boolean {
-  if (handler.statusMessage !== GUARD_STATUS && handler.statusMessage !== AUDIT_STATUS) return false;
-  if (!agentId) return true;
+  const scriptName = handler.statusMessage === GUARD_STATUS
+    ? GUARD_SCRIPT
+    : handler.statusMessage === AUDIT_STATUS
+      ? AUDIT_SCRIPT
+      : undefined;
+  if (!scriptName) return false;
   return [handler.command, handler.commandWindows].some(
-    (command) => typeof command === 'string' && command.includes(agentId),
+    (command) => isManagedCommand(command, scriptName, agentId),
   );
 }
 
@@ -69,9 +97,12 @@ function withoutElydora(groups: JsonObject[], agentId?: string): JsonObject[] {
 
 function findHandler(groups: JsonObject[], statusMessage: string): JsonObject | undefined {
   for (const group of groups) {
-    if (!Array.isArray(group.hooks)) continue;
+    if (!Array.isArray(group.hooks) || !group.hooks.every(isObject)) {
+      throw new Error('Codex hook matcher group must contain a hooks array');
+    }
     const handler = group.hooks.find(
-      (candidate) => isObject(candidate) && candidate.statusMessage === statusMessage,
+      (candidate) => candidate.statusMessage === statusMessage
+        && isElydoraHandler(candidate),
     );
     if (isObject(handler)) return handler;
   }
@@ -84,37 +115,82 @@ function commandReferences(handler: JsonObject, scriptPath: string): boolean {
   );
 }
 
-async function readSettings(configPath: string): Promise<JsonObject | undefined> {
+async function readJsonObject(filePath: string, label: string): Promise<JsonObject | undefined> {
   let raw: string;
   try {
-    raw = await fsp.readFile(configPath, 'utf-8');
+    raw = await fsp.readFile(filePath, 'utf-8');
   } catch (error) {
-    if (isObject(error) && error.code === 'ENOENT') return undefined;
-    throw new Error(`Read Codex hooks config: ${error instanceof Error ? error.message : String(error)}`);
+    if (hasErrorCode(error, 'ENOENT')) return undefined;
+    throw new Error(`Read ${label} at ${filePath}: ${errorMessage(error)}`, {
+      cause: asError(error),
+    });
   }
 
   let value: unknown;
   try {
     value = JSON.parse(raw);
   } catch (error) {
-    throw new Error(`Failed to parse Codex hooks config: ${error instanceof Error ? error.message : String(error)}`);
+    throw new Error(`Failed to parse ${label} at ${filePath}: ${errorMessage(error)}`, {
+      cause: asError(error),
+    });
   }
-  if (!isObject(value)) throw new Error('Codex hooks config must contain a JSON object');
+  if (!isObject(value)) throw new Error(`${label} at ${filePath} must contain a JSON object`);
   return value;
 }
 
-async function writeSettings(configPath: string, settings: JsonObject): Promise<void> {
-  await fsp.mkdir(path.dirname(configPath), { recursive: true });
-  const tempPath = `${configPath}.${process.pid}.${Date.now()}.tmp`;
+async function readSettings(configPath: string): Promise<JsonObject | undefined> {
+  return readJsonObject(configPath, 'Codex hooks config');
+}
+
+async function failWrite(
+  handle: FileHandle | undefined,
+  tempPath: string,
+  label: string,
+  cause: unknown,
+): Promise<never> {
+  const errors = [asError(cause)];
+  if (handle) {
+    try {
+      await handle.close();
+    } catch (error) {
+      errors.push(asError(error));
+    }
+  }
   try {
-    await fsp.writeFile(tempPath, JSON.stringify(settings, null, 2) + '\n', {
-      encoding: 'utf-8',
-      mode: 0o600,
-    });
+    await fsp.unlink(tempPath);
+  } catch (error) {
+    if (!hasErrorCode(error, 'ENOENT')) errors.push(asError(error));
+  }
+  const message = `Write ${label}: ${errorMessage(cause)}`;
+  if (errors.length > 1) throw new AggregateError(errors, message);
+  throw new Error(message, { cause: errors[0] });
+}
+
+async function writeSettings(configPath: string, settings: JsonObject): Promise<void> {
+  const directory = path.dirname(configPath);
+  await fsp.mkdir(directory, { recursive: true, mode: 0o700 });
+  const tempPath = path.join(directory, `.${path.basename(configPath)}.${randomUUID()}.tmp`);
+  let handle: FileHandle | undefined;
+  try {
+    handle = await fsp.open(tempPath, 'wx', 0o600);
+    await handle.writeFile(JSON.stringify(settings, null, 2) + '\n', 'utf-8');
+    await handle.sync();
+    await handle.close();
+    handle = undefined;
     await fsp.rename(tempPath, configPath);
   } catch (error) {
-    await fsp.unlink(tempPath).catch(() => undefined);
-    throw new Error(`Write Codex hooks config: ${error instanceof Error ? error.message : String(error)}`);
+    await failWrite(handle, tempPath, `Codex hooks config at ${configPath}`, error);
+  }
+}
+
+async function removeSettings(configPath: string): Promise<void> {
+  try {
+    await fsp.unlink(configPath);
+  } catch (error) {
+    if (hasErrorCode(error, 'ENOENT')) return;
+    throw new Error(`Remove Codex hooks config at ${configPath}: ${errorMessage(error)}`, {
+      cause: asError(error),
+    });
   }
 }
 
@@ -124,37 +200,73 @@ function getHooks(settings: JsonObject): JsonObject {
   return { ...settings.hooks };
 }
 
+function isOwnedSettings(settings: JsonObject, hooks: JsonObject): boolean {
+  const settingsKeys = new Set(['description', 'hooks']);
+  const hookKeys = new Set(['PreToolUse', 'PostToolUse']);
+  return Object.keys(settings).every((key) => settingsKeys.has(key))
+    && settings.description === OWNED_DESCRIPTION
+    && Object.keys(hooks).every((key) => hookKeys.has(key))
+    && Object.values(hooks).every((value) => Array.isArray(value) && value.length === 0);
+}
+
+async function regularFileExists(filePath: string, label: string): Promise<boolean> {
+  try {
+    const metadata = await fsp.stat(filePath);
+    return metadata.isFile();
+  } catch (error) {
+    if (hasErrorCode(error, 'ENOENT')) return false;
+    throw new Error(`Read ${label} at ${filePath}: ${errorMessage(error)}`, {
+      cause: asError(error),
+    });
+  }
+}
+
+async function requireRuntime(filePath: string, label: string): Promise<void> {
+  if (!filePath) throw new Error(`${label} path is required`);
+  if (!await regularFileExists(filePath, label)) {
+    throw new Error(`${label} is missing: ${filePath}`);
+  }
+}
+
 async function runtimeScriptsExist(guard: JsonObject, audit: JsonObject): Promise<boolean> {
   const root = path.join(os.homedir(), '.elydora');
-  let entries;
+  let entries: Array<{ isDirectory(): boolean; name: string }>;
   try {
     entries = await fsp.readdir(root, { withFileTypes: true });
-  } catch {
-    return false;
+  } catch (error) {
+    if (hasErrorCode(error, 'ENOENT')) return false;
+    throw new Error(`Read Elydora runtime directory at ${root}: ${errorMessage(error)}`, {
+      cause: asError(error),
+    });
   }
 
   for (const directory of entries) {
     if (!directory.isDirectory()) continue;
     const agentDir = path.join(root, directory.name);
-    try {
-      const config = JSON.parse(await fsp.readFile(path.join(agentDir, 'config.json'), 'utf-8'));
-      if (config.agent_name !== AGENT_KEY) continue;
-      const guardPath = path.join(agentDir, 'guard.js');
-      const hookPath = path.join(agentDir, 'hook.js');
-      if (!commandReferences(guard, guardPath) || !commandReferences(audit, hookPath)) continue;
-      await Promise.all([fsp.access(guardPath), fsp.access(hookPath)]);
-      return true;
-    } catch {
-      continue;
-    }
+    const guardPath = path.join(agentDir, GUARD_SCRIPT);
+    const hookPath = path.join(agentDir, AUDIT_SCRIPT);
+    if (!commandReferences(guard, guardPath) || !commandReferences(audit, hookPath)) continue;
+
+    const configPath = path.join(agentDir, 'config.json');
+    const config = await readJsonObject(configPath, 'Elydora runtime config');
+    if (!config || config.agent_name !== AGENT_KEY) continue;
+    const [guardExists, hookExists] = await Promise.all([
+      regularFileExists(guardPath, 'Elydora guard runtime'),
+      regularFileExists(hookPath, 'Elydora audit runtime'),
+    ]);
+    return guardExists && hookExists;
   }
   return false;
 }
 
 export const codexPlugin: AgentPlugin = {
   async install(config: InstallConfig): Promise<void> {
+    if (!config.agentId) throw new Error('agentId is required');
     const configPath = resolveConfigPath();
-    const settings = (await readSettings(configPath)) ?? {};
+    const existingSettings = await readSettings(configPath);
+    await requireRuntime(config.guardScriptPath, 'Elydora guard runtime');
+    await requireRuntime(config.hookScriptPath, 'Elydora audit runtime');
+    const settings = existingSettings ?? { description: OWNED_DESCRIPTION };
     const hooks = getHooks(settings);
 
     hooks.PreToolUse = [
@@ -177,7 +289,11 @@ export const codexPlugin: AgentPlugin = {
     const hooks = getHooks(settings);
     hooks.PreToolUse = withoutElydora(eventGroups(hooks, 'PreToolUse'), agentId);
     hooks.PostToolUse = withoutElydora(eventGroups(hooks, 'PostToolUse'), agentId);
-    await writeSettings(configPath, { ...settings, hooks });
+    if (isOwnedSettings(settings, hooks)) {
+      await removeSettings(configPath);
+    } else {
+      await writeSettings(configPath, { ...settings, hooks });
+    }
   },
 
   async status(): Promise<PluginStatus> {
diff --git a/test/codex-plugin.test.mjs b/test/codex-plugin.test.mjs
index f932af8..60f3d84 100644
--- a/test/codex-plugin.test.mjs
+++ b/test/codex-plugin.test.mjs
@@ -224,6 +224,114 @@ test('Codex status requires both runtime scripts and uninstall preserves other h
   }
 });
 
+test('Codex install preserves user handlers that reuse Elydora status text', async () => {
+  const userGuard = {
+    type: 'command',
+    command: 'user-guard',
+    statusMessage: 'Checking Elydora agent state',
+  };
+  const userAudit = {
+    type: 'command',
+    command: 'user-audit',
+    statusMessage: 'Recording Elydora tool use',
+  };
+  const fixture = await createFixture({ existingSettings: {
+    hooks: {
+      PreToolUse: [{ matcher: 'Bash', hooks: [userGuard] }],
+      PostToolUse: [{ matcher: 'Bash', hooks: [userAudit] }],
+    },
+  } });
+  try {
+    assert.equal(fixture.installResult.code, 0, fixture.installResult.stderr);
+    const settings = JSON.parse(await readFile(fixture.configPath, 'utf-8'));
+    assert.deepEqual(settings.hooks.PreToolUse[0].hooks[0], userGuard);
+    assert.deepEqual(settings.hooks.PostToolUse[0].hooks[0], userAudit);
+    assert.equal(settings.hooks.PreToolUse.length, 2);
+    assert.equal(settings.hooks.PostToolUse.length, 2);
+  } finally {
+    await fixture.close();
+  }
+});
+
+test('Codex install rejects missing runtimes before writing hooks config', async () => {
+  const homeDir = await mkdtemp(path.join(os.tmpdir(), 'elydora-codex-missing-runtime-'));
+  const agentDir = path.join(homeDir, '.elydora', 'agent-1');
+  const configPath = path.join(homeDir, '.codex', 'hooks.json');
+  try {
+    const install = await runPlugin(homeDir, 'install', {
+      agentName: 'codex',
+      agentId: 'agent-1',
+      baseUrl: 'https://api.elydora.com',
+      guardScriptPath: path.join(agentDir, 'guard.js'),
+      hookScriptPath: path.join(agentDir, 'hook.js'),
+    });
+    assert.equal(install.code, 1);
+    assert.match(install.stderr, /runtime is missing/i);
+    await assert.rejects(readFile(configPath), { code: 'ENOENT' });
+  } finally {
+    await rm(homeDir, { recursive: true, force: true });
+  }
+});
+
+test('Codex status surfaces malformed referenced runtime metadata', async () => {
+  const fixture = await createFixture();
+  try {
+    assert.equal(fixture.installResult.code, 0, fixture.installResult.stderr);
+    await writeFile(path.join(fixture.agentDir, 'config.json'), '{ malformed');
+    const status = await runPlugin(fixture.homeDir, 'status', null);
+    assert.equal(status.code, 1);
+    assert.match(status.stderr, /parse Elydora runtime config/i);
+  } finally {
+    await fixture.close();
+  }
+});
+
+test('Codex status surfaces malformed hook matcher groups', async () => {
+  const fixture = await createFixture();
+  try {
+    assert.equal(fixture.installResult.code, 0, fixture.installResult.stderr);
+    const settings = JSON.parse(await readFile(fixture.configPath, 'utf-8'));
+    settings.hooks.PreToolUse[0].hooks = null;
+    await writeFile(fixture.configPath, JSON.stringify(settings, null, 2));
+    const status = await runPlugin(fixture.homeDir, 'status', null);
+    assert.equal(status.code, 1);
+    assert.match(status.stderr, /matcher group must contain a hooks array/i);
+  } finally {
+    await fixture.close();
+  }
+});
+
+test('Codex uninstall removes a hooks config owned entirely by Elydora', async () => {
+  const fixture = await createFixture();
+  try {
+    assert.equal(fixture.installResult.code, 0, fixture.installResult.stderr);
+    const uninstall = await runPlugin(fixture.homeDir, 'uninstall', 'agent-1');
+    assert.equal(uninstall.code, 0, uninstall.stderr);
+    await assert.rejects(readFile(fixture.configPath), { code: 'ENOENT' });
+  } finally {
+    await fixture.close();
+  }
+});
+
+test('Codex install preserves invalid contract shapes before writing', async () => {
+  for (const existingSettings of [
+    { hooks: null },
+    { hooks: { PreToolUse: null } },
+    { hooks: { PreToolUse: [{ hooks: null }] } },
+  ]) {
+    const fixture = await createFixture({ existingSettings });
+    try {
+      assert.equal(fixture.installResult.code, 1);
+      assert.deepEqual(
+        JSON.parse(await readFile(fixture.configPath, 'utf-8')),
+        existingSettings,
+      );
+    } finally {
+      await fixture.close();
+    }
+  }
+});
+
 test('Codex install preserves malformed config for recovery', async () => {
   const fixture = await createFixture({ existingSettings: '{ malformed' });
   try {