diff --git a/gitnexus-claude-plugin/hooks/gitnexus-hook.js b/gitnexus-claude-plugin/hooks/gitnexus-hook.js
index 91c20c9d2..db1b366fe 100644
--- a/gitnexus-claude-plugin/hooks/gitnexus-hook.js
+++ b/gitnexus-claude-plugin/hooks/gitnexus-hook.js
@@ -238,10 +238,30 @@ function runGitNexusCli(args, cwd, timeout) {
   });
 }
 
+/**
+ * Detect whether the hook is running under Codex rather than Claude Code.
+ * Codex sets CODEX_* environment variables in its hook runtime.
+ */
+function isCodexRuntime() {
+  return Boolean(
+    process.env.CODEX_THREAD_ID ||
+    process.env.CODEX_CI ||
+    process.env.CODEX_MANAGED_PACKAGE_ROOT,
+  );
+}
+
 /**
  * Emit a hook response with additional context for the agent.
+ *
+ * Claude Code expects the context nested under its hook-specific output
+ * object; Codex's SDK expects a top-level `additionalContext`. Emitting the
+ * wrong shape makes Codex reject the hook with "invalid post-tool-use JSON".
  */
 function sendHookResponse(hookEventName, message) {
+  if (isCodexRuntime()) {
+    console.log(JSON.stringify({ additionalContext: message }));
+    return;
+  }
   console.log(
     JSON.stringify({
       hookSpecificOutput: { hookEventName, additionalContext: message },
diff --git a/gitnexus/hooks/claude/gitnexus-hook.cjs b/gitnexus/hooks/claude/gitnexus-hook.cjs
index 9793bd7bc..17741da00 100755
--- a/gitnexus/hooks/claude/gitnexus-hook.cjs
+++ b/gitnexus/hooks/claude/gitnexus-hook.cjs
@@ -274,10 +274,30 @@ function handlePreToolUse(input) {
   }
 }
 
+/**
+ * Detect whether the hook is running under Codex rather than Claude Code.
+ * Codex sets CODEX_* environment variables in its hook runtime.
+ */
+function isCodexRuntime() {
+  return Boolean(
+    process.env.CODEX_THREAD_ID ||
+    process.env.CODEX_CI ||
+    process.env.CODEX_MANAGED_PACKAGE_ROOT,
+  );
+}
+
 /**
  * Emit a PostToolUse hook response with additional context for the agent.
+ *
+ * Claude Code expects the context nested under its hook-specific output
+ * object; Codex's SDK expects a top-level `additionalContext`. Emitting the
+ * wrong shape makes Codex reject the hook with "invalid post-tool-use JSON".
  */
 function sendHookResponse(hookEventName, message) {
+  if (isCodexRuntime()) {
+    console.log(JSON.stringify({ additionalContext: message }));
+    return;
+  }
   console.log(
     JSON.stringify({
       hookSpecificOutput: { hookEventName, additionalContext: message },
diff --git a/gitnexus/test/unit/hooks.test.ts b/gitnexus/test/unit/hooks.test.ts
index 892a19483..794f065fb 100644
--- a/gitnexus/test/unit/hooks.test.ts
+++ b/gitnexus/test/unit/hooks.test.ts
@@ -1438,6 +1438,31 @@ describe('PostToolUse staleness detection (integration)', () => {
       const output = parseHookOutput(result.stdout);
       expect(output).not.toBeNull();
     });
+
+    it(`${label}: emits top-level additionalContext under Codex runtime`, () => {
+      fs.writeFileSync(
+        path.join(gitNexusDir, 'meta.json'),
+        JSON.stringify({ lastCommit: 'aaaaaaa0000000000000000000000000deadbeef', stats: {} }),
+      );
+
+      const result = runHook(
+        hookPath,
+        {
+          hook_event_name: 'PostToolUse',
+          tool_name: 'Bash',
+          tool_input: { command: 'git commit -m "test"' },
+          tool_output: { exit_code: 0 },
+          cwd: tmpDir,
+        },
+        undefined,
+        { env: { ...process.env, CODEX_THREAD_ID: 'test-thread' } },
+      );
+
+      // Codex expects top-level additionalContext, NOT hookSpecificOutput.
+      const parsed = JSON.parse(result.stdout.trim());
+      expect(parsed.additionalContext).toContain('stale');
+      expect(parsed.hookSpecificOutput).toBeUndefined();
+    });
   }
 });