CM-73962: Stop sending explicit allow decisions from the Codex hook (#566)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: Altruistus

cycode/cli/apps/ai_guardrails/ides/codex.py

@@ -269,21 +269,18 @@ def parse_hook_payload(self, raw_payload: dict) -> AIHookPayload:
         )
 
     def build_hook_response(self, decision: HookDecision) -> dict:
-        # Codex accepts the same hook response shapes as Claude Code:
-        #  - PROMPT: empty for allow, {"decision": "block", "reason": ...} for deny
-        #  - PreToolUse: hookSpecificOutput.permissionDecision
+        # Codex hook response shapes:
+        #  - allow: empty (no objection). Older Codex runtimes reject an explicit
+        #    PreToolUse permissionDecision "allow" as unsupported and report a
+        #    failed hook on every call, so never send it.
+        #  - PROMPT deny: {"decision": "block", "reason": ...}
+        #  - PreToolUse deny: hookSpecificOutput.permissionDecision
+        if decision.action == DecisionAction.ALLOW:
+            return {}
+
         if decision.event_type == AiHookEventType.PROMPT:
-            if decision.action == DecisionAction.ALLOW:
-                return {}
             return {'decision': 'block', 'reason': decision.user_message or ''}
 
-        if decision.action == DecisionAction.ALLOW:
-            return {
-                'hookSpecificOutput': {
-                    'hookEventName': 'PreToolUse',
-                    'permissionDecision': 'allow',
-                }
-            }
         return {
             'hookSpecificOutput': {
                 'hookEventName': 'PreToolUse',

tests/cli/commands/ai_guardrails/ides/test_codex.py

@@ -97,7 +97,8 @@ def test_build_prompt_responses() -> None:
 def test_build_mcp_execution_allow_and_deny() -> None:
     codex = Codex()
     allow = codex.build_hook_response(HookDecision.allow(AiHookEventType.MCP_EXECUTION))
-    assert allow == {'hookSpecificOutput': {'hookEventName': 'PreToolUse', 'permissionDecision': 'allow'}}
+    # Explicit permissionDecision "allow" is rejected by older Codex runtimes.
+    assert allow == {}
 
     deny = codex.build_hook_response(HookDecision.deny(AiHookEventType.MCP_EXECUTION, 'secret in args!'))
     assert deny == {

tests/cli/commands/ai_guardrails/scan/test_scan_command.py

@@ -265,6 +265,60 @@ def test_copilot_cli_payload_skipped(
         assert json.loads(capsys.readouterr().out) == {}
 
 
+class TestCodexMcpResponses:
+    """Codex PreToolUse responses for MCP calls through scan_command."""
+
+    _PAYLOAD = {  # noqa: RUF012
+        'hook_event_name': 'PreToolUse',
+        'session_id': 'session-123',
+        'turn_id': 'turn-1',
+        'tool_name': 'mcp__grafana__query_loki_logs',
+        'tool_input': {'logql': '{app="api"}'},
+    }
+
+    def test_permitted_mcp_call_returns_empty_response(
+        self,
+        mock_ctx: MagicMock,
+        mocker: MockerFixture,
+        capsys: pytest.CaptureFixture[str],
+        mock_scan_command_deps: dict[str, MagicMock],
+    ) -> None:
+        """Older Codex runtimes reject permissionDecision "allow" as unsupported
+        and report a failed hook on every MCP call - allow must be empty."""
+        mocker.patch('sys.stdin', StringIO(json.dumps(self._PAYLOAD)))
+        mock_scan_command_deps['load_policy'].return_value = {'fail_open': True}
+        mock_scan_command_deps['get_handler'].return_value = MagicMock(
+            return_value=HookDecision.allow(AiHookEventType.MCP_EXECUTION)
+        )
+
+        scan_command(mock_ctx, ide='codex')
+
+        assert json.loads(capsys.readouterr().out) == {}
+
+    def test_blocked_mcp_call_returns_deny_with_reason(
+        self,
+        mock_ctx: MagicMock,
+        mocker: MockerFixture,
+        capsys: pytest.CaptureFixture[str],
+        mock_scan_command_deps: dict[str, MagicMock],
+    ) -> None:
+        mocker.patch('sys.stdin', StringIO(json.dumps(self._PAYLOAD)))
+        mock_scan_command_deps['load_policy'].return_value = {'fail_open': True}
+        mock_scan_command_deps['get_handler'].return_value = MagicMock(
+            return_value=HookDecision.deny(AiHookEventType.MCP_EXECUTION, 'Secret detected in MCP arguments')
+        )
+
+        scan_command(mock_ctx, ide='codex')
+
+        assert json.loads(capsys.readouterr().out) == {
+            'hookSpecificOutput': {
+                'hookEventName': 'PreToolUse',
+                'permissionDecision': 'deny',
+                'permissionDecisionReason': 'Secret detected in MCP arguments',
+            }
+        }
+
+
 class TestSelfDetach:
     """Report-mode scans respawn detached and release the IDE immediately."""