diff --git a/tests/e2e/access_control/access_control_client.py b/tests/e2e/access_control/access_control_client.py
index e95ad1f57ce..7ace036f433 100644
--- a/tests/e2e/access_control/access_control_client.py
+++ b/tests/e2e/access_control/access_control_client.py
@@ -4,6 +4,8 @@ from __future__ import annotations
 
 from dataclasses import dataclass
 
+from pydantic import BaseModel, ValidationError
+
 from proxy_client import ProxyClient
 from e2e_http import StreamingResponse
 from models import (
@@ -19,6 +21,24 @@ MODEL_ACCESS_DENIED_MARKER = "key_model_access_denied"
 ROUTE_NOT_ALLOWED_MARKER = "not allowed to call this route"
 
 
+class ApiErrorDetail(BaseModel):
+    message: str | None = None
+    type: str | None = None
+    code: str | int | None = None
+
+
+class ApiErrorEnvelope(BaseModel):
+    error: ApiErrorDetail
+
+
+def error_envelope(body: str) -> ApiErrorEnvelope | None:
+    """The OpenAI-shaped `{"error": {...}}` a client parses, or None if absent."""
+    try:
+        return ApiErrorEnvelope.model_validate_json(body)
+    except ValidationError:
+        return None
+
+
 @dataclass(frozen=True, slots=True)
 class AccessControlClient:
     proxy: ProxyClient
diff --git a/tests/e2e/access_control/test_access_control_e2e.py b/tests/e2e/access_control/test_access_control_e2e.py
index e24b721d831..af7e9a099fd 100644
--- a/tests/e2e/access_control/test_access_control_e2e.py
+++ b/tests/e2e/access_control/test_access_control_e2e.py
@@ -13,19 +13,18 @@ management route).
 
 from __future__ import annotations
 
-import json
-
 import pytest
 
 from access_control_client import (
     AccessControlClient,
     MODEL_ACCESS_DENIED_MARKER,
     ROUTE_NOT_ALLOWED_MARKER,
+    error_envelope,
 )
 from e2e_config import unique_marker
 from e2e_http import Success, UnauthorizedError, UnknownApiError, unwrap
 from lifecycle import ResourceManager
-from models import ChatBody, ChatMessage, LiteLLMParamsBody
+from models import ChatBody, ChatMessage, ChatResponse, LiteLLMParamsBody
 from proxy_client import ProxyClient
 
 pytestmark = pytest.mark.e2e
@@ -35,16 +34,28 @@ DISALLOWED_MODEL = "gpt-5.5"
 VIRTUAL_KEY_BACKEND = "anthropic/claude-haiku-4-5-20251001"
 
 
-def _is_json(body: str) -> bool:
-    try:
-        json.loads(body)
-        return True
-    except ValueError:
-        return False
-
-
-
 class TestAccessControl:
+    def test_allowed_model_is_permitted(
+        self, client: AccessControlClient, resources: ResourceManager
+    ) -> None:
+        """The allow-list's positive half.
+
+        Without this, every other case in this class passes just as happily
+        against a gateway that denies the allowed model too, because they only
+        ever assert that something was refused.
+        """
+        key = resources.key(models=[ALLOWED_MODEL])
+        result = client.chat_status(
+            key, ALLOWED_MODEL, f"capital of France? {unique_marker()}"
+        )
+        assert result.status_code == 200, (
+            f"key allow-listed for {ALLOWED_MODEL!r} must be able to call it, got "
+            f"{result.status_code}: {result.body[:300]}"
+        )
+        assert ChatResponse.model_validate_json(result.body).choices, (
+            f"200 must carry a real completion, not an error envelope: {result.body[:300]}"
+        )
+
     def test_disallowed_model_is_denied_403(
         self, client: AccessControlClient, resources: ResourceManager
     ) -> None:
@@ -85,7 +96,13 @@ class TestAccessControl:
             f"unknown model must be rejected 400 before forwarding, got "
             f"{result.status_code}: {result.body[:300]}"
         )
-        assert _is_json(result.body), f"400 body must be valid JSON: {result.body[:300]}"
+        envelope = error_envelope(result.body)
+        assert envelope is not None, (
+            f"400 body must be an OpenAI-shaped error envelope, got: {result.body[:300]}"
+        )
+        assert envelope.error.message, (
+            f"400 error must carry a message a client can surface: {result.body[:300]}"
+        )
 
 
 class TestVirtualKeyAuth: