diff --git a/nanobot/providers/openai_compat_provider.py b/nanobot/providers/openai_compat_provider.py
index 9a7da7522..83db7e8f8 100644
--- a/nanobot/providers/openai_compat_provider.py
+++ b/nanobot/providers/openai_compat_provider.py
@@ -157,6 +157,16 @@ def _is_direct_openai_base(api_base: str | None) -> bool:
     return "api.openai.com" in normalized and "openrouter" not in normalized
 
 
+def _responses_circuit_key(
+    model: str | None,
+    default_model: str,
+    reasoning_effort: str | None,
+) -> str:
+    model_name = (model or default_model).lower()
+    effort = reasoning_effort.lower() if isinstance(reasoning_effort, str) else ""
+    return f"{model_name}:{effort}"
+
+
 class OpenAICompatProvider(LLMProvider):
     """Unified provider for all OpenAI-compatible APIs.
 
@@ -434,7 +444,7 @@ class OpenAICompatProvider(LLMProvider):
             return False
 
         # Circuit breaker: skip after repeated failures, probe periodically.
-        key = f"{model_name}:{reasoning_effort or ''}"
+        key = _responses_circuit_key(model, self.default_model, reasoning_effort)
         failures = self._responses_failures.get(key, 0)
         if failures >= _RESPONSES_FAILURE_THRESHOLD:
             tripped = self._responses_tripped_at.get(key, 0.0)
@@ -444,7 +454,7 @@ class OpenAICompatProvider(LLMProvider):
         return True
 
     def _record_responses_failure(self, model: str | None, reasoning_effort: str | None) -> None:
-        key = f"{(model or self.default_model).lower()}:{reasoning_effort or ''}"
+        key = _responses_circuit_key(model, self.default_model, reasoning_effort)
         count = self._responses_failures.get(key, 0) + 1
         self._responses_failures[key] = count
         if count >= _RESPONSES_FAILURE_THRESHOLD:
@@ -455,7 +465,7 @@ class OpenAICompatProvider(LLMProvider):
             )
 
     def _record_responses_success(self, model: str | None, reasoning_effort: str | None) -> None:
-        key = f"{(model or self.default_model).lower()}:{reasoning_effort or ''}"
+        key = _responses_circuit_key(model, self.default_model, reasoning_effort)
         self._responses_failures.pop(key, None)
         self._responses_tripped_at.pop(key, None)
 
diff --git a/tests/providers/test_litellm_kwargs.py b/tests/providers/test_litellm_kwargs.py
index 8304aae8f..47db20398 100644
--- a/tests/providers/test_litellm_kwargs.py
+++ b/tests/providers/test_litellm_kwargs.py
@@ -441,6 +441,35 @@ async def test_direct_openai_responses_404_falls_back_to_chat_completions() -> N
     mock_chat.assert_awaited_once()
 
 
+@pytest.mark.asyncio
+async def test_direct_openai_open_circuit_skips_responses_api() -> None:
+    mock_chat = AsyncMock(return_value=_fake_chat_response("from chat"))
+    mock_responses = AsyncMock(return_value=_fake_responses_response("from responses"))
+    spec = find_by_name("openai")
+
+    with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI") as MockClient:
+        client_instance = MockClient.return_value
+        client_instance.chat.completions.create = mock_chat
+        client_instance.responses.create = mock_responses
+
+        provider = OpenAICompatProvider(
+            api_key="sk-test-key",
+            default_model="gpt-5-chat",
+            spec=spec,
+        )
+        for _ in range(3):
+            provider._record_responses_failure("gpt-5-chat", None)
+
+        result = await provider.chat(
+            messages=[{"role": "user", "content": "hello"}],
+            model="gpt-5-chat",
+        )
+
+    assert result.content == "from chat"
+    mock_responses.assert_not_awaited()
+    mock_chat.assert_awaited_once()
+
+
 @pytest.mark.asyncio
 async def test_direct_openai_stream_responses_unsupported_param_falls_back() -> None:
     mock_chat = AsyncMock(return_value=_fake_chat_stream("fallback stream"))
diff --git a/tests/providers/test_responses_circuit_breaker.py b/tests/providers/test_responses_circuit_breaker.py
index 4787459c7..409aea1d5 100644
--- a/tests/providers/test_responses_circuit_breaker.py
+++ b/tests/providers/test_responses_circuit_breaker.py
@@ -69,3 +69,9 @@ def test_reasoning_effort_keyed_separately(provider):
         provider._record_responses_failure("o3", "high")
     assert provider._should_use_responses_api("o3", "high") is False
     assert provider._should_use_responses_api("o3", "low") is True
+
+
+def test_reasoning_effort_key_is_case_insensitive(provider):
+    for _ in range(_RESPONSES_FAILURE_THRESHOLD):
+        provider._record_responses_failure("o3", "High")
+    assert provider._should_use_responses_api("o3", "high") is False