A model's safety-filter refusal is an error that says so, not an empty reply

Opus's provider filter declined one of nisse-review's ask_expert
questions (which repositories a Gitea token can list) every time. It came
back as a 200 with no text, and the expert reported "unparseable reply:
empty reply". Both LLM clients now raise ModelRefused for
finish_reason content_filter (chat route) and stop_reason refusal
(passthrough). The expert's handback says what happened and what to do
instead.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-core/tests/test_llm.py

@@ -20,6 +20,7 @@ from nisse_core.models.llm import (
     BudgetExhausted,
     LiteLLMClient,
     LLMError,
+    ModelRefused,
     RoleMap,
     TruncatedResponse,
     UpstreamUnavailable,
@@ -295,6 +296,51 @@ def test_anthropic_route_raises_on_truncation() -> None:
         client.complete(EVENTS, tools=None, model="claude-opus-5")
 
 
+# --- the provider's safety filter -------------------------------------------
+
+
+def test_openai_route_raises_when_the_filter_declines() -> None:
+    # As LiteLLM reports Opus's refusal (seen live, #164 follow-up): a 200
+    # with no text, which used to read as an empty answer.
+    declined = httpx.Response(
+        200,
+        json={
+            "model": "claude-opus-5-5",
+            "choices": [
+                {
+                    "finish_reason": "content_filter",
+                    "message": {"role": "assistant", "content": None},
+                }
+            ],
+            "usage": {"prompt_tokens": 612, "completion_tokens": 0},
+        },
+    )
+    client = make_client(lambda request: declined)
+    with pytest.raises(ModelRefused, match="safety filter"):
+        client.complete(EVENTS, tools=None, model="claude-opus-5-5")
+
+
+def test_anthropic_route_raises_when_the_filter_declines() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(
+            200,
+            json={"model": "claude-opus-5-5", "stop_reason": "refusal", "content": []},
+        )
+
+    client = AnthropicPassthroughClient(
+        "https://litellm.test",
+        "tok",
+        transport=httpx.MockTransport(handler),
+        sleep=lambda _s: None,
+    )
+    with pytest.raises(ModelRefused, match="declined to answer"):
+        client.complete(EVENTS, tools=None, model="claude-opus-5-5")
+
+
+def test_a_refusal_is_an_llm_error_so_callers_already_handle_it() -> None:
+    assert issubclass(ModelRefused, LLMError)
+
+
 def test_a_complete_answer_is_not_mistaken_for_truncation() -> None:
     def handler(request: httpx.Request) -> httpx.Response:
         return httpx.Response(