diff --git a/src/audita/framework/llm.py b/src/audita/framework/llm.py
index 0350915..f2d88b9 100644
--- a/src/audita/framework/llm.py
+++ b/src/audita/framework/llm.py
@@ -6,6 +6,8 @@ from typing import Any, Optional, Sequence, Tuple
 from audita.core.config import AuditaConfig, DEFAULT_BASE_URL
 from audita.core.errors import AuditaLLMError
 
+_NO_KEY_PLACEHOLDER = "audita-no-key-required"
+
 
 class OpenAICompatibleStructuredLLMClient:
     def __init__(self) -> None:
@@ -60,7 +62,7 @@ class OpenAICompatibleStructuredLLMClient:
                 ) from exc
 
             openai_client = OpenAI(
-                api_key=config.api_key,
+                api_key=_client_api_key(config),
                 base_url=config.base_url,
                 timeout=config.llm_timeout_seconds,
             )
@@ -92,5 +94,15 @@ def _requires_api_key(config: AuditaConfig) -> bool:
     return _normalized_base_url(config.base_url) == _normalized_base_url(DEFAULT_BASE_URL)
 
 
+def _client_api_key(config: AuditaConfig) -> str:
+    if config.api_key:
+        return config.api_key
+    if _requires_api_key(config):
+        raise AuditaLLMError(
+            "OpenRouter credentials are required but missing for the configured endpoint."
+        )
+    return _NO_KEY_PLACEHOLDER
+
+
 def _normalized_base_url(base_url: str) -> str:
     return base_url.rstrip("/")
diff --git a/tests/test_framework_llm.py b/tests/test_framework_llm.py
index 0f34279..80f2402 100644
--- a/tests/test_framework_llm.py
+++ b/tests/test_framework_llm.py
@@ -223,7 +223,7 @@ def test_missing_api_key_is_allowed_for_nondefault_endpoint(monkeypatch):
         ),
     )
 
-    assert openai_inits == [{"api_key": None, "base_url": "http://localhost:8000/v1", "timeout": 600}]
+    assert openai_inits == [{"api_key": "audita-no-key-required", "base_url": "http://localhost:8000/v1", "timeout": 600}]
     assert create_calls == [
         {
             "model": "meta-llama/Llama-3.1-8B-Instruct",