diff --git a/routstr/upstream/messages_dispatch.py b/routstr/upstream/messages_dispatch.py
index e7429150..1951768d 100644
--- a/routstr/upstream/messages_dispatch.py
+++ b/routstr/upstream/messages_dispatch.py
@@ -36,6 +36,12 @@ from .reasoning_effort import adapt_messages_body_for_litellm
 
 logger = get_logger(__name__)
 
+# Sent in place of a blank upstream key. LiteLLM treats ``""`` as missing and
+# falls back to the provider's env var (e.g. ``OPENAI_API_KEY``), failing with
+# an AuthenticationError for keyless upstreams such as self-hosted
+# OpenAI-compatible servers, which the chat path reaches without auth.
+KEYLESS_UPSTREAM_API_KEY = "no-key"
+
 # Anthropic-Messages-only fields that don't translate to OpenAI
 # Chat Completions. ``litellm.drop_params`` only filters *known*
 # unsupported params; these newer/extension fields get passed through
@@ -545,7 +551,7 @@ async def dispatch_anthropic_messages(
     kwargs: dict = {
         "model": litellm_model,
         "api_base": base_url,
-        "api_key": api_key,
+        "api_key": api_key or KEYLESS_UPSTREAM_API_KEY,
         "stream": upstream_stream,
         **body,
     }
diff --git a/tests/unit/test_messages_litellm_dispatch.py b/tests/unit/test_messages_litellm_dispatch.py
index c9b13bd2..7e2da476 100644
--- a/tests/unit/test_messages_litellm_dispatch.py
+++ b/tests/unit/test_messages_litellm_dispatch.py
@@ -1755,3 +1755,31 @@ async def test_x_cashu_zero_value_rejected_not_forwarded(
     assert body["error"]["code"] == "cashu_token_zero_value"
     # Spent-to-zero token must not be echoed back for retry.
     assert "X-Cashu" not in response.headers
+
+
+@pytest.mark.asyncio
+async def test_dispatch_passes_placeholder_key_for_keyless_upstream() -> None:
+    """A blank upstream key must not reach litellm, which would fall back to
+    OPENAI_API_KEY and fail with an AuthenticationError."""
+    provider = BaseUpstreamProvider(base_url="http://localhost:8000/v1", api_key="")
+    captured_kwargs: dict[str, Any] = {}
+
+    async def fake_acreate(**kwargs: Any) -> AsyncIterator[dict]:
+        captured_kwargs.update(kwargs)
+
+        async def no_events() -> AsyncIterator[dict]:
+            return
+            yield
+
+        return no_events()
+
+    with patch(
+        "litellm.anthropic.messages.acreate",
+        new=AsyncMock(side_effect=fake_acreate),
+    ):
+        await provider._dispatch_anthropic_messages(
+            request_body=_anthropic_request_body(stream=True),
+            model_obj=_make_model(),
+        )
+
+    assert captured_kwargs["api_key"] == "no-key"