fix(providers): require base_url for anthropic-compatible

Copilot review on #661: empty base_url let the SDK fall back to
https://api.anthropic.com, sending compat-shaped requests to the
commercial API. The lane is local-only by definition, and the /v1-strip
edge case already established fail-loudly-over-silent-prod-retarget;
apply the same principle to the empty case. create_client raises an
actionable ValueError; the admin Detect path surfaces it as a clean
error string via probe_model_endpoint's existing handler.

docs/architecture.md

@@ -775,9 +775,11 @@ wire translation as the real Anthropic lane, but every model resolves to
 the `_ANTHROPIC_COMPAT_DEFAULT` capabilities (200K context, 64K output,
 `token_param=max_tokens`, `thinking_mode=none`, no native
 web_search/tool_search, no vision) — the static Claude table never
-applies to local checkpoints. `base_url` is the server root WITHOUT
-`/v1` (the Anthropic SDK appends `/v1/messages`); a trailing `/v1`
-pasted out of openai-compatible habit is stripped automatically. Set a
+applies to local checkpoints. `base_url` is required — the server root
+WITHOUT `/v1` (the Anthropic SDK appends `/v1/messages`); a trailing
+`/v1` pasted out of openai-compatible habit is stripped automatically,
+and an empty value fails at client construction rather than falling
+back to the commercial endpoint. Set a
 placeholder `api_key` (e.g. `"dummy"`) for unauthenticated servers. Tool calling
 needs the server started with `--enable-auto-tool-choice
 --tool-call-parser <family>` plus the matching reasoning parser.

tests/test_provider_anthropic_compat.py

@@ -219,6 +219,16 @@ class TestCompatFactory:
         create_client("anthropic-compatible", base_url="/v1", api_key="")
         mock_anthropic_cls.assert_called_once_with(base_url="/v1")
 
+    @patch("turnstone.core.providers._anthropic._ensure_anthropic")
+    def test_create_client_requires_base_url(self, mock_ensure: MagicMock) -> None:
+        """Empty base_url fails at construction — the local-only lane must
+        never fall back to the SDK's https://api.anthropic.com default."""
+        from turnstone.core.providers import create_client
+
+        with pytest.raises(ValueError, match="anthropic-compatible requires base_url"):
+            create_client("anthropic-compatible", base_url="", api_key="dummy")
+        mock_ensure.return_value.Anthropic.assert_not_called()
+
     @patch("turnstone.core.providers._anthropic._ensure_anthropic")
     def test_create_client_real_lane_base_url_untouched(self, mock_ensure: MagicMock) -> None:
         """The real anthropic lane forwards base_url verbatim — the /v1

turnstone/core/providers/__init__.py

@@ -153,7 +153,15 @@ def create_client(provider_name: str, *, base_url: str, api_key: str) -> Any:
         kwargs: dict[str, str] = {}
         if resolved_key is not None:
             kwargs["api_key"] = resolved_key
-        if provider_name == "anthropic-compatible" and base_url:
+        if provider_name == "anthropic-compatible":
+            # The lane targets local /v1/messages servers; without a
+            # base_url the SDK would default to https://api.anthropic.com
+            # and send compat-shaped requests to the commercial API.
+            if not base_url:
+                raise ValueError(
+                    "anthropic-compatible requires base_url (the server root, "
+                    "e.g. http://your-vllm-host:8000)"
+                )
             # The Anthropic SDK appends /v1/... to base_url, so a
             # /v1-suffixed URL (the openai-compatible convention) would
             # request /v1/v1/messages and 404.  Tolerate the suffix.