diff --git a/coworker/providers/openai_responses.py b/coworker/providers/openai_responses.py
index 566c620b..5b5ab317 100644
--- a/coworker/providers/openai_responses.py
+++ b/coworker/providers/openai_responses.py
@@ -1,4 +1,4 @@
-"""OpenAI Responses provider — native OpenAI models via `/v1/responses`.
+"""OpenAI Responses provider — native and compatible models via `/responses`.
 
 Chat Completions rejects function tools combined with any `reasoning_effort` other than
 `none` on GPT-5.6+ ("use /v1/responses"), which had reasoning pinned OFF for native OpenAI
@@ -8,9 +8,10 @@ reasoning + tools at real effort levels, streamed reasoning summaries (→ the s
 chain-of-thought continuity across tool round-trips via `store: false` +
 `include: ["reasoning.encrypted_content"]` — nothing retained server-side.
 
-Routing: the `openai` provider entry with NO custom base_url builds this class; a custom
-endpoint (Azure, vLLM, any OpenAI-compatible gateway) and every compat vendor keep the
-Chat Completions `OpenAIProvider` (registry.py).
+Routing: the `openai` provider entry with NO custom base_url builds this class. Most custom
+endpoints (Azure, vLLM, and the existing compat vendors) keep the Chat Completions
+`OpenAIProvider`; vendors that explicitly implement the Responses wire can opt into this
+class with their own base URL (registry.py).
 
 Like the other native providers, this is mostly a pair of pure converters from the
 canonical OpenAI-chat-shaped history to Responses `input` items. What the converters
@@ -289,14 +290,16 @@ class OpenAIResponsesProvider(ProviderClient):
         default_model: str = "gpt-5.6-sol",
         api_key: Optional[str] = None,
         secrets: Any = None,
+        base_url: Optional[str] = None,
     ):
         # Same deferred-client contract as OpenAIProvider: built lazily so an engine can be
         # assembled before any key exists; key resolves at call time (explicit → env →
-        # SecretStore). Tests inject a `client` directly. No base_url — a custom endpoint
-        # routes to the Chat Completions provider instead (registry.py).
+        # SecretStore). Tests inject a `client` directly. `base_url` is opt-in: stock OpenAI
+        # leaves it unset, while Responses-compatible vendors can supply their own endpoint.
         self._client = client
         self._api_key = api_key
         self._secrets = secrets
+        self._base_url = (base_url or "").strip().rstrip("/") or None
         self.default_model = default_model
 
     def _ensure_client(self) -> Any:
@@ -310,7 +313,10 @@ class OpenAIResponsesProvider(ProviderClient):
                     "No model API key configured. Set OPENAI_API_KEY in the environment, "
                     "or add your key in Manage → Settings."
                 )
-            self._client = OpenAI(api_key=key)
+            kwargs = {"api_key": key}
+            if self._base_url:
+                kwargs["base_url"] = self._base_url
+            self._client = OpenAI(**kwargs)
         return self._client
 
     def _request_kwargs(
diff --git a/tests/test_openai_responses.py b/tests/test_openai_responses.py
index 9e871d57..1b375273 100644
--- a/tests/test_openai_responses.py
+++ b/tests/test_openai_responses.py
@@ -17,6 +17,43 @@ from coworker.providers.openai_responses import (
     convert_tools,
 )
 
+
+def test_responses_custom_base_url_reaches_sdk(monkeypatch):
+    captured: dict = {}
+
+    def fake_openai(**kwargs):
+        captured.update(kwargs)
+        return SimpleNamespace()
+
+    monkeypatch.setattr("openai.OpenAI", fake_openai)
+    provider = OpenAIResponsesProvider(
+        api_key="ark-key",
+        base_url="https://ark.example/api/v3/",
+    )
+
+    provider._ensure_client()
+
+    assert captured == {
+        "api_key": "ark-key",
+        "base_url": "https://ark.example/api/v3",
+    }
+
+
+def test_stock_openai_responses_path_unchanged(monkeypatch):
+    """Lockdown: stock OpenAI must not receive a vendor base URL."""
+    captured: dict = {}
+
+    def fake_openai(**kwargs):
+        captured.update(kwargs)
+        return SimpleNamespace()
+
+    monkeypatch.setattr("openai.OpenAI", fake_openai)
+    provider = OpenAIResponsesProvider(api_key="openai-key")
+
+    provider._ensure_client()
+
+    assert captured == {"api_key": "openai-key"}
+
 # -- fakes ------------------------------------------------------------------------