xai: drop unsupported 'stop' parameter for grok-3 models

The grok-3 model family rejects the legacy 'stop' parameter with a 400
BadRequestError rather than ignoring it. Override _get_request_payload
on ChatXAI to silently strip 'stop' from the outbound payload when the
model name starts with 'grok-3', so shared configurations across xAI
models don't hard-fail on grok-3.

Other models (e.g. grok-4) continue to forward 'stop' unchanged.

libs/partners/xai/langchain_xai/chat_models.py

@@ -535,6 +535,21 @@ class ChatXAI(BaseChatOpenAI):  # type: ignore[override]
     def _resolve_model_profile(self) -> ModelProfile | None:
         return _get_default_model_profile(self.model_name) or None
 
+    def _get_request_payload(
+        self,
+        input_: LanguageModelInput,
+        *,
+        stop: list[str] | None = None,
+        **kwargs: Any,
+    ) -> dict:
+        payload = super()._get_request_payload(input_, stop=stop, **kwargs)
+        # The grok-3 family rejects the `stop` parameter with a 400 error
+        # rather than ignoring it. Drop it silently so calls that set `stop`
+        # for other models in a shared code path don't hard-fail on grok-3.
+        if self.model_name and self.model_name.startswith("grok-3"):
+            payload.pop("stop", None)
+        return payload
+
     def _stream(self, *args: Any, **kwargs: Any) -> Iterator[ChatGenerationChunk]:
         """Route to Chat Completions or Responses API."""
         if self._use_responses_api({**kwargs, **self.model_kwargs}):

libs/partners/xai/tests/unit_tests/test_chat_models.py

@@ -159,3 +159,22 @@ def test_stream_usage_metadata() -> None:
 
     model = ChatXAI(model=MODEL_NAME, stream_usage=False)
     assert model.stream_usage is False
+
+
+def test_chat_xai_grok_3_strips_stop_from_payload() -> None:
+    """grok-3 family rejects the `stop` param; ensure it's stripped from payload."""
+    llm = ChatXAI(model="grok-3", api_key=SecretStr("test-api-key"), stop=["END"])
+    payload = llm._get_request_payload([HumanMessage(content="hi")])
+    assert "stop" not in payload
+
+    # Also covers forward-compat variants like `grok-3-mini`, `grok-3-fast`, etc.
+    llm_variant = ChatXAI(
+        model="grok-3-mini", api_key=SecretStr("test-api-key"), stop=["END"]
+    )
+    payload_variant = llm_variant._get_request_payload([HumanMessage(content="hi")])
+    assert "stop" not in payload_variant
+
+    # Sanity check: other models still forward `stop`.
+    llm_other = ChatXAI(model="grok-4", api_key=SecretStr("test-api-key"), stop=["END"])
+    payload_other = llm_other._get_request_payload([HumanMessage(content="hi")])
+    assert payload_other.get("stop") == ["END"]