feat(openrouter): surface parallel_tool_calls on bind_tools (#38214)

`ChatOpenRouter.bind_tools()` now accepts a `parallel_tool_calls`
argument to disable parallel tool use.

---

Users binding tools to `ChatOpenRouter` previously had to know that
arbitrary kwargs were forwarded to the OpenRouter SDK in order to
disable parallel tool use (e.g.
`model.bind(parallel_tool_calls=False)`). This made the option hard to
discover.

This exposes `parallel_tool_calls` as an explicit keyword-only argument
on `ChatOpenRouter.bind_tools()`, matching the ergonomics of
`langchain-openai`. When set, it is forwarded to the request exactly as
before; when left as `None` it is omitted, preserving existing behavior.
No change to the underlying request payload or SDK floor — purely an
API/discoverability improvement.

Made by [Open
SWE](https://openswe.vercel.app/agents/053b59c8-baf4-84e1-c003-425e349e014d)

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

libs/partners/openrouter/langchain_openrouter/chat_models.py

@@ -837,6 +837,7 @@ class ChatOpenRouter(BaseChatModel):
         *,
         tool_choice: dict | str | bool | None = None,
         strict: bool | None = None,
+        parallel_tool_calls: bool | None = None,
         **kwargs: Any,
     ) -> Runnable[LanguageModelInput, AIMessage]:
         """Bind tool-like objects to this chat model.
@@ -852,8 +853,13 @@ class ChatOpenRouter(BaseChatModel):
 
                 If `None`, the `strict` argument will not be passed to
                 the model.
+            parallel_tool_calls: Set to `False` to disable parallel tool use.
+                Defaults to `None` (no specification, which allows parallel
+                tool use).
             **kwargs: Any additional parameters.
         """
+        if parallel_tool_calls is not None:
+            kwargs["parallel_tool_calls"] = parallel_tool_calls
         formatted_tools = [
             convert_to_openai_tool(tool, strict=strict) for tool in tools
         ]

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

@@ -1044,6 +1044,20 @@ class TestBindTools:
         tools = bound.kwargs["tools"]
         assert "strict" not in tools[0]["function"]
 
+    def test_bind_tools_parallel_tool_calls_forwarded(self) -> None:
+        """Test that parallel_tool_calls is forwarded to the request kwargs."""
+        model = _make_model()
+        bound = model.bind_tools([GetWeather], parallel_tool_calls=False)
+        assert isinstance(bound, RunnableBinding)
+        assert bound.kwargs["parallel_tool_calls"] is False
+
+    def test_bind_tools_parallel_tool_calls_none_omits_key(self) -> None:
+        """Test that parallel_tool_calls=None does not set the key in kwargs."""
+        model = _make_model()
+        bound = model.bind_tools([GetWeather])
+        assert isinstance(bound, RunnableBinding)
+        assert "parallel_tool_calls" not in bound.kwargs
+
 
 # ===========================================================================
 # with_structured_output tests