feat(openai): add max_tokens to AzureChatOpenAI (#32959)

Fixes #32949

This pattern is [present in
`ChatOpenAI`](https://github.com/langchain-ai/langchain/blob/master/libs/partners/openai/langchain_openai/chat_models/base.py#L2821)
but wasn't carried over to Azure.


[CI](https://github.com/langchain-ai/langchain/actions/runs/17741751797/job/50417180998)

libs/partners/openai/langchain_openai/chat_models/azure.py

@@ -578,6 +578,9 @@ class AzureChatOpenAI(BaseChatOpenAI):
     ``'parallel_tools_calls'`` will be disabled.
     """
 
+    max_tokens: Optional[int] = Field(default=None, alias="max_completion_tokens")  # type: ignore[assignment]
+    """Maximum number of tokens to generate."""
+
     @classmethod
     def get_lc_namespace(cls) -> list[str]:
         """Get the namespace of the langchain object."""
@@ -699,6 +702,15 @@ class AzureChatOpenAI(BaseChatOpenAI):
             "openai_api_version": self.openai_api_version,
         }
 
+    @property
+    def _default_params(self) -> dict[str, Any]:
+        """Get the default parameters for calling Azure OpenAI API."""
+        params = super()._default_params
+        if "max_tokens" in params:
+            params["max_completion_tokens"] = params.pop("max_tokens")
+
+        return params
+
     def _get_ls_params(
         self, stop: Optional[list[str]] = None, **kwargs: Any
     ) -> LangSmithParams:

libs/partners/openai/tests/unit_tests/chat_models/test_azure.py

@@ -139,3 +139,37 @@ def test_chat_completions_api_uses_model_name() -> None:
     assert payload["model"] == "gpt-5"
     assert "messages" in payload  # Chat Completions API uses 'messages'
     assert "input" not in payload
+
+
+def test_max_completion_tokens_parameter() -> None:
+    """Test that max_completion_tokens can be used as a direct parameter."""
+    llm = AzureChatOpenAI(
+        azure_deployment="gpt-5",
+        api_version="2024-12-01-preview",
+        azure_endpoint="my-base-url",
+        max_completion_tokens=1500,
+    )
+    messages = [HumanMessage("Hello")]
+    payload = llm._get_request_payload(messages)
+
+    # Should use max_completion_tokens instead of max_tokens
+    assert "max_completion_tokens" in payload
+    assert payload["max_completion_tokens"] == 1500
+    assert "max_tokens" not in payload
+
+
+def test_max_tokens_converted_to_max_completion_tokens() -> None:
+    """Test that max_tokens is converted to max_completion_tokens."""
+    llm = AzureChatOpenAI(
+        azure_deployment="gpt-5",
+        api_version="2024-12-01-preview",
+        azure_endpoint="my-base-url",
+        max_tokens=1000,  # type: ignore[call-arg]
+    )
+    messages = [HumanMessage("Hello")]
+    payload = llm._get_request_payload(messages)
+
+    # max_tokens should be converted to max_completion_tokens
+    assert "max_completion_tokens" in payload
+    assert payload["max_completion_tokens"] == 1000
+    assert "max_tokens" not in payload