Use max_completion_tokens instead of deprecated max_tokens in OpenAI handler (#3254)

🤖 Generated with Claude Code

Co-authored-by: Marvin Context Protocol <41898282+Marvin Context Protocol@users.noreply.github.com>
Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com>

src/fastmcp/client/sampling/handlers/openai.py

@@ -84,7 +84,7 @@ class OpenAISamplingHandler:
         kwargs: dict[str, Any] = {
             "model": model,
             "messages": openai_messages,
-            "max_tokens": params.maxTokens,
+            "max_completion_tokens": params.maxTokens,
         }
         if params.temperature is not None:
             kwargs["temperature"] = params.temperature

tests/client/sampling/handlers/test_openai_handler.py

@@ -1,7 +1,8 @@
-from unittest.mock import MagicMock
+from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 from mcp.types import (
+    CreateMessageRequestParams,
     CreateMessageResult,
     ModelHint,
     ModelPreferences,
@@ -72,6 +73,39 @@ def test_select_model_from_preferences(prefs, expected):
     assert handler._select_model_from_preferences(prefs) == expected
 
 
+async def test_handler_passes_max_completion_tokens():
+    """Verify the handler uses max_completion_tokens (not max_tokens)."""
+    mock_client = MagicMock(spec=AsyncOpenAI)
+    mock_client.chat = MagicMock()
+    mock_client.chat.completions = MagicMock()
+    mock_client.chat.completions.create = AsyncMock(
+        return_value=ChatCompletion(
+            id="123",
+            created=123,
+            model="gpt-4o-mini",
+            object="chat.completion",
+            choices=[
+                Choice(
+                    message=ChatCompletionMessage(content="hi", role="assistant"),
+                    finish_reason="stop",
+                    index=0,
+                )
+            ],
+        )
+    )
+    handler = OpenAISamplingHandler(default_model="gpt-4o-mini", client=mock_client)
+    messages = [
+        SamplingMessage(role="user", content=TextContent(type="text", text="hello"))
+    ]
+    params = CreateMessageRequestParams(messages=messages, maxTokens=300)
+    await handler(messages, params, context=None)  # type: ignore[arg-type]
+
+    call_kwargs = mock_client.chat.completions.create.call_args
+    assert "max_completion_tokens" in call_kwargs.kwargs
+    assert call_kwargs.kwargs["max_completion_tokens"] == 300
+    assert "max_tokens" not in call_kwargs.kwargs
+
+
 async def test_chat_completion_to_create_message_result():
     mock_client = MagicMock(spec=AsyncOpenAI)
     handler = OpenAISamplingHandler(default_model="fallback-model", client=mock_client)  # type: ignore[arg-type]