fix: strip title fields from tool schemas for Gemini 2.5 Flash compatibility (#3861)

* fix: strip title fields from tool schemas for Gemini compatibility

Gemini 2.5 Flash produces MALFORMED_FUNCTION_CALL when a function
declaration's parameters_json_schema contains 'title' fields (which
Pydantic adds by default). Strip them in _convert_tool_to_google_genai
before passing to the API.

Fixes #3860

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix ty errors and lowest-deps test failure

- Add assertions for non-None before subscripting FunctionDeclaration
  fields (ty check)
- Test compress_schema directly instead of constructing FunctionDeclaration
  which may not support parameters_json_schema in google-genai==1.18.0

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -144,15 +144,19 @@ class GoogleGenaiSamplingHandler:
 def _convert_tool_to_google_genai(tool: MCPTool) -> GoogleTool:
     """Convert an MCP Tool to Google GenAI format.
 
-    Google's parameters_json_schema accepts standard JSON Schema format,
-    so we pass tool.inputSchema directly without conversion.
+    We prune ``title`` fields from the schema because Gemini 2.5 Flash
+    produces ``MALFORMED_FUNCTION_CALL`` when Pydantic's auto-generated
+    title annotations are present.
     """
+    from fastmcp.utilities.json_schema import compress_schema
+
+    schema = compress_schema(tool.inputSchema, prune_titles=True)
     return GoogleTool(
         function_declarations=[
             FunctionDeclaration(
                 name=tool.name,
                 description=tool.description or "",
-                parameters_json_schema=tool.inputSchema,
+                parameters_json_schema=schema,
             )
         ]
     )

tests/client/sampling/handlers/test_google_genai_handler.py

@@ -432,3 +432,31 @@ def test_response_to_result_with_tools_mixed_content():
     assert isinstance(tool_use, ToolUseContent)
     assert tool_use.type == "tool_use"
     assert tool_use.name == "search"
+
+
+# ────────────────────────────────────────────────────────────
+# Test for title stripping (PR #3860)
+# ────────────────────────────────────────────────────────────
+
+
+def test_convert_tool_strips_titles():
+    """_convert_tool_to_google_genai should strip titles from inputSchema.
+
+    We test via compress_schema directly rather than constructing a
+    FunctionDeclaration because older google-genai versions may not
+    support the parameters_json_schema field.
+    """
+    from fastmcp.utilities.json_schema import compress_schema
+
+    input_schema = {
+        "title": "Params",
+        "type": "object",
+        "properties": {
+            "query": {"title": "Query", "type": "string"},
+        },
+    }
+
+    schema = compress_schema(input_schema, prune_titles=True)
+    assert "title" not in schema
+    assert "title" not in schema["properties"]["query"]
+    assert schema["properties"]["query"]["type"] == "string"