address PR feedback

src/fastmcp/server/server.py

@@ -205,7 +205,7 @@ class FastMCP(Generic[LifespanResultT]):
             | None
         ) = None,
         tags: set[str] | None = None,
-        serializer: Callable[[Any], str] | None = None,
+        tool_serializer: Callable[[Any], str] | None = None,
         **settings: Any,
     ):
         self.tags: set[str] = tags or set()
@@ -215,7 +215,6 @@ class FastMCP(Generic[LifespanResultT]):
                 seconds=self.settings.cache_expiration_seconds
             )
         )
-        self._serializer = serializer
 
         self._mounted_servers: dict[str, MountedServer] = {}
 
@@ -229,7 +228,7 @@ class FastMCP(Generic[LifespanResultT]):
         )
         self._tool_manager = ToolManager(
             duplicate_behavior=self.settings.on_duplicate_tools,
-            serializer=self._serializer,
+            serializer=tool_serializer,
         )
         self._resource_manager = ResourceManager(
             duplicate_behavior=self.settings.on_duplicate_resources

src/fastmcp/tools/tool.py

@@ -125,7 +125,7 @@ class Tool(BaseModel):
                 arguments_to_validate=arguments,
                 arguments_to_pass_directly=pass_args,
             )
-            return _convert_to_content(result, _serializer=self.serializer)
+            return _convert_to_content(result, serializer=self.serializer)
         except Exception as e:
             raise ToolError(f"Error executing tool {self.name}: {e}") from e
 
@@ -146,8 +146,8 @@ class Tool(BaseModel):
 
 def _convert_to_content(
     result: Any,
+    serializer: Callable[[Any], str] | None = None,
     _process_as_single_item: bool = False,
-    _serializer: Callable[[Any], str] | None = None,
 ) -> list[TextContent | ImageContent | EmbeddedResource]:
     """Convert a result to a sequence of content objects."""
     if result is None:
@@ -182,8 +182,8 @@ def _convert_to_content(
         return other_content + mcp_types
 
     if not isinstance(result, str):
-        if _serializer is not None:
-            result = _serializer(result)
+        if serializer is not None:
+            result = serializer(result)
         else:
             result = pydantic_core.to_json(result, fallback=str, indent=2).decode()
 

tests/tools/test_tool_manager.py

@@ -401,7 +401,7 @@ class TestCallTools:
             return json.dumps(data)
 
         # Instantiate FastMCP with the custom serializer
-        mcp = FastMCP(serializer=custom_serializer)
+        mcp = FastMCP(tool_serializer=custom_serializer)
         manager = mcp._tool_manager
 
         def get_data() -> dict: