use schema to decide what to do about content conversion

Author: davemssavage

src/mcp/server/fastmcp/server.py

@@ -276,7 +276,8 @@ async def call_tool(
         """Call a tool by name with arguments."""
         context = self.get_context()
         result = await self._tool_manager.call_tool(name, arguments, context=context)
-        converted_result = _convert_to_content(result)
+        schema = self._tool_manager.get_schema(name)
+        converted_result = _convert_to_content(result, schema)
         return converted_result
 
     async def list_resources(self) -> list[MCPResource]:
@@ -879,24 +880,28 @@ async def get_prompt(
 
 def _convert_to_content(
     result: Any,
+    schema: dict[str, Any] | None
 ) -> Sequence[TextContent | ImageContent | EmbeddedResource]:
-    """Convert a result to a sequence of content objects."""
-    if result is None:
-        return []
+    if schema is None:
+        """Convert a result to a sequence of content objects."""
+        if result is None:
+            return []
 
-    if isinstance(result, TextContent | ImageContent | EmbeddedResource):
-        return [result]
+        if isinstance(result, TextContent | ImageContent | EmbeddedResource):
+            return [result]
 
-    if isinstance(result, Image):
-        return [result.to_image_content()]
+        if isinstance(result, Image):
+            return [result.to_image_content()]
 
-    if isinstance(result, list | tuple):
-        return list(chain.from_iterable(_convert_to_content(item) for item in result))  # type: ignore[reportUnknownVariableType]
+        if isinstance(result, list | tuple):
+            return list(chain.from_iterable(_convert_to_content(item, schema) for item in result))  # type: ignore[reportUnknownVariableType]
 
-    if not isinstance(result, str):
-        result = pydantic_core.to_json(result, fallback=str, indent=2).decode()
+        if not isinstance(result, str):
+            result = pydantic_core.to_json(result, fallback=str, indent=2).decode()
 
-    return [TextContent(type="text", text=result)]
+        return [TextContent(type="text", text=result)]
+    else:
+        return result
 
 
 class Context(BaseModel, Generic[ServerSessionT, LifespanContextT]):