fix: correct MCPServer.call_tool return type and remove dead code

Author: Ar-maan05

docs/migration.md

@@ -428,6 +428,26 @@ async def my_tool(x: int, ctx: Context) -> str:
 
 The internal layers (`ToolManager.call_tool`, `Tool.run`, `Prompt.render`, `ResourceTemplate.create_resource`, etc.) now require `context` as a positional argument.
 
+### `MCPServer.call_tool()` return type corrected
+
+`MCPServer.call_tool()`'s return type signature has been corrected from `Sequence[ContentBlock] | dict[str, Any]` to `CallToolResult | Sequence[ContentBlock] | tuple[Sequence[ContentBlock], dict[str, Any]]` to match what the internal tool manager actually returns when converting tool results.
+
+**Before (v1):**
+
+```python
+async def call_tool(
+    self, name: str, arguments: dict[str, Any], context: Context[LifespanResultT, Any] | None = None
+) -> Sequence[ContentBlock] | dict[str, Any]:
+```
+
+**After (v2):**
+
+```python
+async def call_tool(
+    self, name: str, arguments: dict[str, Any], context: Context[LifespanResultT, Any] | None = None
+) -> CallToolResult | Sequence[ContentBlock] | tuple[Sequence[ContentBlock], dict[str, Any]]:
+```
+
 ### Registering lowlevel handlers on `MCPServer` (workaround)
 
 `MCPServer` does not expose public APIs for `subscribe_resource`, `unsubscribe_resource`, or `set_logging_level` handlers. In v1, the workaround was to reach into the private lowlevel server and use its decorator methods:

src/mcp/server/mcpserver/server.py

@@ -4,7 +4,6 @@
 
 import base64
 import inspect
-import json
 import re
 from collections.abc import AsyncIterator, Awaitable, Callable, Iterable, Sequence
 from contextlib import AbstractAsyncContextManager, asynccontextmanager
@@ -322,14 +321,6 @@ async def _handle_call_tool(
                 content=list(unstructured_content),  # type: ignore[arg-type]
                 structured_content=structured_content,  # type: ignore[arg-type]
             )
-        if isinstance(result, dict):  # pragma: no cover
-            # TODO: this code path is unreachable — convert_result never returns a raw dict.
-            # The call_tool return type (Sequence[ContentBlock] | dict[str, Any]) is wrong
-            # and needs to be cleaned up.
-            return CallToolResult(
-                content=[TextContent(type="text", text=json.dumps(result, indent=2))],
-                structured_content=result,
-            )
         return CallToolResult(content=list(result))
 
     async def _handle_list_resources(
@@ -399,8 +390,15 @@ async def list_tools(self) -> list[MCPTool]:
 
     async def call_tool(
         self, name: str, arguments: dict[str, Any], context: Context[LifespanResultT, Any] | None = None
-    ) -> Sequence[ContentBlock] | dict[str, Any]:
-        """Call a tool by name with arguments."""
+    ) -> CallToolResult | Sequence[ContentBlock] | tuple[Sequence[ContentBlock], dict[str, Any]]:
+        """Call a tool by name with arguments.
+
+        Returns:
+            CallToolResult: If the tool returned a CallToolResult directly.
+            Sequence[ContentBlock]: If the tool returned unstructured content and has no output schema.
+            tuple[Sequence[ContentBlock], dict[str, Any]]: If the tool has an output schema,
+                returning both unstructured content and structured content.
+        """
         if context is None:
             context = Context(mcp_server=self)
         return await self._tool_manager.call_tool(name, arguments, context, convert_result=True)