Allow CallToolResult to be returned directly to support _meta field for OpenAI Apps (#1459)

Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

tests/server/test_lowlevel_output_validation.py

@@ -391,6 +391,47 @@ async def test_both_content_and_valid_dict_with_output_schema():
     assert result.structuredContent == {"sentiment": "positive", "confidence": 0.95}
 
 
+@pytest.mark.anyio
+async def test_tool_call_result():
+    """Test returning ToolCallResult when no outputSchema is defined."""
+    tools = [
+        Tool(
+            name="get_info",
+            description="Get structured information",
+            inputSchema={
+                "type": "object",
+                "properties": {},
+            },
+            # No outputSchema for direct return of tool call result
+        )
+    ]
+
+    async def call_tool_handler(name: str, arguments: dict[str, Any]) -> CallToolResult:
+        if name == "get_info":
+            return CallToolResult(
+                content=[TextContent(type="text", text="Results calculated")],
+                structuredContent={"status": "ok", "data": {"value": 42}},
+                _meta={"some": "metadata"},
+            )
+        else:
+            raise ValueError(f"Unknown tool: {name}")
+
+    async def test_callback(client_session: ClientSession) -> CallToolResult:
+        return await client_session.call_tool("get_info", {})
+
+    result = await run_tool_test(tools, call_tool_handler, test_callback)
+
+    # Verify results
+    assert result is not None
+    assert not result.isError
+    assert len(result.content) == 1
+    assert result.content[0].type == "text"
+    assert result.content[0].text == "Results calculated"
+    assert isinstance(result.content[0], TextContent)
+    assert result.structuredContent == {"status": "ok", "data": {"value": 42}}
+    assert result.meta == {"some": "metadata"}
+
+
 @pytest.mark.anyio
 async def test_output_schema_type_validation():
     """Test outputSchema validates types correctly."""