Fix proxy tool result meta attribute forwarding (#2526)

Proxied tool results now properly forward the meta attribute from upstream servers through ProxyToolManager and ProxyTool.

Co-authored-by: Jeremiah Lowin <[email protected]>

Author: dherrman

src/fastmcp/server/proxy.py

@@ -117,6 +117,7 @@ async def call_tool(self, key: str, arguments: dict[str, Any]) -> ToolResult:
                 return ToolResult(
                     content=result.content,
                     structured_content=result.structured_content,
+                    meta=result.meta,
                 )
 
 

tests/server/proxy/test_proxy_server.py

@@ -14,6 +14,7 @@
 from fastmcp.client.transports import FastMCPTransport, StreamableHttpTransport
 from fastmcp.exceptions import ToolError
 from fastmcp.server.proxy import FastMCPProxy, ProxyClient
+from fastmcp.tools.tool import ToolResult
 from fastmcp.tools.tool_transform import (
     ToolTransformConfig,
 )
@@ -216,6 +217,23 @@ async def test_error_tool_raises_error(self, proxy_server):
             async with Client(proxy_server) as client:
                 await client.call_tool("error_tool", {})
 
+    async def test_call_tool_forwards_meta(self, fastmcp_server, proxy_server):
+        """Test that metadata from proxied tool results is properly forwarded."""
+
+        @fastmcp_server.tool
+        def tool_with_meta(value: str) -> ToolResult:
+            """A tool that returns metadata in its result."""
+            return ToolResult(
+                content=f"Result: {value}",
+                meta={"custom_key": "custom_value", "processed": True},
+            )
+
+        async with Client(proxy_server) as client:
+            result = await client.call_tool("tool_with_meta", {"value": "test"})
+
+        assert result.content[0].text == "Result: test"  # type: ignore[attr-defined]
+        assert result.meta == {"custom_key": "custom_value", "processed": True}
+
     async def test_proxy_can_overwrite_proxied_tool(self, proxy_server):
         """
         Test that a tool defined on the proxy can overwrite the proxied tool with the same name.