fix: ProxyTool crashes on non-TextContent error responses (#3926)

* fix: handle non-TextContent error responses in ProxyTool

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Avoid serializing binary content into ToolError messages

Use type name instead of str(content) to prevent dumping
large base64 payloads into error messages.

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* ruff format fix

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: strawgate

src/fastmcp/server/providers/proxy.py

@@ -158,7 +158,13 @@ async def run(
                     name=backend_name, arguments=arguments, meta=meta
                 )
             if result.isError:
-                raise ToolError(cast(mcp.types.TextContent, result.content[0]).text)
+                first = result.content[0] if result.content else None
+                if isinstance(first, mcp.types.TextContent):
+                    raise ToolError(first.text)
+                elif first is None:
+                    raise ToolError("Tool returned an error with no content")
+                else:
+                    raise ToolError(f"Tool returned an error ({type(first).__name__})")
             # Preserve backend's meta (includes task metadata for background tasks)
             return ToolResult(
                 content=result.content,

tests/server/providers/proxy/test_proxy_server.py

@@ -330,6 +330,36 @@ 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_error_tool_with_image_content(self, proxy_server):
+        """Non-TextContent error responses should not crash with AttributeError."""
+        error_result = mcp_types.CallToolResult(
+            content=[
+                mcp_types.ImageContent(
+                    type="image", data="abc123", mimeType="image/png"
+                )
+            ],
+            isError=True,
+        )
+        with patch.object(
+            Client, "call_tool_mcp", new_callable=AsyncMock, return_value=error_result
+        ):
+            with pytest.raises(ToolError):
+                async with Client(proxy_server) as client:
+                    await client.call_tool("error_tool", {})
+
+    async def test_error_tool_with_empty_content(self, proxy_server):
+        """Error responses with empty content should not crash."""
+        error_result = mcp_types.CallToolResult(
+            content=[],
+            isError=True,
+        )
+        with patch.object(
+            Client, "call_tool_mcp", new_callable=AsyncMock, return_value=error_result
+        ):
+            with pytest.raises(ToolError):
+                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."""