fix: set isError=True on error ToolResults

Author: anasstahr

mcp_proxy_for_aws/middleware/error_handling.py

@@ -23,6 +23,13 @@
 logger = logging.getLogger(__name__)
 
 
+class _FailedToolResult(ToolResult):
+    """A ToolResult that signals an error via the MCP isError flag."""
+
+    def to_mcp_result(self) -> mt.CallToolResult:
+        return mt.CallToolResult(content=self.content, isError=True)
+
+
 class ConnectionErrorMiddleware(Middleware):
     """Middleware that ensures tool calls never hang and always return a response.
 
@@ -78,6 +85,6 @@ def _is_credential_error(error: Exception) -> bool:
 
     @staticmethod
     def _error_result(message: str) -> ToolResult:
-        return ToolResult(
+        return _FailedToolResult(
             content=[mt.TextContent(type='text', text=message)],
         )

tests/unit/test_error_handling_middleware.py

@@ -22,7 +22,7 @@
 from fastmcp.tools.tool import ToolResult
 from mcp import McpError
 from mcp.types import ErrorData
-from mcp_proxy_for_aws.middleware.error_handling import ConnectionErrorMiddleware
+from mcp_proxy_for_aws.middleware.error_handling import ConnectionErrorMiddleware, _FailedToolResult
 from typing import Optional
 from unittest.mock import AsyncMock, Mock
 
@@ -67,6 +67,7 @@ async def test_passes_through_on_success(self):
         result = await middleware.on_call_tool(context, call_next)
 
         assert result is expected
+        assert not isinstance(result, _FailedToolResult)
         call_next.assert_awaited_once_with(context)
 
     @pytest.mark.asyncio
@@ -80,6 +81,8 @@ async def test_catches_exception_returns_error_result(self):
 
         result = await middleware.on_call_tool(context, call_next)
 
+        assert isinstance(result, _FailedToolResult)
+        assert result.to_mcp_result().isError is True
         assert len(result.content) == 1
         text = _get_text(result)
         assert 'Connection closed' in text
@@ -97,6 +100,8 @@ async def hang_forever(context: MiddlewareContext[mt.CallToolRequestParams]) ->
 
         result = await middleware.on_call_tool(context, hang_forever)
 
+        assert isinstance(result, _FailedToolResult)
+        assert result.to_mcp_result().isError is True
         assert len(result.content) == 1
         text = _get_text(result)
         assert 'slow_tool' in text
@@ -114,6 +119,7 @@ async def test_credential_error_suggests_profile(self):
 
         result = await middleware.on_call_tool(context, call_next)
 
+        assert result.to_mcp_result().isError is True
         text = _get_text(result)
         assert 'expired or invalid AWS credentials' in text
         assert '--profile' in text
@@ -127,6 +133,7 @@ async def test_non_credential_error_no_suggestion(self):
 
         result = await middleware.on_call_tool(context, call_next)
 
+        assert result.to_mcp_result().isError is True
         text = _get_text(result)
         assert '--profile' not in text