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Fix: Correctly detect MCP tool errors #14937

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Merged
scidomino merged 1 commit into from
Dec 11, 2025
+45 −0
Merged

Fix: Correctly detect MCP tool errors #14937

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38 changes: 38 additions & 0 deletions packages/core/src/tools/mcp-tool.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -223,6 +223,44 @@ describe('DiscoveredMCPTool', () => {
},
);

it('should return a structured error if MCP tool reports a top-level isError (spec compliant)', async () => {
const tool = new DiscoveredMCPTool(
mockCallableToolInstance,
serverName,
serverToolName,
baseDescription,
inputSchema,
);
const params = { param: 'isErrorTopLevelCase' };
const functionCall = {
name: serverToolName,
args: params,
};

// Spec compliant error response: { isError: true } at the top level of content (or response object in this mapping)
const errorResponse = { isError: true };
const mockMcpToolResponseParts: Part[] = [
{
functionResponse: {
name: serverToolName,
response: errorResponse,
},
},
];
mockCallTool.mockResolvedValue(mockMcpToolResponseParts);
const expectedErrorMessage = `MCP tool '${serverToolName}' reported tool error for function call: ${safeJsonStringify(
functionCall,
)} with response: ${safeJsonStringify(mockMcpToolResponseParts)}`;
const invocation = tool.build(params);
const result = await invocation.execute(new AbortController().signal);

expect(result.error?.type).toBe(ToolErrorType.MCP_TOOL_ERROR);
expect(result.llmContent).toBe(expectedErrorMessage);
expect(result.returnDisplay).toContain(
`Error: MCP tool '${serverToolName}' reported an error.`,
);
});
Comment on lines +226 to +262
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@gemini-code-assist gemini-code-assist Bot Dec 11, 2025

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high

The new test case for spec-compliant MCP errors only validates the case where isError is a boolean true. To ensure the fix is robust and prevent future regressions, it's important to also test the case where isError is the string 'true', as the implementation correctly handles both. I suggest converting this test to use it.each to cover both scenarios, similar to the existing test for legacy error formats.

    it.each([
      { isErrorValue: true, description: 'true (bool)' },
      { isErrorValue: 'true', description: '"true" (str)' },
    ])(
      'should return a structured error if MCP tool reports a top-level isError: $description (spec compliant)',
      async ({ isErrorValue }) => {
        const tool = new DiscoveredMCPTool(
          mockCallableToolInstance,
          serverName,
          serverToolName,
          baseDescription,
          inputSchema,
        );
        const params = { param: 'isErrorTopLevelCase' };
        const functionCall = {
          name: serverToolName,
          args: params,
        };

        // Spec compliant error response: { isError: true } at the top level of content (or response object in this mapping)
        const errorResponse = { isError: isErrorValue };
        const mockMcpToolResponseParts: Part[] = [
          {
            functionResponse: {
              name: serverToolName,
              response: errorResponse,
            },
          },
        ];
        mockCallTool.mockResolvedValue(mockMcpToolResponseParts);
        const expectedErrorMessage = `MCP tool '${serverToolName}' reported tool error for function call: ${safeJsonStringify(
          functionCall,
        )} with response: ${safeJsonStringify(mockMcpToolResponseParts)}`;
        const invocation = tool.build(params);
        const result = await invocation.execute(new AbortController().signal);

        expect(result.error?.type).toBe(ToolErrorType.MCP_TOOL_ERROR);
        expect(result.llmContent).toBe(expectedErrorMessage);
        expect(result.returnDisplay).toContain(
          `Error: MCP tool '${serverToolName}' reported an error.`,
        );
      },
    );


it.each([
{ isErrorValue: false, description: 'false (bool)' },
{ isErrorValue: 'false', description: '"false" (str)' },
Expand Down
7 changes: 7 additions & 0 deletions packages/core/src/tools/mcp-tool.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -133,6 +133,13 @@ class DiscoveredMCPToolInvocation extends BaseToolInvocation<
}

if (response) {
// Check for top-level isError (MCP Spec compliant)
const isErrorTop = (response as { isError?: boolean | string }).isError;
if (isErrorTop === true || isErrorTop === 'true') {
return true;
}

// Legacy check for nested error object (keep for backward compatibility if any tools rely on it)
const error = (response as { error?: McpError })?.error;
const isError = error?.isError;

Expand Down
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