`call_tool_chain` return value is always empty in MCP bridge; data only accessible via `console.log`

[cmtonkinson]

Bug Report

Summary

When using call_tool_chain via the @utcp/code-mode-mcp MCP bridge, the result field in the response is always empty ({}). Data returned via return statements inside the tool chain is silently dropped. The only way to retrieve output is by using console.log(JSON.stringify(...)) and reading from the logs array.

Environment

• @utcp/code-mode-mcp: 1.2.0
• MCP client: Claude Desktop (claude-ai/0.1.0, protocolVersion 2025-11-25)
• Node.js: v24.14.0
• npm: 11.9.0
• macOS (arm64)
• Underlying MCP server: mcp/github via Docker (stdio transport)

Steps to Reproduce

1. Configure @utcp/code-mode-mcp as an MCP server in Claude Desktop with a valid .utcp_config.json
2. Invoke call_tool_chain with a code block that returns a value:

(async () => {
  const repos = await github.github_search_repositories({ query: "user:cmtonkinson", perPage: 5 });
  return repos;
})()

3. Inspect the MCP tool response

Actual Behavior

The MCP tool response contains:

{
  "success": true,
  "nonMcpContentResults": {},
  "logs": []
}

The result / nonMcpContentResults field is empty. The return value is silently discarded.

Workaround

Using console.log(JSON.stringify(result)) instead of return result causes the data to appear in the logs array:

(async () => {
  const repos = await github.github_search_repositories({ query: "user:cmtonkinson", perPage: 5 });
  console.log(JSON.stringify(repos));
})()

Response:

{
  "success": true,
  "nonMcpContentResults": {},
  "logs": ["{\"total_count\":11,\"items\":[...]}"]
}

Expected Behavior

Per the README documentation:

const { result, logs } = await client.callToolChain(`
  const repos = await github.search_repositories({ query: "user:example" });
  return repos;
`);
console.log(result); // should contain the returned value

The result field should contain the return value of the executed code block.

Additional Notes

• The bug appears to be in the MCP bridge layer (code-mode-mcp) specifically — the underlying @utcp/code-mode TypeScript library likely serializes result correctly, but it is lost when the MCP tool response is constructed.
• Top-level await also does not work inside call_tool_chain despite documentation suggesting it should — an explicit async IIFE wrapper is required.
• Related: issue Example from readme incorrect #7 notes several other discrepancies between the README examples and actual behavior.

[h3xxit]

Good catch. Feel free to make a PR for the fix.

Or @universal-tool-calling-protocol/contributors if anyone else wants to take this