Transform Responses

Tool middleware in mcpose wraps request execution. A small function in the middle can inspect results and inject supplementary context.

Transforming Output

To augment tool results, call next(req) and return a new result object with modified content:

import type { ToolMiddleware } from 'mcpose';

export const withSource: ToolMiddleware = async (req, next) => {
  const result = await next(req);
  return {
    ...result,
    content: [
      ...result.content,
      {
        type: 'text',
        text: 'Source: internal docs',
      },
    ],
  };
};

Configuring the Proxy

Pass the middleware function to the toolMiddleware array in ProxyOptions:

import { createBackendClient, startProxy } from 'mcpose';
import { withSource } from './middleware/with-source.mjs';

const backend = await createBackendClient({
  command: 'node',
  args: ['./upstream-server.mjs'],
});

await startProxy(backend, {
  toolMiddleware: [withSource],
});

When a client calls any tool, withSource runs after the upstream server returns. The client receives the upstream data alongside the injected source annotation.

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Rejection reasons

PII redaction + audit