@contextio/logger

Capture-to-disk plugin for @contextio/proxy. Writes every LLM API request/response pair as a JSON file with atomic writes (write to .tmp, then rename).

Install

npm install @contextio/logger

Usage

import { createProxy } from '@contextio/proxy';
import { createLoggerPlugin } from '@contextio/logger';

const logger = createLoggerPlugin({
  captureDir: '~/.contextio/captures',  // default
  maxSessions: 20,                       // prune old sessions on startup; 0 = keep all
});

const proxy = createProxy({
  port: 4040,
  plugins: [logger],
});

await proxy.start();
console.log(`Captures written to ${logger.captureDir}`);

Capture format

Each file is a complete request/response pair:

claude_a1b2c3d4_1739000000000-000001.json

Filename: {source}_{sessionId}_{timestamp}-{counter}.json

{
  "timestamp": "2026-02-15T20:50:00.815Z",
  "sessionId": "67bb9e8f",
  "source": "claude",
  "provider": "anthropic",
  "apiFormat": "anthropic-messages",
  "targetUrl": "https://api.anthropic.com/v1/messages",
  "requestBody": { "model": "claude-sonnet-4-20250514", "messages": ["..."] },
  "responseStatus": 200,
  "responseIsStreaming": true,
  "responseBody": "data: {\"type\":\"content_block_delta\",...}",
  "timings": { "total_ms": 2002 }
}

Session retention

When maxSessions is set, the plugin groups captures by session ID on startup and removes the oldest sessions, keeping only the most recent N.

License

MIT