A Tiny LLM Request Recorder I Use to Reproduce Production Failures

Most LLM failures are easy to describe and surprisingly hard to reproduce. A user reports that the model returned an empty answer. A tool call disappeared halfway through a stream. One provider rejected a request that worked everywhere else. Then I open the logs and find something like this: LLM request failed: 400 Bad Request Technically true. Operationally useless. The missing piece is usually the exact request shape: model, parameters, message roles, tool definitions, timeout behavior, and the raw provider response. I wanted something smaller than a full observability platform, so I built a request recorder around fetch . It stores enough information to inspect or replay a failed call without logging the API key. What the recorder captures For each request, I want: a unique request ID timestamp and duration URL and model sanitized request body HTTP status raw response body network or timeout errors I deliberately do not record the Authorization header. Prompt content is also redacted by default. Full payload capture must be enabled explicitly because storing production prompts can create a much worse problem than the bug being investigated. The recorder This example runs on Node.js 18 or newer and has no external dependencies. Create recorded-fetch.mjs : import { randomUUID } from " node:crypto " ; import { mkdir , writeFile } from " node:fs/promises " ; import path from " node:path " ; function sanitize ( value , captureContent ) { if ( Array . isArray ( value )) { return value . map (( item ) => sanitize ( item , captureContent )); } if ( ! value || typeof value !== " object " ) { return value ; } const result = {}; for ( const [ key , child ] of Object . entries ( value )) { const normalizedKey = key . toLowerCase (); if ( normalizedKey . includes ( " api_key " ) || normalizedKey . includes ( " apikey " ) || normalizedKey . includes ( " authorization " ) ) { result [ key ] = " [REDACTED] " ; continue ; } if ( ! captureContent && ( normalizedKey === " content "