Lightweight Express Error Middleware and a TypeScript CLI Probe

Express already has an error handler, and it logs a line. The CLI next to it logs a different line. Neither asks what the developer was doing, and neither leaves a draft. Adding a heavy agent for both paths is more code than the bug.

5xx responses often pass through next(err) or simply finish the response with a status. A logging middleware that only wraps next(err) misses a handler that sent 500 and returned. A CLI probe that only wraps Node exceptions misses exit 137. You want one client with two hooks, both typed.

Aethel.express() returns an Express-compatible middleware: (req, res, next). On finish, status codes of 500 and above are captured as HTTP_<status>. A throw before next is captured as UNCAUGHT_MIDDLEWARE and re-passed to next(err).

Aethel.next() registers uncaughtException and unhandledRejection for the Next.js Node instrumentation hook. nextMiddleware() is the request helper when you already have a middleware chain that returns a response status.

bashnpx --yes @aethellayer/sdk init

Express:

tsimport express from "express";import { Aethel } from "@aethellayer/sdk"; const app = express();const aethel = new Aethel({ apiKey: process.env.AETHEL_API_KEY! });app.use(aethel.express());

Next.js instrumentation:

tsexport async function register() {  if (process.env.NEXT_RUNTIME === "nodejs") {    const { Aethel } = await import("@aethellayer/sdk");    new Aethel({ apiKey: process.env.AETHEL_API_KEY! }).next();  }}

The CLI uses the same package. The constructor sends one start signal when the key is set. That signal does not open an inbox thread. Captured 5xx responses do.