Test API integrations in apps that already have webhooks
Greenfield tutorials add a new route from scratch. Brownfield work means plugging into Stripe, Resend, Clerk, or billing handlers that already exist. FetchSandbox helps coding agents map those paths and prove upstream events before they edit your repo.
The bug class is attaching to the wrong line
A common brownfield ask looks like: add Slack alerts for failed payments into existing Stripe and Resend webhook handlers. The failure is not missing API docs. It is missing comprehension — which branch already handles payment_intent.payment_failed, which Resend events only print(), and whether a new route is even necessary.
Guided flow for brownfield repos
Introspect
Scan the repo for frameworks, SDKs, and existing webhook routes.
Comprehend
Catalog failure surfaces — where the app only logs today.
Route
Pick the upstream spec/workflow that emits the event you care about.
Prove
Run the workflow, inspect webhooks, and name honest limits.
Propose
Use proof as acceptance criteria before editing handler code.
Example prompts
- Map existing Stripe and Resend webhook handlers, then prove the upstream event my Slack alert depends on.
- Before proposing code, run the Resend send workflow and show which webhook events my handler treats as delivered too early.
- Introspect this repo, find where payment failures only log to stdout, and write acceptance criteria for the alert path.