What Telltale does
Telltale is an API drift and schema detection Chrome extension. It quietly watches the fetch and XMLHttpRequest traffic your web app already makes, infers a schema for each endpoint from the responses it observes, and alerts you the instant a response stops matching what it has learned — a missing field, a changed type, an unexpected null, a new status code or an unfamiliar field. There's nothing to write up front: install it, browse normally, and it learns your contract as you go.
How API drift detection works
Most API breakage is caught late — in a failing UI, a customer report, or a brittle test. Telltale catches it the moment it happens in the browser tab:
- Learn — it observes real responses and infers endpoint schemas automatically, no pre-written spec required.
- Detect — confidence-gated drift detection flags missing fields, type changes, unexpected nulls, new status codes and new fields.
- Export — turn the learned contract into TypeScript types, Zod schemas, JSON Schema or OpenAPI.
- Compare — environment-aware comparison surfaces contract differences across staging and production.
Local-first and privacy-aware
Telltale runs entirely in your browser. Traffic is analyzed locally — nothing is sent to a backend — and heuristic PII redaction helps keep sensitive values out of the inferred contracts.
Who it's for
Frontend and QA engineers who want to know the instant an API contract shifts, without standing up test infrastructure or waiting on backend involvement. It pairs well with API tests built from real traffic in XHRScribe and code-level integration tests from APItiser. Free on the Chrome Web Store; works in any Chromium browser.
FAQ
Do I need to write a schema or spec first? No — Telltale infers the contract from observed traffic with zero configuration.
Does my API data leave the browser? No — it's local-first, with heuristic PII redaction on the contracts it builds.
How is it different from XHRScribe? XHRScribe turns captured traffic into runnable API tests; Telltale continuously watches traffic and alerts you when a response drifts from the learned contract.
Is it free? Yes, free on the Chrome Web Store.