Fixing "No signatures found matching the expected signature" in Stripe webhooks

This exact error string sends people to Stack Overflow constantly: No signatures found matching the expected signature for payload. It's Stripe's constructEvent telling you the HMAC it computed doesn't match the v1 value in your Stripe-Signature header. Almost always one of four things:

1. Wrong secret. Each endpoint (and each of live/test mode) has its own whsec_.... Pasting the secret from a different endpoint, or from test mode into a production handler, is the single most common cause.
2. Body already parsed as JSON before verification. express.json() or similar middleware re-serializes the body — different key order or whitespace — before your webhook route sees it. Stripe signs the exact raw bytes it sent, so you need the untouched raw body on that specific route.
3. A proxy or CDN normalizing the body. Re-encoded gzip or rewritten line endings change the byte length. Compare what your handler receives against the length in Stripe's Dashboard webhook attempt log.
4. Checking v0 instead of v1. The header carries both; Stripe's own SDKs verify v1.

Instead of guessing which one applies to you, I built a tool that does the actual HMAC-SHA256 computation in your browser (Web Crypto API, nothing sent to any server) and tells you exactly where the mismatch is:

Try the signature validator

Paste your raw payload, your Stripe-Signature header, and your signing secret — it recomputes the signature client-side and shows you whether it's a secret mismatch, a missing v1, or something else.

If it points at the fix, that's it, you're done for free. If you want the other 6 checks we run before calling a Stripe integration production-ready (idempotency, Checkout session edge cases, Charges vs PaymentIntents), that's a one-page checklist: 7-point checklist, $2.