## TL;DR

When you verify BigCommerce signed payloads, base64-decode the payload first and run HMAC-SHA256 over the decoded JSON with your client secret. Verifying against the still-encoded string never matches.

## Steps

1. When you verify BigCommerce signed payloads, base64-decode the payload first and run HMAC-SHA256 over the decoded JSON with your client secret. Verifying against the still-encoded string never matches. Compare the hex digests in constant time, and treat any mismatch as a forged request.

## When to use

You are seeing this: Verifying against the still-encoded string never matches. Use this skill when you run into "BigCommerce app callbacks: verify the signature against the decoded payload".

## When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

## Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

## Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.