What to run
Open JSON Formatter. It stays in the browser.
How to read the result
Formatted output means it parsed.
The error line is the first break: trailing comma, single quotes, or a comment.
Pretty-print does not mean the schema is correct.
What to open next
Open JWT Decoder if it came from a token. It stays in the browser and does not verify the signature.
Open Text Diff against the last known-good body. It stays in the browser.