Common signs of invalid base64
Length not a multiple of 4 after removing whitespace. Characters outside A–Z, a–z, 0–9, +, /, =, -, _. Stray quotes or commas from JSON copy-paste.
- Unexpected = in the middle of the string
- data: prefix still attached
- Truncated copy (string ends abruptly)
Fixing invalid base64
base64app auto-repair normalizes whitespace, strips data URIs, converts URL-safe alphabet, fixes padding, and reports each change. Use /repair-base64/ for the dedicated repair workflow.