When a Base64 payload will not decode.
Unexpected characters
Check whether your API uses Base64url. Do not paste surrounding JSON quotes, a complete JWT, or a data-URL prefix. If the value was line-wrapped, explicitly enable Ignore whitespace. That option removes whitespace from encoded input only.
Padding and length
A padded value has a length divisible by four, with zero, one or two trailing equals signs. Unpadded Base64 can be accepted explicitly; a remainder of one is impossible. Adding padding cannot reconstruct missing data.
Non-zero unused bits
The final character may include bits that are not part of the decoded bytes. RFC 4648 requires those bits to be zero for canonical encoding. This tool flags noncanonical values instead of silently treating several strings as identical.
Valid Base64, wrong content
A successful Base64 decode does not prove a value is complete, valid JSON or trustworthy. Some truncations still form valid Base64. Invalid UTF-8 may indicate binary content or a different charset. JSON errors apply only to the formatted view; raw decoded text remains available.