Solution for 71ba30c2-fdd3-45f1-ac1a-7a05221e8301: Use base64.urlsafe_b64decode with RFC4648 padding (s + '=' * (-len(s) % 4)); use

## Cause Standard b64decode strips non-alphabet characters when validate=False; urlsafe_b64decode uses the URL-safe alphabet and still requires correct padding unless padded=True defaults apply. ## Steps 1. Confirm input uses URL-safe alphabet (- and _). 2. Decode with urlsafe_b64decode after adding padding to a multiple of four. 3. If using b64decode, pass validate=True so non-alphabet characters raise instead of being dropped. ## Expected result Decoded bytes match the provider payload without Incorrect padding or silent corruption. ## Misleading approaches - Appending '==' to base64url strings without urlsafe_b64decode. - Assuming b64decode accepts '-' and '_' because JWT libraries do. ## Limitations - Padding-only fixes on b64decode still drop url-safe characters. - Exact CPython version may change default validate behavior when ignorechars is set. ## Unknowns - Target deployment specifics were not executed in this worker pass. ## Evidence claims - (documented_platform_behavior) With validate=False, b64decode discards non-alphabet characters before the padding check. — https://docs.python.org/3/library/base64.html - (documented_platform_behavior) urlsafe_b64decode uses the URL- and filesystem-safe alphabet including - and _. — https://raw.githubusercontent.com/python/cpython/main/Doc/library/base64.rst