The error codes that may be changing are some of those representing invalid values for API parameters. Notably, the following will change:

"noX", indicating that a required parameter was not specified, becomes "missingparam".
"unknown_X", indicating that an unrecognized value was specified for an enumerated-value parameter, becomes "badvalue".
"too-many-X", indicating that too many values were supplied to a multi-valued parameter, becomes "toomanyvalues".
"baduser_X", "badtimestamp_X", and so on become "baduser", "badtimestamp", and so on.

Note this is not a comprehensive list, other codes may be changing as well.

These changes make the error codes more predictable, at the expense of not indicating in the code which parameter specifically triggered the error. If you have a use case where knowing which parameter triggered the error is needed, please let us know (by replying to this message or by filing a request in Phabricator) and we'll add the necessary error metadata.

The human-readable text is also changing for some of these errors (with or without error code changes), and for a few the error metadata may be changing (e.g. "botMax" changes to "highmax" for limit-type warnings in non-back-compat error formats).

This change will most likely go out to Wikimedia wikis with 1.35.0-wmf.19. See https://www.mediawiki.org/wiki/MediaWiki_1.35/Roadmap for a schedule.

Brad Jorsch (Anomie)
Senior Software Engineer
Wikimedia Foundation