Gerrit change 133955, which I plan to merge on Thursday for deployment to WMF wikis with 1.24wmf7, will cause the API to report an error when action=edit is used with the "redirect" parameter and none of "appendtext", "prependtext", or "section=new" are used. This will occur regardless of whether the page being edited is actually a redirect.

API action=edit has a "redirect" parameter, which will automatically edit the target page if the supplied title is a redirect. The original intention of this feature was to be used in combination with appendtext or section=new for posting notifications to user talk pages without having to first check whether the page is a redirect (see bug 24330). But as currently implemented it will perform the edit even when replacement page text is supplied, which is extremely prone to error: while reviewing the patch proposing this change, I reviewed several scripts and bots that were logged as submitting API requests with this combination of parameters and every one would make erroneous edits when the target page is a redirect.

Clients not using the "redirect" parameter to action=edit or clients that use it in combination with "appendtext", "prependtext", or "section=new" will not be affected by this change. Affected clients have several options for coping with this change, such as:

Stop supplying the "redirect" parameter to action=edit, which will edit the redirect page itself.
When fetching the page content to transform it for the edit, use the "redirect" parameter to action=query and submit the edit to the target page directly (and without using the "redirect" parameter to action=edit) rather than via the redirect title.
Use appendtext, prependtext, or section=new instead of loading the page's wikitext and appending or prepending content on the client side.
Detect the redirect when fetching the page content and treat the unexpected redirect as an error condition. Or handle the error returned by action=edit in whatever manner is appropriate to your script, bot, or tool.

See https://www.mediawiki.org/wiki/MediaWiki_1.24/Roadmap for the deployment schedule.

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Brad Jorsch (Anomie)
Software Engineer
Wikimedia Foundation