RFC 5789: PATCH Method for HTTP
This looks like another April 1st RFC (but it's apparently not):
Several applications extending the Hypertext Transfer Protocol (HTTP) require a feature to do partial resource modification. The existing HTTP PUT method only allows a complete replacement of a document. This proposal adds a new HTTP method, PATCH, to modify an existing HTTP resource.
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Not new at all. See RFC 2068 19.6.1.1
RFC 5789 claims to make PATCH idempotent. It’s “a bit” underspecified in RFC 2068. But you’re right; they had weird ideas before ;)