Before-Save Flow (API v66+) Throws INVALID_FIELD_FOR_INSERT_UPDATE on Field Update When Called from an Unsupported API Version

You may observe one or more of the following:

• Before-save record-triggered flows (v66+) throw the error: Invalid target field for field update (errorCode: INVALID_FIELD_FOR_INSERT_UPDATE) when updating a field via a REST API caller
• The error occurs during record updates via a REST API caller using a version older than when the field was introduced, but not during:
  • Record inserts (creates) via the same integration
  • Manual UI updates
  • REST API calls using a version equal to or newer than the field's introduction version
• The issue began after upgrading the flow to API version 66 (Spring '26)
• Reverting the flow API version to 65 or below resolves the error
• The flow executes successfully in-memory; the error is thrown at the platform commit layer post-flow execution
  
  

Root Cause

Prior to Flow API v66, before-save flows could update fields regardless of whether those fields were accessible in the calling client's API version. This was a platform-level security/access bug.

Flow API v66 (Spring '26) introduced a fix that added an explicit check: if a field is not accessible in the API version used by the calling client, the platform will reject the write at the commit layer with INVALID_FIELD_FOR_INSERT_UPDATE. The fix is API-versioned, meaning it only applies when the flow is running at v66 or higher.