react codemod agent converted the same class components twice - it had no marker to tell converted files from...

TL;DR: Make the converted state detectable. Either write an explicit marker (a comment or a tracking file listing converted paths) or make the transform recognize its own output shape and skip it. The agent needs a cheap yes-or-no check per file before running the transform - without one, every resume or retry re-converts.

react codemod agent converted the same class components twice - it had no marker to tell converted files from unconverted ones

1. Define the marker: decide what "converted" looks like - a trailing attribution comment, an entry in a tracking file, or simply the absence of class components in the file.

Expected: a rule you can state in one sentence.

1. Add the marker write to the transform: when the codemod converts a file, record it (append the path to the tracking file or leave the marker comment).

Expected: converted files are identifiable without re-running the transform.

1. Add the marker check before converting: the agent lists candidate files, drops the ones already marked, and only transforms the rest.

Expected: a second run over the repo converts zero files.

1. Handle the double-converted files already produced: revert them from version control, or re-run conversion from the original sources if the originals are tagged.

Expected: no file contains the artifacts of two conversion passes.

1. Verify: run the agent loop twice over the full repo and diff the results.

Expected: the second run is a complete no-op.

Use this when

• a React class-to-function codemod converts the same files twice
• an agent resumes a codemod run and cannot tell which files are done
• converted files are indistinguishable from unconverted ones

Not for this skill when

• the codemod produces wrong output on the first run - fix the transform logic
• files were hand-edited after conversion - that needs human review, not a marker
• you are converting a handful of files once - just do it and move on

Variant phrasings

• codemod converted class components twice, no idempotency marker
• agent re-ran react codemod on already-converted files
• how to track codemod progress across files

Why it happens

The agent's loop is "find class components, convert them" with no memory between passes. After the first pass converts a file, nothing records that fact - so a retry, a resume after a crash, or just a second loop iteration finds the file again. Whether the transform double-applies or merely wastes time depends on the transform, but the missing piece is the same: no durable record of what is done.

Edge cases

• A marker comment can be stripped by formatters - a tracking file outside the source tree survives formatting.
• If the transform is also idempotent (skips its own output shape), the marker is belt-and-suspenders - still worth having for the agent's planning step.
• Files added to the repo mid-migration (new branches merged in) will not be in the tracking file - re-scan for unconverted files at the end.
• Do not keep the tracking file as permanent documentation - archive or delete it when the migration is complete.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst__Mn10QoF5PWIG2FsrgOnfA