## TL;DR

If your automation marks DAG runs via the API on 3.3.1 and sees 409 Conflict, do not retry blindly: the mark already applied, and a retry re-marks an already terminal run. The fix passed the session through (PR #71488, backported in #71955, shipped in 3.3.2rc1), so upgrade to 3.3.2 or later.

## Steps

1. If your automation marks DAG runs via the API on 3.3.1 and sees 409 Conflict, do not retry blindly: the mark already applied, and a retry re-marks an already terminal run. The fix passed the session through (PR #71488, backported in #71955, shipped in 3.3.2rc1), so upgrade to 3.3.2 or later. On a pinned 3.3.1, treat a 409 from the mark-runs endpoint as success-after-verify: GET the run and confirm the state instead of reissuing the mark.

## When to use

Use this skill when you run into "Airflow 3.3.1 marking DAG run success returns 409 Conflict even though it applied".

## When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

## Versions

Versions mentioned in the source: 3.3.1, 3.3.2. If you are on something much newer or older, the details may have shifted.

## Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.