Does sume/auto pick a different video model on a retry? No: same route

No. A replay of the same sume/auto request with the same Idempotency-Key returns the original job, and the model choice cannot flip on a retry: Sume resolves it as a pure function of the normalized request and the catalog version. The same body gets the same price and the same route.

The poll response reports "model": "sume/auto", and Sume does not disclose which family served the request.

When to pin and when to leave it on auto

Use sume/auto when you care about getting a good clip within the supported controls and do not need a named model. Pin a catalog id such as seedance-2.5 or wan-3.0 when you need a clip over 15 seconds, a specific reference-input set, or the same look across a series.

The auto create controls default to 720p and 8 seconds, with 3-10 second clips at 16:9 or 9:16. A 30-second job is a pinned-model job.

A replay test

Send the same request twice with one key and compare the ids. They should match, and the second call should not add a charge.

import os, requests
H = {"Authorization": f"Bearer {os.environ['SUME_API_KEY']}",
     "Idempotency-Key": "auto-replay-demo-001"}
body = {"model": "sume/auto", "prompt": "A vertical product clip, natural light",
        "aspect_ratio": "9:16", "duration": 5}
ids = []
for _ in range(2):
    r = requests.post("https://api.sume.com/v1/videos", headers=H,
                      json=body, timeout=30)
    r.raise_for_status()
    ids.append(r.json()["id"])
print(ids, ids[0] == ids[1])

Do not infer the family

Sume says not to use traits of an output to guess which family ran. If the family matters to your product, pin the model id.