StepAudio 3 Music preview is free, then retired: keep your API stable

StepFun's StepAudio 3 Music is exposed as stepaudio-3-music-preview on an asynchronous HTTP task API (POST /v1/audio/music), billed free for a limited time. The model page says the preview name is used during the free trial, the preview version is retired when the trial ends, and a paid version is added. So the practical rule is: keep the model id out of your code, and plan for the id and the price to change on a date you do not control. On Sume the stable id is sume/music-auto, described in the Music Router docs.

What exactly is changing on StepFun's side?

Two things, per the page: the model name (a formal paid version replaces the preview) and billing (free to paid). The page does not give a date, so do not hard-code one. Treat any pipeline that depends on free output as one that will need a budget line later.

Which Sume ids stay stable?

The Music Router docs say sume/music-auto is the default and Sume picks the engine (Lyria 3.5 today); you can also pin lyria-3.5 or lyria-3-pro from GET /v1/music-router/models. The older sume/music-1.0 is retiring gradually: its routes keep working and keep job.model = sume/music-1.0, but every request resolves through the router, and new integrations should call POST /v1/music-router/generate.

How do I keep my own integration stable?

Store the id in config. Read job.request.routed_model if you need to know which engine ran; on Sume job.model echoes the id you requested. Pin an explicit engine only if you need that engine and accept that it may leave the catalog; unknown ids fail with 400 model_not_found and a catalog_url. For the same advice about another vendor's retired models, see Suno old models retired.