Music Router prompt limit: 5,000 characters, and what to put in them

Short answer

A Music Router prompt can be 1 to 5,000 characters. That is far more than most briefs need, and the useful question is not how to fill it but what has to be in it, because the request has no other controls. Per the Music Router docs there is no duration field, no seed, no guidance setting and no accepted negative prompt.

Where each control lives

Everything that shapes the track goes in the one text field: length, structure, instruments and exclusions. The docs list three rules.

• Length: write it in the prompt, for example "a 2-minute track" or section markers such as [0:00-0:30] Intro: .... Sending duration or duration_seconds is rejected.
• Exclusions: put them in the positive prompt, for example "Instrumental, no vocals." A non-empty negative_prompt returns HTTP 400 with public_reason=negative_prompt_unsupported.
• Image: image_url is optional and must be a public HTTPS image.

What a good brief contains

The music brief template in the docs uses seven axes: emotion, genre or lineage, tempo as a number, key and mode, two to four instruments with texture, an arc with one named moment, and era or production. The docs call them creative directions, not guaranteed output values, so listen to the result.

A seven-axis brief with timed sections is usually 400 to 900 characters. The script below counts a draft and flags the two things the API refuses, so you catch them before you submit. The limit is checked on the character count of the prompt; the counter uses Python's len, which matches for ordinary text but counts some emoji differently from the API.

prompt = (
    "Hushed, slightly melancholic neo-soul, 72 BPM, D minor. "
    "Rhodes through tape wow, soft sub bass, brushed snare, muted trumpet. "
    "[0:00-0:12] Sparse Rhodes only. [0:12-0:30] Trumpet answers, bass enters. "
    "A 30-second track. Instrumental, no vocals."
)
payload = {"model": "sume/music-auto", "prompt": prompt}

problems = []
if not 1 <= len(prompt) <= 5000:
    problems.append(f"prompt length {len(prompt)} is outside 1-5000")
for banned in ("duration", "duration_seconds"):
    if banned in payload:
        problems.append(f"remove {banned}: write the length in the prompt")
if payload.get("negative_prompt"):
    problems.append("remove negative_prompt: write exclusions positively")
print(len(prompt), "characters;", problems or "no problems found")

Which model id

The router resolves sume/music-auto to an engine, Lyria 3.5 today, or you can pin lyria-3.5 or lyria-3-pro from GET /v1/music-router/models. An unknown model fails with 400 model_not_found and a catalog_url. Every request resolves through the router now, including those sent to the older Music 1.0 route.

After a rejection

If a prompt is rejected for policy, change the flagged content and keep the musical brief; the docs advise against flattening it into a generic bed. Retry only inside the budget you authorized, since each generation is billed.

What fits in 5,000 characters

Most music briefs are far shorter than the limit. A brief in the docs example style, with genre, tempo, key, instruments, one named moment and a no-vocals line, is a few hundred characters. The 5,000 limit starts to matter when you write a timed structure with a line per section, such as an intro, a build and an ending, each with its own time range.

Timed sections are the supported way to control length, since duration is rejected. A 2-minute track can be written as a 2-minute request in the prompt, or as time ranges like [0:00-0:30] Intro. Eight such lines at 150 characters each use 1,200 characters, well inside the limit.

Because the limit is a character count, count what you send, spaces and punctuation included, before you submit rather than after a rejection.

• Check the length in code: len(prompt) must be between 1 and 5000.
• Put exclusions in the positive prompt, since negative_prompt is rejected.
• Long is not better; add a line only if it changes what you hear.