Prompt length limits across Firefly and Sume

Sume documents one hard prompt cap: music prompts take 1 to 5000 characters. Image prompts must be a non-empty string and the video docs list prompt as a required string; neither page states a character limit. Adobe's September 2026 Firefly update says longer prompts are now allowed for Image and Video on the web.

What Firefly changed

Adobe's announcement says Firefly now supports a longer prompt length for Image and Video on web, and that credit usage can drop when you fit a complete idea into one prompt instead of iterating through several short ones. The post as read does not give a character count for the new limit, so this article does not quote one.

What Sume documents per model family

The table lists only what the Sume docs state. A blank means the docs name no limit, not that no limit exists.

Length and cost on Sume music

A long music prompt does not cost more. The music docs state a fixed $0.125 per accepted generation, and the price does not vary by prompt length or optional image conditioning. That makes a detailed brief free of extra cost, which is the opposite of the incentive Firefly describes for credits.

The music docs also recommend a structured brief: emotion, genre, tempo as a number, key, two to four instruments, an arc with one named moment, and era. Seven short clauses fit well inside 5000 characters.

Why one complete prompt usually wins

Longer prompts help when a scene has several constraints: camera move, lighting, wardrobe, product placement and mood. Splitting those across three short prompts and three generations multiplies cost on any per-generation model, so it is worth writing the complete brief once, as Adobe suggests for credits.

The same advice has a limit. Models can ignore details that sit deep in a long prompt, and the Sume docs describe music direction axes as creative directions, not guaranteed settings. Verify the output instead of assuming that every clause was honored.

A practical rule for clients

Do not rely on a limit that is not documented. For image and video, validate length on your side against a number you choose, and handle a 400 response from the API if a request is rejected. For music, reject anything over 5000 characters before you submit.

• Music: enforce 5000 characters client-side
• Image and video: keep prompts to what the scene needs and handle 400 errors
• Re-read the model page when a vendor announces a longer limit