The Genre Prior

You put "-pedal steel" in the Exclude field on a modern country prompt. Pedal steel showed up anyway. Both times Red Lab ran it. Meanwhile "no guitar" on dark synthwave held in four runs out of four. Same kind of instruction, opposite results, and the difference isn't how firmly you typed it.

Sunday's newsletter covered the rewrite: under v6, your style prompt can be rewritten before the model generates, and your instruction can vanish in the expansion. This post is the other half. What happens to the instruction that survives?

The prior underneath the prompt

It lands on ground that isn't neutral. Every genre word carries a prior: the model's default expectation of what that genre contains. Pedal steel for modern country. Drums under the verse for arena rock. Gated toms for anything labeled 80s. Name the genre and you've loaded the whole picture, and your surviving instruction gets negotiated against it.

The principle that came out of v6 testing is one sentence: an instruction binds in proportion to how weakly the genre's default opposes it. So the question to ask before writing a constraint isn't whether it's phrased positively or negatively. It's whether the genre you just named wants the opposite.

Evidence, sized honestly

Here's the full size of it, because I'd rather show it than round it up. Three Red Lab cases: "no guitar" on dark synthwave held four of four, "-pedal steel" on modern country failed two of two, and "no drums until the chorus" on arena rock failed two of two, with verse transient density measuring 81 and 87 percent of chorus density and the first drum hit landing inside the first vocal bar. Two independent field cases show the same shape: an 80s prompt returned gated tom fills through three different phrasings of "no gated drums," and a seven-genre tester found timed instructions mostly honored, except where the genre opposed them. In every case logged, the constraint lost on the genre's most defining element. That's an observed pattern at small counts, not a law, and if your sessions contradict it, I want the receipts.

The prior bends the singer too. Field reports found rock prompts returning a country-inflected voice regardless of settings, with a Custom Model holding the vocal character the prompt couldn't. If your voice keeps drifting toward the genre's default singer, more adjectives in the vocals line won't move it.

What actually moves it

Four fixes, in the book's order of reliability. Move the constraint into the songsheet as a section tag, because timed and structural instructions belong in the lyrics field, not the style field; the evidence here is suggestive rather than controlled, and the book says so, but the songsheet is the surface built for structure and it's the route taught first. Encode the absence positively: "solo acoustic guitar and upright bass only, every section" gives the model a smaller machine to operate, instead of a banned part it keeps reaching for. Change the genre word, because the prior comes with the label; if "modern country" pulls pedal steel, "acoustic Americana" may not. And use the Exclude field anyway, as a pressure valve rather than a lock: it's visible, it costs nothing, and on an element the genre doesn't insist on, it will likely hold.

The full constraint hierarchy, including how the prior interacts with the rewrite from Sunday's piece, is Chapter 6 of Unlock Suno: The Complete Guide, v6 Edition.

The working rule fits in one line. Before you write a constraint, ask whether the genre you just named wants the opposite. When it does, the fix is never a louder adjective. Move the structure to the songsheet, hand the model a smaller machine, or change the label the prior rides in on.

Josh, Founder, JG BeatsLab