How to Write an Avatar System Prompt That Survives 100 Posts

The first ten posts with a new avatar always look great. The eleventh starts to look slightly different. By post thirty, you're squinting wondering if it's the same character. By post fifty, it isn't.

This isn't a model problem. It's a prompt problem. Your avatar's "system prompt" — the description you reuse to keep the character consistent — is doing too much work that the model can't actually hold across a long session.

What an avatar system prompt actually has to do

It has to lock in three things: identity (who is this character), style (how they look), and behavior (what they do on camera). Most avatar prompts I see lock in one and let the other two drift.

The fix is not a longer prompt. The fix is being explicit about the boundary between identity and style. Identity is "Jeff is a 44-year-old laid-back tech-dad in Montgomery, AL." Style is "short brown hair, backwards cap, hoodie, muted earth tones." Those are different sentences. They go in different sections. Mixing them is what kills consistency.

The structure that holds

I write every avatar system prompt in five blocks, separated by blank lines:

IDENTITY: name, age, occupation, location, summary in one sentence.
APPEARANCE: hair, skin, signature accessories, color palette. Pick five details and never change them.
VIBE: what energy do they carry. "Calm, dry, slightly burnt out" beats a paragraph of adjectives.
WHAT THEY DO ON CAMERA: the actions you'll see. "Sits at a desk, holds coffee, turns to camera, mid-shrug." Specific verbs, not categories.
WHAT THEY DON'T DO: the negative space. "Never points at the camera. Never gives a thumbs up. Never poses."

The one rule that matters

Whatever you write in those five blocks, never edit them mid-batch. If you find a problem on post twelve, write it down, finish the batch, then revise the system prompt for the next batch. Editing mid-stream is what causes the slow drift you hate. The model is consistent — your prompt is the variable.

I've gone over a hundred posts with a single avatar system prompt and the character still looks like the same guy. The reason isn't that the prompt is special. The reason is that I stopped touching it.

— Jeff