The 4 reasons a character slips
A character breaks role because the conversation grew past its memory, because your message asked the assistant rather than the character, because the scene had no situation to hold it, or because a filter interrupted. The first 3 are yours to fix and the fourth is not.
The break usually reads as the model losing interest. It is more mechanical than that, and 3 of the 4 causes have a direct remedy.
The description fell out of memory
The character sheet sits at the top of the conversation, and a long conversation pushes it out of view. The character then drifts towards a neutral voice, which is exactly what the model does with no instruction. Restating the sentence every 20 or 30 messages costs nothing and prevents this.
You addressed the assistant, not the character
A question such as can you make him angrier speaks to the system about the character. The reply arrives from the system, and the scene has ended. Writing the direction inside the scene, or in a bracketed aside, keeps the frame intact.
The scene had no situation
A character with nothing to do falls back on chat. Scenes hold together when there is a place, a moment and something at stake, and they dissolve into conversation when there is not. This is why a strong opening line does more for consistency than a strong description.
The filter interrupted
The fourth cause looks different: the voice switches abruptly to a formal register and refers to itself. That is not the character slipping; it is a different layer speaking. Recognising it saves you from rewriting a description that was never the problem.
Before you start
Does telling it to stay in character work?
As a reminder, yes, and it does not restore a description that fell out of memory. Repeating the description works better than repeating the instruction.
Why does it slip more in long sessions?
Because the budget fills. This is the same mechanism that makes any long conversation forget its opening, and roleplay simply notices it sooner.
Is a shorter description more stable?
Yes, measurably. A single sentence survives being repeated; a page does not get repeated at all.
Open a scene with a place, a moment and something at stake.
Start a character chat