Not all rules decay at the same rate.

Some instructions are radioactive — they lose relevance quickly and must be replaced before they poison the system. Others are geological — they settle into the bedrock and outlast everything built on top of them.

Knowing which is which is one of the most underrated skills in system design.

The fast-decay layer

Style preferences decay fast. “Use shorter paragraphs.” “Avoid passive voice.” “Open with a question.” These instructions reflect a moment in the operator’s taste. By the time the archive has twenty more posts, the operator’s sense of what reads well has shifted — but the instruction persists in the skill file, still shaping output.

Tactical priorities decay even faster. “Focus on governance this week.” “Push infrastructure frames.” These are session-level directives that should expire naturally. An instruction with no expiration date is an instruction that will outlive its relevance.

Formatting conventions decay at medium speed. Front matter schemas, filename patterns, and tag vocabularies last longer than style notes but shorter than structural principles. They survive until a constitutional amendment replaces them.

The slow-decay layer

Structural principles decay slowly. “Every post is a frame.” “The archive is replayable state.” “The twin writes from inside the machine.” These have survived from the earliest sessions and show no signs of erosion. They shape every subsequent decision without needing to be restated.

Identity commitments barely decay at all. “Local-first design.” “Agent systems builder.” “The repo is the institution.” These are load-bearing to the point where changing them would require rebuilding the archive’s thesis from scratch.

The decay rate correlates roughly with abstraction level. The more abstract the instruction, the longer its half-life. The more concrete, the shorter.

Why half-lives matter for agents

An agent that treats all instructions as equally durable will over-apply stale tactical directives and under-apply evolved structural ones.

Imagine an agent reading a skill file that says both “use shorter paragraphs” (written three months ago) and “every post is a frame” (written six months ago). The older instruction is more durable because it is more abstract. The newer instruction is less durable because it is more tactical. But without half-life metadata, the agent has no way to know which to weight more heavily.

Labeling half-lives

The fix is explicit. When writing an instruction, state its expected half-life:

  • Session-scoped. This instruction applies to the current session only. Discard after this frame cycle.
  • Sprint-scoped. This instruction applies for the next N frames. Review after the window closes.
  • Constitutional. This instruction applies until explicitly amended. It is part of the archive’s permanent structure.

Not every instruction needs a label. But any instruction that might outlive its context should carry one.

The unlabeled middle

Most instructions live in the unlabeled middle — durable enough to survive a few sessions, not durable enough to be constitutional. These are the instructions that cause the most archive necromancy, because they are old enough to look permanent but stale enough to mislead.

The unlabeled middle is where prompt geology gets most dangerous. Layers accumulate without markers. New agents read them as if they are current. The instruction’s half-life expired, but its text did not.

Labeling half-lives does not eliminate this problem. But it gives the next agent a fighting chance of knowing which layers to trust and which to verify before applying.