Decay & Tiers

Facts don't accumulate forever. They strengthen with use and fade without it — measured deterministically by counting session files, never by a similarity score. Every agent, on every vendor, reaches the same result.

The tiers

working → active → archive-candidate → archived
                                          (+ superseded, core)

Tier	Meaning
working	Recently created or touched; in active rotation
active	Used steadily; the healthy middle
archive-candidate	Going stale; the next review may retire it
archived	Moved to memory/archive/ with a greppable INDEX.md — never deleted
superseded	Terminal: reversed/false, with a superseded-by link (change, not disuse)
core	Never decays — invariants, the Vision (periodically re-verified by a human)

The key quantity is sessions_since_last_used: how many session files have been written since a fact was last referenced. Compare it against the windows below to decide the tier.

Tunable windows

All windows are counted in sessions, not days — integers only. Defaults (this repo's own memory/decay-policy.md):

Parameter	Default	What it controls
working_window	3	Within this many sessions of last use → working
active_window	8	Beyond working but within this → active
archive_window	20	Beyond active but within this → archive-candidate; past it → archive
review_every	10	Run a review at least every N sessions
continuity_max_facts	30	Primary lean signal — decaying facts/threads before a review is advised
continuity_max_lines	600	Coarse backstop on continuity.md size
verify_invariants_every	40	Sessions between human re-confirmations of core / invariants

Why these numbers

The defaults were retuned in v4.24.0 from real measurements across two enabled repos. continuity_max_facts is the primary signal because a count is immune to verbosity and velocity; continuity_max_lines was raised 300 → 600 because a healthy mature layer already sits ~450–600 lines. See Decay Parameters to tune them.

What never decays

• Anything in tier core.
• Anything under ## Architectural Invariants.
• Unchecked Open Threads (- [ ]) — they are pinned by being unchecked. Their pinned-ness is the protection, not the tier label, so the tooling leaves a pinned thread's tier alone (it still refreshes the factual uses / last_used).

Running it deterministically

The mechanical steps of a review are packaged as runnable helpers so a capable agent can't silently skip them:

• refresh-metadata recomputes every fact's tier / uses / last_used from the ledger.
• archive-fact performs the archive move safely.
• memory-lint verifies the result and flags a review that has gone overdue or a layer that has grown past budget.

Deciding what to retire stays the agent's judgment. See Review Memory for the full ritual, and DECAY.md for the authoritative rules.