# Reference — facts worth keeping

Durable information surfaced in conversation that would otherwise be lost: measured numbers, external
constraints, how a tool actually behaves, links worth having. One file per subject.

Not decisions (`docs/decisions.md`), not proposals (`ideas/`), not the present state of the code
(`docs/state.md`). A fact only true today belongs in `state.md`; a rule belongs in the doc owning the
topic.

Each file states **how the fact was established** — measured, read from a spec, or asserted — so a
later reader knows whether to trust it or re-check it. An unattributed number is a rumour.

Scratch code illustrating a fact is welcome; `verify.sh` ignores `.go` here entirely, so it need not
compile.

Out of agent context by default, this index included. Opened only when the human names the topic.

## Index

- [agent-session-costs.md](agent-session-costs.md) — how context and token cost accumulate in an agent session