Pattern 9

Context is a budget, not a constant

Problem. Everything auto-loaded into a session — instruction files, the memory index, skill descriptions, hook strings — costs tokens on every turn, in every session. No single addition is large, so the total grows a few percent a week, and unattended agents spend with nobody watching. Quality erodes before any cost alarm fires.

Pattern. Meter it like money. A baseline counter measures every always-loaded source individually and keeps history. A trend alarm fires when the baseline beats its rolling median by a set margin, because accretion is the common failure, not the blowout. Unattended runs carry hard spend ceilings sized as belts (10–50x a normal cycle), multi-agent fan-outs are bounded by construction, index files carry explicit size ceilings, and a standing note tells the runtime what must survive context compaction.

Why this beats the obvious. The obvious control is a size warning on the main instruction file: one source, one absolute threshold. The real failure is distributed (a dozen sources each growing slightly) and relative (this month versus last), so per-source measurement with trend detection catches what a static ceiling misses. Attribution is the payoff: a total says something grew; the breakdown says what to trim.

Cost. A counter and its history to maintain, estimates that drift from true tokenizer counts, and ceilings that need sizing judgment — a cap set as a governor instead of a belt aborts legitimately heavy runs.

Where it lives: samples/scripts/ghost_token_counter.py (the per-source baseline) and samples/scripts/token_report.py (spend telemetry feeding the audit's trend rule).

Seen in 3 of 8 teardowns · Practise: learn/02