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+# ADR-0003: cost accounting — instrument and attribution model
+
+status: accepted
+date: 2026-07-31
+tier: L (structural L, chaos d10=2 → no override)
+research: [CB-RES-0002](../research/CB-RES-0002-cost-accounting.md) (approved)
+review: history/260731-cost-accounting-{challenge,response}.md
+supersedes: the "uncomputable" disposition of AM-12 / M-D2-CST in
+[CB-EV-0001](../evidence/CB-EV-0001-game-kernel.md)
+
+## Context
+
+M-D2-CST was fully specified in CB-WP-0001 and never instrumented, so D2
+claims about implementation efficiency rest on nothing measured. The survey
+established that the data existed the whole time, and that reading it
+correctly is harder than it looks: this workplan's own opening figure was
+wrong by 2.7×, and the survey's first draft set an acceptance target only a
+*broken* collector could hit.
+
+## Decision
+
+### D1 — Instrument: session transcript JSONL (C1)
+
+Cost is computed from `~/.claude/projects/<slug>/**/*.jsonl`, deduplicated
+by `requestId`, priced per message against
+`benchmarks/baselines/model-prices.toml`.
+
+Rejected: the State Hub as a *source* (its schema has no cache fields, so it
+cannot represent 88% of spend, and its recorded numbers are ~100% in error
+against the same work); the status bar (not machine-readable from a tool
+call); the billing API (no session or task attribution, no admin key). The
+hub remains the durable **sink**; the billing API remains an optional
+external reconciliation check if an admin key ever exists.
+
+### D2 — Attribution: git commit intervals, scoped by session
+
+A message is attributed to the task named by the **next commit at or after
+it**, within its own session:
+
+```text
+interval := (prev_commit_time, this_commit_time]   # ending-at-commit
+scope    := sessionId                              # never wall-clock alone
+task     := the T## tag in the commit subject, else UNATTRIBUTED
+```
+
+Ending-at-commit is the only convention consistent with the loop's
+commit-at-end-of-task pattern; the alternative shifts every task's cost one
+interval. Session scoping is not optional: two sessions overlap 4 h 13 m on
+this repo carrying ~$12 that no wall-clock join can separate.
+
+Rejected: **explicit session markers** at task start/end — they require the
+agent to remember, and the failure this workplan exists to fix was caused by
+exactly that kind of remembered step; nothing that depends on discipline
+gets to be the primary index. Rejected: **hub status transitions** as the
+time index — they are written after the fact, sometimes in a batch, and one
+was written for CB-WP-0001 nine minutes after the work it bounds.
+
+### D3 — Reported shape: composition, not a total
+
+Every report carries the cost split by component (input / output / cache
+read / cache write per TTL) alongside the total. A single total would have
+concealed the finding that motivated the workplan.
+
+## Expected advantage per dimension
+
+| Dim | Expectation vs the C2 baseline | Basis |
+|---|---|---|
+| **D1 ease of specification** | **worse.** The hub is one API call; this is a parser with a dedup rule, a TTL-aware price model, a session scope, and a commit join. Four moving parts against one. | measured: the survey needed three corrections to get the parse right |
+| **D2 efficiency** | **better, decisively.** C2's recorded numbers are ~100% in error on the same work ($0.03-equivalent recorded against $92.21 actual). C1 is exact by construction. Cost to produce: one file read, ~1 s. | measured |
+| **D3 speed** | **parity.** 2,040 lines / 5.1 MB parsed in <1 s; a hub call is a network round trip. Neither is a bottleneck. | measured |
+| **D4 optionality** | **better.** The transcript is a file on disk in a documented shape; the hub is a service that must be running. The collector degrades to "no data" rather than "wrong data" when a transcript is absent. | reasoned |
+
+An honest summary: **we are buying accuracy with specification complexity.**
+That trade is right here only because the alternative is not "a simpler
+correct number" but "a number that is wrong by two orders of magnitude",
+which is what CB-WP-0001 actually recorded.
+
+## Known failure modes of the chosen model
+
+Stated rather than discovered later. Each becomes a test in T04.
+
+1. **33% of spend has no task.** Only 14 of 33 commits name a task; the rest
+   hold $30.32 of $92.21. Per-task tables are a view over two-thirds of the
+   money and must say so wherever reported.
+2. **Work spanning a boundary is assigned whole to the later task.** A
+   message before a commit belongs to that commit's task even if the thinking
+   began earlier. Accepted: the loop commits per task, so the error is
+   bounded by one interval (p90 17.7 min, max 36.8 min).
+3. **Uncommitted work is invisible.** Cost incurred after the last commit
+   has no enclosing interval. Reported as an open remainder, never dropped.
+4. **`/compact` is safe; resumed and concurrent sessions are the risk.**
+   Compaction stays within one file and one session. Two agents on one repo
+   are separable only because `sessionId` exists — this is why D2 scopes by
+   it.
+5. **The price sheet cannot express a time-boxed rate.** Sonnet's intro
+   price is a TOML comment. $0.17 at the pin; the schema defect is the real
+   issue and is deferred to T03 with a stated deadline of 2026-08-31, when
+   the intro rate expires and the sheet becomes silently wrong.
+6. **Dedup is load-bearing in the dangerous direction.** If the format ever
+   splits one response across two `requestId`s, the collector *under*-reports
+   and nothing looks wrong. T04 asserts the dedup invariant at runtime
+   (identical `usage` within a group) rather than trusting the survey's
+   one-time check.
+7. **The subagent tree is a separate enumeration.** Missing it under-reports
+   silently; it was $0.66 here and will not stay small on a fan-out pass.
+
+## Consequences
+
+- **Gate satisfied:** T04 may now write collector code. Nothing before this.
+- T03 specifies the metrics with this ADR's contracts; T05's acceptance test
+  is $92.87 as two components, with a 33% unattributed line.
+- The hub's token API keeps receiving events, now as a lossy projection of a
+  computed number rather than as an estimate. Its schema gap (no cache
+  fields) is recorded as a limitation of the dashboard, not of the metric.
+- **Raised out of scope, for the ralph-workplan skill:** its instruction to
+  "read tokens from the Claude Code status bar" asks an agent for a figure
+  it cannot read, and is the proximate cause of the hub's bad numbers. The
+  skill lives outside this repo; flagged for the maintainer.
diff --git a/workplans/CB-WP-0002-cost-accounting.md b/workplans/CB-WP-0002-cost-accounting.md
index c5a82f5..72693c3 100644
--- a/workplans/CB-WP-0002-cost-accounting.md
+++ b/workplans/CB-WP-0002-cost-accounting.md
@@ -87,7 +87,7 @@ accuracy and the hub to lead on durability.
 
 ```task
 id: CB-WP-0002-T02
-status: todo
+status: done
 priority: high
 state_hub_task_id: "eae248ab-f29f-4f11-9d20-e8145b0d822d"
 ```