# CB-EV-0002: cost accounting capability: meta.loop.cost-accounting spec: [CostAccounting.md](../specs/CostAccounting.md) v1.0 decision: [ADR-0003](../decisions/ADR-0003-cost-accounting.md) research: [CB-RES-0002](../research/CB-RES-0002-cost-accounting.md) instrument: `make cost-pin` (`tools/cb-cost.py --pin fc76445`) measured: 2026-07-31 Subject of measurement: the CB-WP-0001 session, pinned at commit `fc76445` (`2026-07-31T02:17:59Z`). All numbers below are emitted by the tool, not transcribed by hand (CA-15). --- ## 1. Acceptance table | ID | Metric | Target | Measured | Verdict | |---|---|---|---|---| | AC-1 | pinned total, as two components | $93.15 = $92.03 + $1.11 | **$92.03 main + $1.11 subagent = $93.15** | **met** | | AC-2 | reconciliation residual | $0.00 | **$0.000000** | **met** | | AC-3 | unattributed share reported | present, 33% | **32.4%, own line** | **met** | | AC-4 | composition reported | 5 components | **5 of 5** | **met** | | AC-5 | dedup violation aborts | non-zero exit | **abort raised** | **met** | | AC-6 | zero responses refuses to report | non-zero exit | **0 rows, no number emitted** | **met** | | AC-7 | subagent tree enumerated | >0 found | **2 of 4 transcripts** | **met** | | AC-8 | 5m cache priced at 1.25× | $1.25/100k @ fable | **$1.2500 (1h would be $2.0000)** | **met** | | AC-9 | streamed partial output → final | 5,5,195 → 195 | **195** | **met** | No `unmeasured` rows. AC-1's target was corrected four times before and after this run; §4 records why, because the sequence is more useful than the final number. ## 2. Composition — the finding the metric exists for ```text input 690 tok $ 0.00 0.0% output 323,643 tok $ 11.13 11.9% cache_read 80,611,798 tok $ 59.67 64.1% write_5m 37,467 tok $ 0.47 0.5% write_1h 1,672,854 tok $ 21.87 23.5% TOTAL $ 93.15 ``` **88.0% of spend is cache; 11.9% is output.** The ratio of context re-read to text written is 249:1. A single total would have shown $93.15 and concealed all of it — which is exactly what CB-WP-0001's `M-D2-TOK` would have done, and why that metric is now demoted. ## 3. Per-task attribution ```text UNATTRIBUTED $ 30.14 32.4% T08 $ 21.02 22.6% T07 $ 12.01 12.9% T03 $ 9.91 10.6% T04 $ 8.00 8.6% T05 $ 7.19 7.7% T06 $ 3.58 3.8% T09 $ 1.29 1.4% ``` **Stated limit (CA-10):** 32.4% of cost sits in commits whose subject carries no `T##` tag, so this table is a view over 67.5% of spend. That is a property of commit hygiene, not of the collector. T08 (the GROUND aggregate, six code iterations) at $21.02 is the most expensive task and was also the one that produced the most rework — the tuple-map hash panic, the discarded `setup.patch`, and the 5.6×-wrong benchmark all originated there. Expensive *and* error-dense: the correlation is worth watching, not yet a conclusion from n=1. ## 4. What it does not support - **The per-task figures are not comparable across passes.** They mix models (opus/fable/sonnet) at different price points and different cache states. The dollar figure is comparable; a token count is not. - **AC-3's 32.4% is a fixture pin, not a quality target.** Improving commit tagging will move it, and that is the desired direction. - **This is one session.** Every ratio here (cache share, $/turn, the compaction effect in §5) is n=1 and should be treated as a hypothesis until a second pass reproduces it. - ~~**The price sheet cannot express a time-boxed rate.**~~ **Fixed 2026-07-31 by CB-WP-0003 T11** (CA-16/CA-17). Applying the sonnet promotional rate moved AC-1 from $93.32 to **$93.15** — the $0.17 this section predicted, now collected rather than merely noted. - **AC-1 is not invoice-verified.** No admin key exists, so the Anthropic billing API could not independently confirm the total. The transcript counters are the same ones billing uses, but that is an argument, not a reconciliation. ## 5. The question that could not be answered before **"What does `/compact` actually cost, and is a long session quadratic?"** CB-WP-0003 T04 asserts that cost ≈ turns × mean_context and that mean_context grows with turns, making long sessions quadratic. The data **qualifies that claim**: it is true only between compactions. The session compacted twice, and the transcript records both directly: | compaction | pre-tokens | post-tokens | reduction | |---|---|---|---| | C1 `00:07:14Z` (manual) | 542,991 | 19,974 | **27×** | | C2 `02:22:23Z` (manual) | 344,954 | 19,035 | **18×** | Cost per turn across the boundary: | segment | turns | mean context | total | $/turn | |---|---|---|---|---| | start → C1 | 136 | 304,178 | $62.19 | **$0.457** | | C1 → C2 | 202 | 193,493 | $30.01 | **$0.149** | **The 202 turns after the first compaction cost less than half of the 136 turns before it — a 3.1× drop in cost per turn.** Context growth *is* the cost driver, and compaction is the control on it. At the pre-compact rate, those 202 turns would have cost ~$92 instead of $30. Two consequences for CB-WP-0003: 1. **T04's "one task per session" recommendation is not the only remedy, and may not be the cheapest one.** Compaction achieved a 27× context reduction inside a running session at the cost of one summarization call. A fresh session pays a cold-start re-read of the committed artifacts; compaction pays a summary. Which is cheaper is now a measurable question rather than a matter of taste, and T04 should measure it before prescribing. 2. **The quadratic claim should be restated as bounded-quadratic:** cost grows with context between compactions and resets at each one. The failure mode is not "a long session" but "a long *uncompacted* session". ## 6. Retrospective note The positive control paid for itself on its first execution, which is the strongest evidence this project has produced for the InnerLoop v1.0 rule that added it. CA-02 asserted that `usage` is identical across the lines of one `requestId` — verified twice on the main transcript, by the survey (206/206 groups) and independently by the adversarial reviewer. It is false in the `subagents/` tree, where `output_tokens` is a running count (`5, 5, 195`). The assertion fired, the run aborted, and the tool refused to print a number. Under the prior first-wins rule it would have printed a plausible one. The generalization that failed is worth naming: **a property verified on the largest sample was assumed to hold on the smallest one.** The main transcript is 338 of 346 responses, so 206/206 felt conclusive; the violation lives entirely in the 8 responses nobody checked separately. Cost of the adversarial review this pass: **$1.11**, against a $93.15 pass. It found three approval-blocking defects, one of which (the subagent exclusion) would have made this evidence file certify a broken collector. Second consecutive pass where a ~1% spend on review changed the outcome. That is now two data points for CB-WP-0003 T03.