clay-borg/evidence/CB-EV-0002-cost-accounting.md

# 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** | <!-- fact:pinned_total --><!-- fact:pinned_main --><!-- fact:pinned_subagent -->
| 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.