clay-borg/specs/CostAccounting.md

# Cost Accounting

Status: **v1.0** — 2026-07-31. Derived from
[CB-RES-0002](../research/CB-RES-0002-cost-accounting.md) (approved) and
[ADR-0003](../decisions/ADR-0003-cost-accounting.md). Makes M-D2-CST
computable; supersedes its "uncomputable" disposition in CB-EV-0001.

Defines how the USD cost of agentic work is measured and attributed, so
that D2 claims about implementation efficiency are falsifiable.

---

## 1. The cost model

### 1.1 Unit of billing

The unit is one **API response**, identified by `requestId`. It is *not*
one JSONL line: a response is written as up to six lines split by content
block (`thinking`, `text`, `tool_use`), and every line repeats the same
`usage` object.

> **CA-01.** Cost is computed over responses deduplicated by `requestId`.
> Summing per line is a defect; it inflates by ≈1.9× on measured data.

> **CA-02.** Dedup is asserted, not assumed. Within a `requestId` group the
> model and every **input-side** counter (`input_tokens`,
> `cache_read_input_tokens`, both `ephemeral_*` fields) must be identical —
> they are charged once per response. A divergence aborts the run.

> **CA-02a.** `output_tokens` is exempt from CA-02 and resolves to the
> **maximum** across the group, not the first value. In streamed transcripts
> early lines carry a *partial* count and only the last line carries the
> final total.

Rationale: if the format splits a response in a way dedup does not expect,
the error is silent and *under*-reports. The dangerous direction gets the
assertion.

*CA-02a exists because the assertion fired on real data the first time it
ran.* The survey verified identical `usage` across 206/206 groups in the
main transcript and generalized it; the `subagents/` tree does not behave
that way — one response reads `output_tokens` 5, 5, 195 across its three
lines. First-wins scored it at 5. That error moved the acceptance target
by $0.45.

### 1.2 Price formula

Per response, against `benchmarks/baselines/model-prices.toml`:

```text
cost = input_tokens              × price.input
     + output_tokens             × price.output
     + cache_read_input_tokens   × price.input × cache.read       (0.10)
     + ephemeral_5m_input_tokens × price.input × cache.write_5m   (1.25)
     + ephemeral_1h_input_tokens × price.input × cache.write_1h   (2.00)
```

> **CA-03.** Each response is priced at **its own** `message.model` rate. A
> session may mix models; CB-WP-0001 used three.

> **CA-04.** Cache writes are priced **per TTL**. The top-level
> `cache_creation_input_tokens` aggregate equals `ephemeral_5m +
> ephemeral_1h` and must never be priced at a single multiplier — doing so
> inflated a measured subagent transcript by 43%.

> **CA-05.** A response whose model is absent from the price sheet is
> reported as an unpriced line with its token counts, never dropped and
> never priced at a default.

### 1.3 Scope of a measurement

> **CA-06.** A measurement enumerates **every** transcript for the repo:
> `~/.claude/projects/<slug>/*.jsonl` and every
> `<session>/subagents/agent-*.jsonl`. Subagent cost is not in the main
> file and is invisible to a collector that reads one path.

> **CA-07.** Every committed number states its **pin** — a timestamp or
> commit. Transcripts are append-live: the file grows as the measuring
> session writes to it, and an unpinned total is not reproducible.

## 2. The attribution contract

> **CA-08.** A response is attributed to the task named by the next commit
> at or after it, within its own session:
> `interval := (prev_commit_time, this_commit_time]`, scoped by `sessionId`.

> **CA-09.** Timestamps are converted, never offset-subtracted. Commit
> times are parsed from `%cI` and converted to UTC; this repo carries two
> distinct offsets.

> **CA-10.** A response in a commit whose subject carries no `T##` tag is
> attributed to `UNATTRIBUTED`, which is **reported as its own line** in
> every table. On CB-WP-0001 this is 33% of cost ($30.32 of $92.21) — a
> per-task table is a view over roughly two-thirds of the money and says so
> wherever it appears.

> **CA-11.** Cost after the last commit is an **open remainder**, reported
> separately from `UNATTRIBUTED`. It is work not yet committed, not work
> without a task.

> **CA-12.** Attribution never keys on wall-clock alone. Two sessions
> overlapped 4 h 13 m on this repo carrying ~$12; only `sessionId`
> separates them.

## 3. Reported shape

> **CA-13.** Every report carries **composition** — the split across input,
> output, cache read, cache write 5m, cache write 1h — alongside the total.
> A total alone would have concealed the finding that motivated this work:
> 88.4% of spend is cache, at 256:1 cache-read to output tokens.

> **CA-14.** Reconciliation is asserted. Attributed + unattributed + open
> remainder + unpriced must equal the transcript total to the cent. A
> mismatch aborts rather than reporting.

> **CA-15.** Tables that a tool can emit are emitted by the tool. Every
> committed number in this capability's evidence is produced by a command,
> not typed. *(Adversarial review of CB-RES-0002 found every computed
> figure correct to the cent and three hand-typed markdown tables wrong.)*

## 4. Acceptance metrics

Each row names the command that produces its number, per InnerLoop §Step 4.
`cb-cost` is `tools/cb-cost` (T04).

| ID | Metric | Target | Instrument |
|---|---|---|---|
| **AC-1** | reproduces the pinned CB-WP-0001 total | **$93.32** = $92.21 main + $1.11 subagent | `make cost-pin` |
| **AC-2** | reconciliation residual (CA-14) | **$0.00** exactly | same command, `reconciled: ok` line |
| **AC-3** | unattributed share reported (CA-10) | present, and **33%** on the pinned run | `cb-cost --pin fc76445 --by-task` |
| **AC-4** | composition reported (CA-13) | all five components present | `cb-cost --pin fc76445 --composition` |
| **AC-5** | dedup invariant asserted (CA-02) | violation exits non-zero | `make cost-test` |
| **AC-6** | positive control: refuses to report on zero responses | exits non-zero | `make cost-test` |
| **AC-7** | subagent tree included (CA-06) | omitting it changes AC-1 by $1.11 | `make cost-test` |
| **AC-8** | per-TTL cache pricing (CA-04) | 5m-only transcript prices at 1.25× | `make cost-test` |
| **AC-9** | streamed partial output resolves to final (CA-02a) | 5,5,195 → 195, not 5 | `make cost-test` |

**AC-5 through AC-8 are the positive control.** Per InnerLoop v1.0 §Step 5,
a harness must assert it did the work it reports. `--self-test` runs each
assertion against a fixture whose expected value is known and fails loudly;
`make cost` runs it before any reported number.

## 5. Metric feasibility check

Per InnerLoop §Step 4, the acceptance table is checked against the
contracts in this same spec:

- AC-1's $92.87 is reachable only if CA-06 holds (both trees enumerated).
  Under a main-file-only collector the target is unreachable — this is the
  defect the adversarial review caught, where a target of $92.21 would have
  been hit *only* by a broken collector.
- AC-3's 33% is a property of CB-WP-0001's commit subjects, not of the
  collector. It is a regression pin on the fixture, not a quality target;
  improving tagging discipline will change it, and that is expected.
- CA-07 (pinning) makes AC-1 reproducible; without it the target drifts
  upward on every run and the test is meaningless.

## 6. Known limitations, stated with the numbers

- **Per-task cost covers ~67% of spend.** Structural: 19 of 33 commits
  carry no task tag. Reported per CA-10, never silently dropped.
- **Work spanning a commit is assigned whole to the later task.** Bounded
  by one interval: p50 6.9 min, p90 17.7 min, max 36.8 min.
- **The price sheet cannot express a time-boxed rate.** Sonnet's intro
  price is a TOML comment, so the sheet is silently wrong for sonnet-priced
  work ($0.17 at the pin, 0.19%). **This becomes an error, not a rounding
  issue, on 2026-08-31** when the intro rate expires and the comment and
  the data disagree in the other direction. Tracked as a schema defect
  against the price sheet.
- **The State Hub cannot store what this spec measures.** Its token event
  schema has `tokens_in`/`tokens_out` and no cache fields, so the dashboard
  necessarily shows a lossy projection. This is a limitation of the sink,
  not of the metric; `make cost` remains the authority.

## 7. Revisions to M-D2-CST

`specs/MetricsAndScenarios.md` §1a is superseded by this spec. M-D2-CST is
redefined from "tokens × pricepoint" — which named no instrument and was
never computed — to: **USD per completed workplan task, per CA-08
attribution, produced by `make cost`.** M-D2-TOK is retained but demoted:
tokens are the input to the cost model, not a comparable figure across
models or across cache states.