tegwick
b96cd94a64
Normative spec for the cost model (dedup by requestId, per-model and per-TTL pricing), the attribution contract (ending-at-commit intervals scoped by sessionId), and the reported shape (composition, not a total). Every acceptance row names the command that produces its number, per InnerLoop Step 4. AC-5..AC-8 are the positive control: cb-cost --self-test must fail on a dedup violation, on zero responses, on a missing subagent tree, and on 5m cache priced at the 1h rate. The feasibility check earns its place here: AC-1's $92.87 is reachable only if CA-06 holds. The earlier $92.21 target was reachable only by a collector with the exact blind spot the survey documented. MetricsAndScenarios 1a superseded. Two of its rules struck through rather than deleted, because both were wrong in instructive ways: "if the cache split is unknown, count all input at full price" would have priced 80.5M cache reads at 10x, and "the hub already records tokens" named as a source what is only ever a lossy sink. M-D2-TOK demoted -- tokens are not comparable across models or cache states. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
168 lines
7.9 KiB
Markdown
168 lines
7.9 KiB
Markdown
# Cost Accounting
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Status: **v1.0** — 2026-07-31. Derived from
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[CB-RES-0002](../research/CB-RES-0002-cost-accounting.md) (approved) and
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[ADR-0003](../decisions/ADR-0003-cost-accounting.md). Makes M-D2-CST
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computable; supersedes its "uncomputable" disposition in CB-EV-0001.
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Defines how the USD cost of agentic work is measured and attributed, so
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that D2 claims about implementation efficiency are falsifiable.
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---
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## 1. The cost model
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### 1.1 Unit of billing
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The unit is one **API response**, identified by `requestId`. It is *not*
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one JSONL line: a response is written as up to six lines split by content
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block (`thinking`, `text`, `tool_use`), and every line repeats the same
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`usage` object.
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> **CA-01.** Cost is computed over responses deduplicated by `requestId`.
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> Summing per line is a defect; it inflates by ≈1.9× on measured data.
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> **CA-02.** Dedup is asserted, not assumed. Within a `requestId` group,
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> `usage` must be identical across lines and the model must not vary. A
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> violation aborts the run rather than producing a number.
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Rationale for CA-02: if the format ever splits one response across two
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`requestId`s, dedup *under*-reports and nothing looks wrong. The dangerous
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direction gets the assertion.
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### 1.2 Price formula
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Per response, against `benchmarks/baselines/model-prices.toml`:
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```text
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cost = input_tokens × price.input
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+ output_tokens × price.output
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+ cache_read_input_tokens × price.input × cache.read (0.10)
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+ ephemeral_5m_input_tokens × price.input × cache.write_5m (1.25)
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+ ephemeral_1h_input_tokens × price.input × cache.write_1h (2.00)
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```
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> **CA-03.** Each response is priced at **its own** `message.model` rate. A
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> session may mix models; CB-WP-0001 used three.
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> **CA-04.** Cache writes are priced **per TTL**. The top-level
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> `cache_creation_input_tokens` aggregate equals `ephemeral_5m +
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> ephemeral_1h` and must never be priced at a single multiplier — doing so
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> inflated a measured subagent transcript by 43%.
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> **CA-05.** A response whose model is absent from the price sheet is
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> reported as an unpriced line with its token counts, never dropped and
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> never priced at a default.
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### 1.3 Scope of a measurement
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> **CA-06.** A measurement enumerates **every** transcript for the repo:
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> `~/.claude/projects/<slug>/*.jsonl` and every
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> `<session>/subagents/agent-*.jsonl`. Subagent cost is not in the main
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> file and is invisible to a collector that reads one path.
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> **CA-07.** Every committed number states its **pin** — a timestamp or
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> commit. Transcripts are append-live: the file grows as the measuring
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> session writes to it, and an unpinned total is not reproducible.
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## 2. The attribution contract
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> **CA-08.** A response is attributed to the task named by the next commit
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> at or after it, within its own session:
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> `interval := (prev_commit_time, this_commit_time]`, scoped by `sessionId`.
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> **CA-09.** Timestamps are converted, never offset-subtracted. Commit
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> times are parsed from `%cI` and converted to UTC; this repo carries two
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> distinct offsets.
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> **CA-10.** A response in a commit whose subject carries no `T##` tag is
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> attributed to `UNATTRIBUTED`, which is **reported as its own line** in
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> every table. On CB-WP-0001 this is 33% of cost ($30.32 of $92.21) — a
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> per-task table is a view over roughly two-thirds of the money and says so
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> wherever it appears.
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> **CA-11.** Cost after the last commit is an **open remainder**, reported
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> separately from `UNATTRIBUTED`. It is work not yet committed, not work
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> without a task.
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> **CA-12.** Attribution never keys on wall-clock alone. Two sessions
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> overlapped 4 h 13 m on this repo carrying ~$12; only `sessionId`
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> separates them.
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## 3. Reported shape
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> **CA-13.** Every report carries **composition** — the split across input,
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> output, cache read, cache write 5m, cache write 1h — alongside the total.
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> A total alone would have concealed the finding that motivated this work:
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> 88.4% of spend is cache, at 256:1 cache-read to output tokens.
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> **CA-14.** Reconciliation is asserted. Attributed + unattributed + open
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> remainder + unpriced must equal the transcript total to the cent. A
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> mismatch aborts rather than reporting.
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> **CA-15.** Tables that a tool can emit are emitted by the tool. Every
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> committed number in this capability's evidence is produced by a command,
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> not typed. *(Adversarial review of CB-RES-0002 found every computed
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> figure correct to the cent and three hand-typed markdown tables wrong.)*
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## 4. Acceptance metrics
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Each row names the command that produces its number, per InnerLoop §Step 4.
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`cb-cost` is `tools/cb-cost` (T04).
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| ID | Metric | Target | Instrument |
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|---|---|---|---|
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| **AC-1** | reproduces the pinned CB-WP-0001 total | **$92.87** = $92.21 main + $0.66 subagent | `cb-cost --repo clay-borg --pin fc76445` |
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| **AC-2** | reconciliation residual (CA-14) | **$0.00** exactly | same command, `reconciled: ok` line |
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| **AC-3** | unattributed share reported (CA-10) | present, and **33%** on the pinned run | `cb-cost --pin fc76445 --by-task` |
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| **AC-4** | composition reported (CA-13) | all five components present | `cb-cost --pin fc76445 --composition` |
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| **AC-5** | dedup invariant asserted (CA-02) | violation exits non-zero | `cb-cost --self-test` |
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| **AC-6** | positive control: refuses to report on zero responses | exits non-zero | `cb-cost --self-test` |
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| **AC-7** | subagent tree included (CA-06) | omitting it changes AC-1 by $0.66 | `cb-cost --self-test` |
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| **AC-8** | per-TTL cache pricing (CA-04) | 5m-only transcript prices at 1.25× | `cb-cost --self-test` |
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**AC-5 through AC-8 are the positive control.** Per InnerLoop v1.0 §Step 5,
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a harness must assert it did the work it reports. `--self-test` runs each
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assertion against a fixture whose expected value is known and fails loudly;
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`make cost` runs it before any reported number.
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## 5. Metric feasibility check
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Per InnerLoop §Step 4, the acceptance table is checked against the
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contracts in this same spec:
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- AC-1's $92.87 is reachable only if CA-06 holds (both trees enumerated).
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Under a main-file-only collector the target is unreachable — this is the
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defect the adversarial review caught, where a target of $92.21 would have
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been hit *only* by a broken collector.
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- AC-3's 33% is a property of CB-WP-0001's commit subjects, not of the
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collector. It is a regression pin on the fixture, not a quality target;
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improving tagging discipline will change it, and that is expected.
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- CA-07 (pinning) makes AC-1 reproducible; without it the target drifts
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upward on every run and the test is meaningless.
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## 6. Known limitations, stated with the numbers
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- **Per-task cost covers ~67% of spend.** Structural: 19 of 33 commits
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carry no task tag. Reported per CA-10, never silently dropped.
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- **Work spanning a commit is assigned whole to the later task.** Bounded
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by one interval: p50 6.9 min, p90 17.7 min, max 36.8 min.
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- **The price sheet cannot express a time-boxed rate.** Sonnet's intro
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price is a TOML comment, so the sheet is silently wrong for sonnet-priced
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work ($0.17 at the pin, 0.19%). **This becomes an error, not a rounding
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issue, on 2026-08-31** when the intro rate expires and the comment and
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the data disagree in the other direction. Tracked as a schema defect
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against the price sheet.
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- **The State Hub cannot store what this spec measures.** Its token event
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schema has `tokens_in`/`tokens_out` and no cache fields, so the dashboard
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necessarily shows a lossy projection. This is a limitation of the sink,
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not of the metric; `make cost` remains the authority.
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## 7. Revisions to M-D2-CST
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`specs/MetricsAndScenarios.md` §1a is superseded by this spec. M-D2-CST is
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redefined from "tokens × pricepoint" — which named no instrument and was
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never computed — to: **USD per completed workplan task, per CA-08
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attribution, produced by `make cost`.** M-D2-TOK is retained but demoted:
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tokens are the input to the cost model, not a comparable figure across
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models or across cache states.
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