# CB-RES-0002: agentic cost accounting

capability: meta.loop.cost-accounting
status: approved         # adversarial review 2026-07-31: 16 findings, 15 conceded
tier: L (structural L, chaos d10=2 → no override)
runnable-baseline: invoked — every candidate below was exercised against the
CB-WP-0001 session on this machine, not cited
review-trail: history/260731-cost-accounting-{research,challenge,response}.md

Survey of instruments that can attribute the USD cost of agentic work to a
unit of work, so that M-D2-CST (`specs/MetricsAndScenarios.md` §1a) becomes
computable. CB-WP-0001 specified that metric completely and recorded it as
*uncomputable*; the premise of this workplan is that the data existed the
whole time.

That premise survives. The workplan's **numbers do not** — see §Correction.

---

## Correction to this workplan's own Purpose section

CB-WP-0002's Purpose reports the CB-WP-0001 session at **$248.46**, from
131,863,164 cache-read tokens priced at Fable 5. Both halves are wrong, and
in the same direction — too high. The survey found this by re-deriving the
number rather than adopting it.

**Error 1 — per-line summation double-counts.** A single API response is
written to the transcript as *several* JSONL lines, split by content block
(`thinking`, `text`, `tool_use`), and **every one of those lines repeats the
complete `usage` object**. Measured on the CB-WP-0001 transcript: 657
assistant lines carry only 346 distinct `requestId`s. Group sizes run 1–6:

| lines per requestId | 1 | 2 | 3 | 4 | 5 | 6 | total |
|---|---|---|---|---|---|---|---|
| groups | 140 | 120 | 74 | 6 | 5 | 1 | **346** |
| lines | 140 | 240 | 222 | 24 | 25 | 6 | **657** |

Both rows are checksummed against the file: groups sum to 346, lines to 657.

Positive control on the dedup: across all 206 multi-line groups the `usage`
object is byte-identical (206/206 identical, 0 differing), and no group
mixes models. Every assistant line carries a `requestId` — there is no null
key for `setdefault` to collapse. The duplication is a transcript-format
artifact, not repeated billing. Summing per line inflates by ≈1.9×.

**Error 2 — single-model pricing on a multi-model session.** The session ran
three models, not one. Both columns are shown because the gap between them
*is* error 1:

| model | JSONL lines | API responses (deduped) | pinned cost |
|---|---|---|---|
| claude-opus-5 | 382 | 213 | $36.19 (39%) |
| claude-fable-5 | 250 | 118 | **$55.50 (60%)** |
| claude-sonnet-5 | 24 | 14 | $0.52 (0.6%) |
| `<synthetic>` | 1 | 1 | $0.00 |
| **total** | **657** | **346** | |

§1a *already required* per-model pricing; the Purpose section did not apply
its own rule. Note the inversion, which matters more than the correction:
**Fable is 35% of the calls and 60% of the dollars; Opus is 60% of the calls
and 39% of the dollars.** A count-majority is not a cost-majority — the
error this document is about, one level down.

The `<synthetic>` entry is an error placeholder carrying a valid `requestId`
and a complete **all-zero** `usage` object, not a missing one. Its dollar
impact is exactly $0.00, but a collector guarding on `if not usage` and one
guarding on `if model not in prices` take different branches; T04 must name
which.

**Corrected totals** for the same transcript, all three methods run over
the identical unpinned line set so the methods are comparable:

| method | responses | output | cache read | cost |
|---|---|---|---|---|
| per-line, all-Fable (the Purpose method) | 657 | 700,690 | 161,408,840 | $289.12 |
| per-line, per-model | 657 | 700,690 | 161,408,840 | $210.05 |
| **deduped, per-model (correct)** | **346** | **318,230** | **81,100,498** | **$93.15** |

**Third error, found while verifying the second: the transcript is a live
file.** Re-running the deduped figure minutes later returned 356
responses and $94.04 — this survey's own session appends to the same
JSONL it is measuring. An unpinned total is not a repeatable number. The
acceptance target is therefore pinned by timestamp:

| CB-WP-0001, pinned ≤ `2026-07-31T02:17:59Z` (commit `fc76445`) | value |
|---|---|
| responses | 339 (206 opus-5, 118 fable-5, 14 sonnet-5, 1 synthetic) |
| output | 313,900 tok → $10.66 |
| cache read | 80,453,702 tok → $59.59 |
| cache write 1h | 1,672,854 tok → $21.95 |
| input | 676 tok → $0.00 |
| main transcript | **$92.21** — 88.4% cache, 256:1 cache-read:output |
| + subagent tree (7 responses, ran 23:11–23:14Z, inside the pin) | $1.11 |
| **TRUE TOTAL** | **$93.32** |

*(The subagent figure was $0.66 when this survey was written. T04's
positive control found the cause: `output_tokens` is a running count in
the `subagents/` tree, so first-wins dedup under-counted it. See
`specs/CostAccounting.md` CA-02a.)*

The subagent line is not a footnote. C1's blind-spot finding says a
collector reading only the main file under-reports; a target of $92.21 would
have been hit only by a collector *with* that blind spot, and failed by a
correct one. The acceptance target is **$93.32, stated as its two
components**, so a collector reading one tree is diagnosed rather than
merely failed.

The reported figure was **~2.7× the real cost**. This is the fourth
instance of the harness-does-nothing error class from
`history/260731-inner-loop-retrospective.md`, wearing a new coat: not a
harness that measured nothing, but an arithmetic that measured the same
thing twice. Both produce a number that looks fine.

The qualitative headline survives the correction and gets stronger: cache
reads are **81.1M tokens against 318k of output**, ~255:1. Cost in an
agentic loop is context × turns.

---

## Candidates

### C1 — Session transcript JSONL

`~/.claude/projects/<slug>/<session>.jsonl`, one JSON object per line.
Assistant lines carry `message.usage` with exact billing counters:
`input_tokens`, `output_tokens`, `cache_read_input_tokens`, and
`cache_creation.{ephemeral_1h,ephemeral_5m}_input_tokens`, plus
`message.model`, `requestId`, and an ISO-8601 `timestamp`.

- **Granularity:** per API response, once deduplicated by `requestId`.
- **Accuracy:** exact — these are the counters the invoice is computed from.
  There is no sampling or rounding.
- **Verified non-issue:** `usage.iterations[]` is a sub-breakdown, not an
  additional charge. Every assistant line carries `usage`; the iteration
  outputs sum exactly to the top-level `output_tokens` in every case, and
  no message had more than one iteration. Summing `iterations` *instead of*
  the top-level fields is safe; summing *both* would double-count.
- **Cache-write rates are per-TTL and must not be aggregated.**
  `cache_creation_input_tokens` equals `ephemeral_5m + ephemeral_1h` in all
  775 usage lines across both transcripts, and the two bill at different
  multipliers (1.25 vs 2.0). Pricing the top-level aggregate at a single
  rate is a silent error: the subagent's writes are **entirely 5m** (37,467
  tokens), and pricing them at 1h inflates that transcript by **+43%**
  ($0.658 → $0.939). The main session happens to be all-1h, so the pinned
  total is insensitive — but fan-out passes are exactly where 5m dominates.
- **Survives compaction:** yes. `/compact` writes a summary message into the
  same file (`isCompactSummary`, `compactMetadata`) and the session
  continues; no usage is lost. Compaction is visible as an event, so its
  cost is itself measurable.
- **Attribution:** none built in — a transcript is a flat message stream
  with timestamps. It must be joined against an external time index.
- **Blind spot 1 — subagents are a separate tree.** Subagent cost is **not**
  in the main transcript. `isSidechain` is `false` on all 657 lines;
  subagent work lives in `<session>/subagents/agent-*.jsonl` with an
  `agent-*.meta.json` naming the agentType and model. CB-WP-0001 spawned one
  (the adversarial review). A collector reading only the main file silently
  under-reports.
- **Blind spot 2 — one repo, several transcripts, overlapping in time.**
  The project directory holds more than one session. `f1eb1147` overlaps
  `8cbd5701` for **4 h 13 m**, carrying ~$6 on each side — ~$12 that no
  wall-clock join can separate, beginning 14 seconds after the pin. A
  collector must therefore key attribution on **`sessionId`, not only time**,
  and must enumerate every transcript for the repo rather than one file.

### C2 — Custodian State Hub token API

`record_token_event`, the `update_task_status` token tiers, and
`get_token_summary`. Exercised against CB-WP-0001's workplan
(`a1b434dc-…`), which returned:

```text
tokens_in 362,000   tokens_out 39,100   event_count 7   by model: claude-fable-5
```

- **Granularity:** per task — the best of any candidate, and the only one
  that is natively *about* the unit of work.
- **Accuracy:** poor, and structurally so. Three independent defects:
  1. **The schema has no cache fields.** `tokens_in`/`tokens_out` cannot
     represent the finding this workplan exists to report. Cache read alone
     is **64.6% of cost** and all cache is **88.4%**; the hub cannot express
     either at any fidelity.
  2. **The recorded numbers are estimates.** 7 events for 9 tasks, at
     round figures — the skill's Tier-3 heuristic (1000/500) and Tier-1
     eyeball estimates. Against a deduped transcript output of 318,230,
     the hub's 39,100 is off by ~8×; against total input it is off by
     ~450×.
  3. **Model attribution is wrong.** Everything is filed under
     `claude-fable-5` on a session that was majority Opus 5.
- **Survives compaction:** yes — it is server-side and independent of the
  client.
- **Verdict:** durable and task-shaped, but its numbers are unusable as a
  cost source. Its role is as a **sink** for numbers computed elsewhere,
  not a source. Even as a sink it can only carry a lossy projection until
  the schema grows cache fields.

### C3 — Claude Code status bar

- **Granularity:** whole session, live.
- **Accuracy:** unknown and unauditable — it is rendered text.
- **Machine-readable:** no. Not configured here (`statusLine` is absent
  from `~/.claude/settings.json`), and it is not reachable from inside a
  tool call regardless.
- **Verdict:** eliminated. The ralph-workplan skill's "read tokens from the
  status bar" instruction is the proximate cause of C2's bad numbers — it
  asks an agent to report a figure it cannot read, and an agent that cannot
  read it estimates instead. This should be raised against the skill.

### C4 — Anthropic usage / billing API

- **Granularity:** organization and API-key, by day.
- **Accuracy:** authoritative — it *is* the invoice.
- **Attribution:** none to a task, and none to a session. Cannot separate
  clay-borg from the other twenty-plus projects on this machine.
- **Availability:** requires an admin key; none is configured here.
- **Verdict:** not usable for M-D2-CST, but valuable as an **external
  reconciliation check** if an admin key is ever provisioned — it is the
  only candidate that can catch a systematic error in C1's price model.
  Left as a stated non-dependency.

### C5 — Git commit history (attribution index, not a cost source)

Not a cost instrument; the missing half of C1. The loop already commits per
task iteration with the task in the subject line, giving durable, timestamped
boundaries at exactly the granularity M-D2-CST wants:

```text
a09d76f 2026-07-31T02:14:34+02:00 T08 iter 1: scenario runner executes; …
b58a913 2026-07-31T02:19:34+02:00 T08 iter 2: Reveal, Resolve, End; …
```

- **Interval convention:** *ending-at-commit*, `(prev_commit, this_commit]`.
  This is the only convention consistent with commit-at-end-of-task; under
  the alternative every task's cost shifts one interval.
- Spacing over the 33 pinned commits: min 0.1, p50 **6.9**, p90 17.7, max
  **36.8** minutes. Usually finer than a task; the 37-minute max is not.
- Durable, versioned, and free; requires no change to how work is done.
- **Coverage is the real limit, and it is sized:** only **14 of 33** commits
  name a task (`T##`) in the subject. The other 19 — `chore(consistency)`,
  `CI:`, `AM-4:`, workplan additions — hold **$30.32 of $92.21, or 33% of
  cost**. Attribution to a *task* therefore covers two-thirds of spend at
  best; the remainder is real work that must be reported as its own line,
  not discarded.
- **Boundary conditions verified clean:** 0 responses before the first
  commit, 0 after the last, exactly 1 empty interval (`Initial commit`).
- **Known hazards:** offsets are not uniform — `git log` shows `+02:00` ×34
  and `+00:00` ×1, so a collector must parse `%cI` and convert, never
  subtract a fixed offset. Commits made outside a session create empty
  intervals.

---

## Baselines (benchmark-to-beat)

| Dimension | Baseline holder | Metric | Value | Provenance |
|---|---|---|---|---|
| D1 ease of specification | C2 hub | fields needed to record a task's cost | 4 (`task_id`, `tokens_in`, `tokens_out`, `model`) — but cannot express cache | measured (API schema) |
| D2 efficiency | C2 hub | cost of producing a number | ~0 (one API call) — number is an estimate, off by ~8× on output | measured |
| D2 efficiency | C1 transcript | cost of producing a number | one file read, 5.1 MB, ~1 s; exact | measured |
| D3 speed | C1 transcript | parse of a full session | 2,040 lines / 5.1 MB in <1 s in CPython | measured |
| D2 efficiency (**the deciding row**) | C1 transcript | **error against the billing counters** | **$0.00 — the transcript *is* the counter set; C2's error on the same work is $92.21 − $0.03 recorded ≈ 100%** | measured |
| D4 optionality | C4 billing API | authority | invoice-grade, zero attribution, admin key absent (asserted, not measured — used only to keep C4 as an optional check) | availability measured |

**Benchmark-to-beat for the collector:** reproduce **$93.32** for repo
`clay-borg` pinned at `2026-07-31T02:17:59Z` — as its two components,
**$92.21 main transcript + $1.11 subagent tree** — from the committed price
sheet, with the unattributed remainder reported as its own line (expected
**33%**, $30.32) and reconciliation asserted rather than assumed.

---

## Verdict

**C1 (transcript) leads on accuracy and is the only exact candidate.**
**C5 (git commits) supplies the attribution index C1 lacks.** C2 is the
durable sink. C3 is eliminated. C4 is an optional external check.

The expected shape is therefore: enumerate **every** transcript for the repo
including the `subagents/` tree → dedup by `requestId` → price per message at
its own model's rate and **per cache TTL** from
`benchmarks/baselines/model-prices.toml` → attribute to a task by joining
`(prev_commit, this_commit]` intervals **within a `sessionId`** → emit
per-task cost, an unattributed-remainder line, and a composition breakdown →
push a lossy summary to C2.

**What none of them do well — the surpass opportunity.** Every candidate
reports *totals*. None reports **composition**, and composition is where
the actionable finding lives: 81.1M cache-read tokens against 318k of
output means cost is driven by how much context is re-read per turn, which
no total can show. A metric that had reported only dollars would have been
correct and useless.

**Risks in the baselines themselves.**

1. **The $248.46 figure was wrong and was nearly adopted as this
   workplan's acceptance target.** T05's reconciliation test must be
   against a number this survey re-derived, not against the Purpose
   section. The Purpose section needs correcting.
2. **Dedup is load-bearing.** If the transcript format ever splits one
   response across two `requestId`s, dedup silently under-reports —
   the opposite error, and the more dangerous one. The collector must
   assert its dedup assumption (identical usage within a group) at
   runtime rather than trusting this survey's one-time check.
3. **Subagent transcripts are a separate tree.** Measured: CB-WP-0001's
   one subagent (adversarial review, Fable 5, 7 responses, 158,096 cache
   reads) cost **$1.11**, invisible to any collector reading only the
   main file. Small here; not small for a pass that fans out.
4. **The price sheet has a 90-day staleness rule** (§1a) and no automated
   check. Every number this capability produces inherits that.
5. **The transcript is append-live.** It is written by the session that
   reads it, so any total is a reading at an instant. Every committed
   number from this capability states its pin (timestamp or commit), and
   the collector takes a pin argument rather than defaulting to "all".
6. **Concurrent sessions are not hypothetical — they are in this data.**
   `f1eb1147` and `8cbd5701` overlap for 4 h 13 m on the same repo, ~$12
   inseparable by wall-clock. (The session that wrote the first draft of
   this sentence *was* the overlap.) Attribution keys on `sessionId` first,
   time second. Compaction inside a task is safe — it stays in one file and
   one session.
7. **A third of spend has no task, structurally.** 19 of 33 commits carry no
   task tag; $30.32 of $92.21. Any per-task cost table is a view over ~two
   thirds of the money, and must say so wherever it is reported — the same
   limit-with-the-number rule the coverage gate carries.
8. **The price sheet cannot express a time-boxed rate.** Sonnet's intro
   price lives in a TOML *comment*, so a collector reading the sheet
   silently uses the wrong number ($0.17 at the pin, 0.19%). Small now, and
   the same class of defect this survey levels at C2: a schema that cannot
   hold the fact it needs. Raised for T03.
9. **Hand-typed tables are the actual failure surface.** Every computed
   figure in this survey reproduced to the cent under adversarial
   re-derivation; three hand-written markdown tables did not (a group-size
   row that failed its own checksum, a per-line count labelled as deduped,
   a stale 87%). Numbers a tool can emit should be emitted by the tool.
   Raised for T03 and T06.