/cost-diff

name: cost-diff
description: Snapshot delta between two cost-summary JSON outputs. PR-level cost regression detection — answers "what changed between these two specific snapshots?". Pairs with cost-summary's stable JSON contract.
argument-hint: "--baseline <baseline.json> --current <current.json> [--alert-on-pct N] [--alert-on-usd N] [--alert-on-class-pct <class>:N[,<class>:N]] [--format table|json]"
allowed-tools: Bash
disable-model-invocation: true

PR-level cost regression detection. Where cost-counterfactual compares to HYPOTHETICAL baselines (always-haiku/sonnet/opus) and cost-burn compares latest bucket to PRIOR MEAN, cost-diff compares two SPECIFIC known-good snapshots.

| Question | Skill | |---|---| | "What would we have spent at always-X?" | `cost-counterfactual` | | "Is daily burn accelerating vs prior mean?" | `cost-burn` | | "Did THIS PR add spend vs main?" | **`cost-diff`** ← this |

Algorithm

Implementation: [`scripts/diff.mjs`](../../scripts/diff.mjs). Consumes the stable JSON contract from `cost summary --format json`.

1. Load `--baseline` and `--current` JSON snapshots. 2. Sanity check: both must have `total_cost_usd` + `sessionCount` (cost-summary shape). 3. Per-key delta: `byTier` (haiku/sonnet/opus) and `byModel` (each model). 4. Each entry tagged `added` / `removed` / `changed` based on baseline / current zero-ness. 5. Sort table by `|delta|` descending so the biggest movers are at the top. 6. `--alert-on-pct N`: exit 1 when `total_pct > N`. 7. `--alert-on-usd N`: exit 1 when `total_delta_usd > N`. Both can be set; first to trigger wins.

PR-gate workflow

# Capture baseline (e.g. on main, via the cost-tracker-smoke CI workflow)
cost summary --format json > baseline.json

# On the PR branch, capture current state
cost summary --format json > current.json

# Compare; fail the PR if total spend grew >10% OR >$5
cost diff --baseline baseline.json --current current.json \
          --alert-on-pct 10 --alert-on-usd 5.00

The combination of both flags catches:

• **Percent-only fires**: a small absolute change but a meaningful shift

(e.g. doubling from $0.10 to $0.20 hits +100% but only +$0.10).

• **USD-only fires**: a large absolute change with a small percent

(e.g. growing from $100 to $110 is only +10% but +$10).

Either signal can fail the PR independently — they're OR'd.

--alert-on-class-pct (iter 86)

The two USD-level thresholds above miss a regression class: when ONE token type grows disproportionately even though total spend grows modestly. Example: a PR introduces a verbose context-cache pattern, total spend grows only 10% (under --alert-on-pct 50), but `cache_write` tokens grow 900%. The iter-82 driver hides inside the USD signal.

`--alert-on-class-pct cache_write:50` exits 1 when cache_write tokens grow more than 50% baseline → current. Multiple classes can be checked in one flag (comma-separated):

cost diff --baseline baseline.json --current current.json \
          --alert-on-class-pct cache_write:50,output:25

First class to breach wins. Valid classes: `input | output | cache_write | cache_read`.

Recommended PR-gate triad:

cost diff --baseline ... --current ... \
          --alert-on-pct 25 \
          --alert-on-usd 5.00 \
          --alert-on-class-pct cache_write:100

Three orthogonal signals — `pct` (total grew), `usd` (large absolute jump), `class-pct` (composition shifted). Each catches what the others miss; AND-of-OR semantics means any one firing fails the PR.

Smoke transcript (synthetic baseline + current)

| Total spend       | $1.000000 | $1.500000 | +$0.500000 | 50.00% |
| Sessions          | 10        | 13        | +3         | 30.00% |

## By tier
| opus   | $0      | $0.60   | +$0.600000 | new      | added   |
| sonnet | $0.70   | $0.50   | -$0.200000 | -28.57%  | changed |
| haiku  | $0.30   | $0.40   | +$0.100000 | 33.33%   | changed |

Notice the table is **sorted by absolute delta**, not alphabetically — the biggest mover (opus newly added) bubbles to the top. Operators reading top-down see "what mattered" first.

Exit codes

| Exit | Meaning | |---|---| | 0 | No alert, OR no thresholds set | | 1 | --alert-on-pct or --alert-on-usd threshold exceeded | | 2 | Config error (missing files, invalid JSON, malformed snapshot) |

Status column

| Status | Meaning | |---|---| | `added` | This tier/model was $0 in baseline, >$0 in current | | `removed` | This tier/model was >$0 in baseline, $0 in current | | `changed` | Both baseline and current >$0; delta is the difference |

Entries with `baseline === 0 && current === 0` are dropped (nothing to report).

Composition with cost-summary

cost-diff is the SECOND HALF of a contract that cost-summary started: the stable JSON shape from `cost summary --format json`. Both pieces have been frozen — adding fields to summary is fine; renaming or removing isn't.

If you're consuming snapshots elsewhere (dashboards, alerting), the same shape works — `cost-diff` is just one consumer.