SKILL

When cost-anomaly flags a session as a >3.5σ outlier, the next question is "which MESSAGES were expensive?". cost-session answers that.

Algorithm

Implementation: scripts/session.mjs.

1. Resolve session jsonl: --session-id <id> (scans ~/.claude/projects/*/) or --latest (default; picks most-recently-modified jsonl).
2. Parse all assistant messages with usage blocks.
3. Cost each message via shared PRICING (_prices.mjs).
4. Sort descending by cost_usd, surface top-N (default 20).
5. Compute p50/p90/p99 of message costs for in-session percentile context.
6. Flag the top message if it's >2× the p99 — that's an in-session outlier.

Why this matters: cache writes are the silent cost

Example real session, top message:

| # | Model    | In | Out | Cache W | Cache R | Cost      |
| 1 | opus-4-7 | 6  | 569 | 881898  | 0       | $16.58    |

Without the Cache W column it looks like "569 output tokens cost $16" — that's wrong by 380×. The actual cost is ephemeral 1h cache write at opus pricing: 881,898 tokens × $18.75/1M = $16.54.

Operators reading the table see immediately: "the model wrote 881K tokens to ephemeral cache". From there the question becomes "why did we cache 881K tokens of context for a 6-input request?" — that's a real engineering signal.

Drill-down workflow

# Step 1: find outliers across all sessions
cost anomaly --alert-on-outliers 1 || cost anomaly  # see which session-ids

# Step 2: drill into the flagged session
cost session --session-id <flagged-id> --top 10

# Step 3: open that jsonl at the timestamp the top message reports,
# inspect the prompt + tool calls

Percentile context

Top of output:

| p50 (median) message | $0.85 |
| p90 message          | $1.45 |
| p99 message          | $1.74 |

Lets operators ask "is this top message a 2× outlier or a 380× one?" without having to compute it themselves. The "top is >2× p99" footer fires when the answer is "yes, this is an in-session outlier worth investigating".

--since filter

Useful for drilling into a specific time range within a long session:

cost session --since 2026-06-16T13:00:00Z --top 5

Only messages with timestamp >= --since are considered.

Edge cases

• No costed assistant messages → "No costed assistant messages" + exit 0.
• --session-id not found in any project's jsonls → exit 2 with error.
• --top must be a positive integer → exit 2.