Cost logger over-count fix (cumulative re-sum) — MC #103671

Cost logger over-count fix (cumulative re-sum → idempotent per-session)

Book: System Architecture Status: Fixed + verified — MC #103671 (2026-06-15) Commit: ae045e589 (~/.claude)

The bug

~/.claude/hooks/claude-cli-cost-hook.sh is a Stop hook. Every time it fires (end of each turn) it parses the entire session transcript and sums input_tokens + cache_creation across all assistant messages, then INSERTed a fresh cost_events row with that cumulative total.

Because the transcript grows each turn, every firing logged an ever-larger cumulative snapshot of the same session. Across a day one session produced dozens of rows, so SUM(cost_usd) counted the same early tokens repeatedly.

Evidence (tool-verified, costs.db)

• Today Opus SUM = $14,686 (129 events) vs MAX single = $478.
• One event logged 6,959,199 input tokens — physically impossible (context max 1M) → proves cumulative re-sum.
• All-time: 180 events >1.5M input tokens.

Impact

1. Killswitch / userprompt-cost-guard.sh read SUM(cost_usd) for today → fired on phantom spend. Enabling the guard would have blocked every prompt. (Likely why the guard was previously removed — wrong fix.)
2. cost-tracker.js SUM-based reporting inflated ~30×.

The fix

Before INSERT, DELETE any prior row for the same session_id (read from metadata.session_id, scoped to source='claude-cli'), so each session contributes exactly one row — the latest cumulative. 'unknown' sessions skip the replace (avoid collapsing distinct parse-failures). No schema change.

if session_id and session_id != 'unknown':
    DELETE FROM cost_events
    WHERE source='claude-cli' AND json_extract(metadata,'$.session_id') = ?
INSERT ...

Verification

• Hook run 3× on a fixed transcript → 1 row (was 3), cost $0.1425 (exact: 3000 in / 1300 out @ opus 15/75 per-1M).
• One-time historical dedupe (keep max-cost row per session): claude-cli rows 4060 → 287 (= distinct sessions); today Opus SUM $10,997 → $1,437. costs.db backed up pre-dedupe; PRAGMA integrity_check = ok.

Important follow-on (not a bug)

After correction, today's real Opus spend ≈ $1,437 — still 3× the $500 daily ceiling and above the $1000 killswitch. So there is a genuine cost signal, not pure phantom. Decision needed: raise the ceiling to reflect Opus-1M pricing reality, or treat as overspend. userprompt-cost-guard.sh restoration (MC #103654) stays paused until that ceiling decision, else it legitimately blocks.