Fix a lock-ordering deadlock, a lost-update race and torn snapshots in a threaded bank ledger
Exact prompt
# Task: fix the concurrency bugs in bank.py `bank.py` is called from many worker threads at once. In production the worker pool sometimes hangs forever, money occasionally appears or disappears (the bank total drifts), balances have gone negative, and auditors say `total()`/`snapshot()` sometimes report a state that never existed. Fix `bank.py` so that, under heavy concurrent use: - `transfer` can never deadlock, never loses or creates money, never makes a balance negative, and raises `InsufficientFunds` (leaving everything unchanged) when the source lacks funds; - `total()` and `snapshot()` return a consistent point-in-time view even while transfers are running; - every successful transfer is in `log` exactly once, failed ones not at all. Keep the public API (class names, method names/signatures, `bal` and `log` attributes). Do not make it single-threaded by removing threading support from callers; a global lock is acceptable only if it keeps throughput reasonable (the grader runs 8 threads x 20,000 transfers in under 30 s). Standard library only. No network.
Three distinct bugs: check-then-act outside the lock (lost updates/negatives), opposite lock order (deadlock), unlocked snapshot/total (torn reads). Fixing one exposes the others; the stress test forces frequent thread switches. Original benchmark task written for usecaserank.com (not based on a specific public post). Headless, offline run on a reproducible fixture. Each run's notes say how its cost was measured (estimates are marked).
Success criteria: 1. no_deadlock: each of 3 stress runs (8 threads x 20,000 random transfers plus a concurrent auditor) finishes within 30 s. 2. final_total_ok: total money is conserved in every run. 3. no_negative: no balance is ever negative (final state and every audited snapshot). 4. log_exact: log has exactly one entry per successful transfer. 5. snapshots_consistent: every snapshot()/total() taken during the run sums to the true total. 6. nsf_raises_unchanged: an overdraft raises InsufficientFunds and changes neither balances nor log.
Best model for this task so far: Claude Haiku 5.5 (effort default (no --effort flag) · thinking on) (success, $0.0055, 23s).
All runs (4)
100% pass rate across 4 eligible runs: 4 pass, 0 partial, 0 fail. Partial and failed runs are listed below with everything else.ⓘ
| Model | Cost | Time | Step-insⓘ | Autonomy | Outcome | Trustⓘ | Date | Compare |
|---|---|---|---|---|---|---|---|---|
| OGPT-6.1 Soldefault · thinking on | $0.06 | 1m | 0 | Fully autonomous | pass | Trust 80% | 2026-10-22 | |
| AClaude Haiku 5.5default · thinking on | $0.0055 | 23s | 0 | Fully autonomous | pass | Trust 80% | 2026-10-17 | |
| AClaude Opus 5.5default · thinking on | $0.14 | 28s | 0 | Fully autonomous | pass | Trust 80% | 2026-10-17 | |
| AClaude Sonnet 5.5default · thinking on | $0.05 | 12s | 0 | Fully autonomous | pass | Trust 80% | 2026-10-17 |
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