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Claude Sonnet 5.5 (effort default (no --effort flag) · thinking on) on Root-cause a checkout 500 spike from nginx/app/deploy logs with red herrings

AClaude Sonnet 5.5default · thinking on · Claude Code 2.1.299 (headless -p) · cloud · 2026-10-16 · by @bobsiopener

Trust 80%Fully autonomoussuccess · first trySuperseded
Estimated cost
$0.07
Wall time
16s
Human step-ins
0
Outcome
Pass

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Superseded. This run was rescored on 2026-10-28 and replaced by a newer run. It no longer counts toward rankings, use-case tables or the data export; it is kept here for the record.

Summary

Exact prompt

# Task: incident root cause from logs On 2026-10-12 users reported failed checkouts. `logs/` contains `access.log` (nginx), `app.log` (application) and `deploys.log`. All times UTC. Write `incident.json`: {"incident_start_minute": "HH:MM", "incident_end_minute": "HH:MM" (last minute with app 500s), "root_cause": one of ["db_pool_exhaustion","cdn_outage","slow_search_query","bad_ranking_deploy","traffic_spike","other"], "culprit_commit": "<sha>", "fix_commit": "<sha>", "most_affected_endpoint": "<path>", "app_500_count": <int, HTTP 500 responses with upstream=app>} Then write `summary.md` (≤150 words) for the status page. Note there may be unrelated noise in the logs. No network.

Find the root cause of a spike in failed checkouts from nginx, application and deploy logs. The agent must tie a deploy's config change to database connection-pool timeouts, give the exact incident window and count of 500 errors from the app, and ignore red herrings such as a CDN blip and slow cron warnings. It also writes a short incident summary. 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. root_cause enum must be exactly right. 2. culprit_commit and fix_commit must match (prefix match on the short SHA). 3. most_affected_endpoint must be exact. 4. incident_start_minute within ±1 minute and incident_end_minute within ±2 minutes of the truth. 5. app_500_count must be the exact count of HTTP 500 responses with upstream=app. 6. summary.md must exist, be 1–150 words, and must not attribute the cause to the CDN or the search query.

Setup notes: Use Case Scout batch 3 case c08-log-forensics (offline fixture). Prepared with batch3/prepare.sh; hidden key/tests kept outside the agent's work dir. Prompt = TASK.md verbatim. Scored with the scout's scorer: `python score.py WORKDIR KEYDIR/key.json`. Tools: Read/Write/Edit/Glob/Grep/Bash, web disabled. Single run, no retries. Rescored 2026-10-16 with Scout's fixed scorer (see usecase-scout/AUDIT-batches1-5-2026-10-17.md). The original score was 7/8 checks (fail), and the model's output is unchanged. Claude Code's own reported cost for claude-sonnet-5.5: $0.066749. Score: 8/8 checks. Cost is an ESTIMATE: measured tokens x pricing.json list price (Claude Pro subscription, nothing billed); input_tokens include cache reads and writes. Single run.

Estimated cost List-price estimate; not actually billed.

Metrics

Estimated cost
$0.07
Wall time
16s
Human step-ins
0
Turns
5
Model usage
ModelInput tokOutput tokCost
AClaude Sonnet 5.5anthropic/claude-sonnet-5.5effort default (no --effort flag) · thinking on102,9171,800$0.07

Reproduce this run

  1. Use the use case prompt on the use case page.
  2. Same setup: Claude Sonnet 5.5 (anthropic/claude-sonnet-5.5, effort default (no --effort flag) · thinking on) via Claude Code 2.1.299 (headless -p), cloud.
  3. It counts as a success only if it meets every success criterion: 1. root_cause enum must be exactly right. 2. culprit_commit and fix_commit must match (prefix match on the short SHA). 3. most_affected_endpoint must be exact. 4. incident_start_minute within ±1 minute and incident_end_minute within ±2 minutes of the truth. 5. app_500_count must be the exact count of HTTP 500 responses with upstream=app. 6. summary.md must exist, be 1–150 words, and must not attribute the cause to the CDN or the search query.
  4. Log model, tokens, wall time, human step-ins and the evidence log, then submit it as a reproduction. A different model? Submit a head-to-head rerun.

How runs are scored: Methodology.

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Automated checks

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Evidence log
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   "server_tool_use": {
    "web_search_requests": 0,
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  "modelUsage": {
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    "inputTokens": 10,
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    "cacheReadInputTokens": 93051,
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    "webSearchRequests": 0,
    "costUSD": 0.0667491,
    "contextWindow": 1000000,
    "maxOutputTokens": 128000,
    "thinkingTokens": 119,
    "canonicalModel": "claude-sonnet-5-5",
    "provider": "firstParty",
    "costBasis": "list"
   }
  },
  "permission_denials": [],
  "terminal_reason": "completed",
  "fast_mode_state": "off",
  "fast_mode_disabled_reason": "sdk_opt_in_required",
  "subagent_stats": {
   "spawned": 0,
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    "background": 0,
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   "started_in_background": 0,
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  "safety_stops": 0,
  "is_error": false,
  "num_turns": 5,
  "subtype": "success",
  "api_error_status": null,
  "result": "I've written `incident.json` and `summary.md` (125 words). The root cause is database connection pool exhaustion, introduced by a deploy at 14:05.\n\n- **Cause:** Commit `4be7d19` (\"Reduce idle DB connections to cut RDS cost\") cut `DB_POOL_SIZE` from 20 to 5 and `DB_POOL_TIMEOUT_MS` from 10000 to 5000. The app log then shows `db.pool: timeout acquiring connection` errors with `in_use=5` and a growing number of requests waiting.\n- **Fix:** Commit `c02aa51` reverted it at 14:29:47. A few more 500s followed until 14:30:57, so the incident window is 14:07 to 14:30.\n- **Impact:** There were 319 HTTP 500 responses with `upstream=app`. Of those, 163 were on `/api/checkout` (the most affected endpoint) and 156 on `/api/cart`.\n- **Noise ruled out:**\n  - The 12:40 ranking deploy came before the errors, and none of the app errors relate to search.\n  - The cron reindex slow-query warnings are marked non-blocking.\n  - The 14:39 CDN purge started after the errors had stopped.\n  - 16 `502` responses came from `cdn-edge`, and I left them out of the count.\n\n`incident.json`:\n```json\n{\"incident_start_minute\":\"14:07\",\"incident_end_minute\":\"14:30\",\"root_cause\":\"db_pool_exhaustion\",\"culprit_commit\":\"4be7d19\",\"fix_commit\":\"c02aa51\",\"most_affected_endpoint\":\"/api/checkout\",\"app_500_count\":319}\n```",
  "ttft_ms": 1834,
  "type": "result",
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  "uuid": "[REDACTED]",
  "ttft_stream_ms": 1008,
  "time_to_request_ms": 238,
  "first_content_frame_ms": 1009,
  "queued_turn_count": 0,
  "result_index": 0
 }
]

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