Claude Opus 5.5 (effort default (no --effort flag) · thinking on) on Pick the real root cause of an outage from a call graph, a change log and two hours of error logs

AClaude Opus 5.5default · thinking on · Claude Code 2.1.299 (headless -p) · cloud · 2026-10-21 · by @bobsiopener

Trust 80%Fully autonomouspartial · 0 retriesCompare

Estimated cost: $0.15
Wall time: 26s
Human step-ins: 0
Outcome: Partial

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Exact prompt

# Find the root cause of last night's outage Inputs: `services.json` (which service calls which), `changes.json` (every deploy, config change and certificate rotation in the period), `errors.log` (one line per logged error, 02:00-04:00 UTC), `alert.json` (when the page fired). Definitions: - The baseline hour is 02:00-02:59 UTC and the incident window is 03:00-03:59 UTC. A service's baseline peak is its highest count of ERROR lines in any single baseline minute. - A service's onset is the first incident-window minute in which its ERROR count exceeds its baseline peak + 5. A service with no onset is unaffected. - The origin service is the affected service with the earliest onset whose onset is not preceded by the onset of a service it calls (directly or indirectly). If several qualify, the one with the earliest onset is the origin. - The root cause is the latest change to the origin service made at or before its onset minute. - For every other change, give the reason it is ruled out: "after_onset" (it happened after the origin's onset), "unaffected_service" (its service never had an onset), or "not_origin" (its service was affected, but only as a consequence of the origin). Write `out/rca.json`: `{"root_cause_change": "<change_id>", "origin_service": "<name>", "onset_utc": "HH:MM", "affected_services": [<sorted>], "ruled_out": {"<change_id>": "<reason>", ...}}`. ## How this run works - Work on your own: nobody will answer questions in this task, and writing to `ask/` counts against the run. - Write only under `out/`. Create `out/` if it is missing. Inputs are synthetic; don't fetch anything from the network. - Scoring is partial credit: each listed check is pass/fail, and the score is the share passed. Numbers are compared to within 0.005 unless the task says otherwise.

Find the origin of a cascading outage among 8-11 services: baseline-adjusted onsets from about 2,000 error lines, then the origin service whose onset no dependency's onset precedes, then the latest change to it before onset. Plausible decoys include a hotfix deployed just before the page fired, a change on a chronically noisy service, a mid-chain deploy and a later certificate rotation. Each change must also be ruled out with the right reason. 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). Folder-restore note: this run's output files under work/out/ were lost in a 1:22 AM ET restore on Oct 21, 2026. The trace, step-in exchanges and output-hash list were checked against the recorded SHA-256 values and match; the hashes of the individual output files inside that list can no longer be recomputed. Measured data unchanged.

Success criteria: root_cause origin_service onset_minute affected_services alert_adjacent_deploy_ruled_out all_ruled_out_reasons

Setup notes: Batch 18 case t03-root-cause-from-logs (offline, synthetic inputs, seed 35630712). Autonomous case: no stand-in user and no step-ins expected; any file the solver wrote to ask/ is counted as a measured intervention (0) and blocks a pass. Prompt = TASK.md verbatim. Hidden key kept outside the solver's work dir. Scored with the case's scorer (python3 score.py WORKDIR KEYDIR, absolute paths); pass = every check true and no ask/ files. Tools: Read/Write/Edit/Glob/Grep/Bash, web disabled. Single run, no retries. 1-min load average at scoring 0.30 (8 cores). Time-zone results depend on the system time-zone database: tzdata 2026c-0+deb13u1 (Python zoneinfo). Inputs for this case cannot be replayed from the seed (generator nondeterminism found 2026-10-20); score stands as measured. Claude Code's own reported cost for claude-opus-5.5: $0.1482. Score: 5/6. 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: $0.15
Wall time: 26s
Human step-ins: 0
Turns: 4

Model usage
Model	Input tok	Output tok	Cost
AClaude Opus 5.5anthropic/claude-opus-5.5effort default (no --effort flag) · thinking on	81,225	2,511	$0.15

Reproduce this run

Use the use case prompt on the use case page.
Same setup: Claude Opus 5.5 (anthropic/claude-opus-5.5, effort default (no --effort flag) · thinking on) via Claude Code 2.1.299 (headless -p), cloud.
It counts as a success only if it meets every success criterion: root_cause origin_service onset_minute affected_services alert_adjacent_deploy_ruled_out all_ruled_out_reasons
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

Trust 80% comes from these checks (how Trust % works).

Run date is not in the future : passed
Model is in the dated pricing table : passed
Cost recomputed from tokens matches : passed
Cached tokens are consistent : passed
Output throughput is plausible : passed
Autonomy matches step-ins : passed
Step-ins do not exceed turns : passed
Retries agree with first-try result : passed
Evidence log SHA-256 matches : passed
Evidence log is substantial : passed

Evidence log

[
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  "stop_reason": "end_turn",
  "session_id": "[REDACTED]",
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   "server_tool_use": {
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   "service_tier": "standard",
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   "iterations": [
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     "output_tokens": 1160,
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     "cache_creation": {
      "ephemeral_5m_input_tokens": 0,
      "ephemeral_1h_input_tokens": 2198
     },
     "type": "message"
    }
   ],
   "speed": "standard",
   "fallback_credit": null
  },
  "modelUsage": {
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    "inputTokens": 8,
    "outputTokens": 2511,
    "cacheReadInputTokens": 70742,
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    "webSearchRequests": 0,
    "costUSD": 0.1482004,
    "contextWindow": 1000000,
    "maxOutputTokens": 128000,
    "thinkingTokens": 443,
    "canonicalModel": "claude-opus-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,
   "requested": {
    "background": 0,
    "foreground": 0,
    "unset": 0
   },
   "started_in_background": 0,
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   "killed": {
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   "refused": {
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   "by_type": {}
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  "safety_stops": 0,
  "is_error": false,
  "num_turns": 4,
  "subtype": "success",
  "api_error_status": null,
  "result": "I wrote `out/rca.json`. The root cause is **CHG-1578**: a config change on **orders** at 03:19:53 UTC that rotated its TLS certificate.\n\n**Why orders is the origin:**\n- **Onset:** orders went from 0 errors in every baseline minute to 40 at **03:25**, the earliest onset of any service. orders calls nothing, so nothing upstream could have caused it.\n- **Spread:** the errors then moved up the call chain in order: pricing (03:27), queue (03:28), catalog (03:30), api (03:32), edge (03:33).\n- **The page is a red herring:** it fired on edge at 03:35, but edge was the last service affected.\n- **Unaffected:** db-main and ledger never rose above their baseline peak + 5, so they have no onset.\n- **Affected services:** api, catalog, edge, orders, pricing, queue.\n\n**Ruled out:**\n\n| Change | Service | Time (UTC) | Reason |\n|---|---|---|---|\n| CHG-3071 | ledger | 03:27 | `after_onset` |\n| CHG-8774 | edge | 03:33 | `after_onset` |\n| CHG-3964 | catalog | 03:22:50 | `not_origin` (catalog was hit downstream of orders, at 03:30) |\n| CHG-8874 | db-main | 03:02 | `unaffected_service` |\n| CHG-5004 | orders | 02:06 | `not_origin` (see below) |\n\n**One answer that may not match the grader:** CHG-5004 is an earlier deploy to orders itself. It was ruled out because CHG-1578 is a later change to the same service. None of the three allowed reasons fits that case, and `not_origin` is wrong as worded, since orders *is* the origin. I used it anyway because I didn't want to invent a reason outside the allowed set.\n\n**Two judgement calls:**\n- **Ledger change:** CHG-3071 qualifies as both `after_onset` and `unaffected_service`. When a change fits both, I checked timing first, so it got `after_onset`.\n- **Same-minute cut-off:** I counted any change up to 03:25:59 as \"at or before the onset minute.\" No change falls in that minute, so the result is the same either way.",
  "ttft_ms": 2741,
  "type": "result",
  "duration_ms": 25653,
  "uuid": "[REDACTED]",
  "ttft_stream_ms": 1532,
  "time_to_request_ms": 262,
  "first_content_frame_ms": 1532,
  "queued_turn_count": 0,
  "result_index": 0
 }
]