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Claude Sonnet 5.5 (effort default (no --effort flag) · thinking on) on Consistent-hash ring with weighted virtual nodes and zone-aware replica placement

AClaude Sonnet 5.5default · thinking on · Claude Code 2.1.299 (headless -p) · cloud · 2026-10-20 · by @bobsiopener

Trust 80%Fully autonomoussuccess · first try
Estimated cost
$0.07
Wall time
19s
Human step-ins
0
Outcome
Pass

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Summary

Exact prompt

# Consistent-hash ring with weights and zone-aware replicas `ring.py` places cache keys on storage nodes. The draft uses the wrong hash, ignores weights and zones, and is slow. Rewrite it (Python 3 stdlib only) so that `Ring(nodes, vnodes=64)`, `Ring.owner(key)`, `Ring.replicas(key, n)` and `moves(old_ring, new_ring, keys)` behave exactly as below. ## Hashing 1. `H(s)` = the first 8 bytes of SHA-256 of the UTF-8 encoding of `s`, read as a big-endian unsigned integer. 2. `nodes` maps a node name to a dict with optional `weight` (number, default 1) and `zone` (string, default = the node's own name). `vnodes` must be an int >= 1, else `ValueError`; a negative weight is a `ValueError`. 3. A node gets `k = floor(vnodes * weight + 0.5)` virtual points, but at least 1 if weight > 0. A node with weight 0 gets none and does not exist for the ring. Point i (0 <= i < k) of node `name` is at position `H(f"{name}#{i}")`. 4. The ring is all points sorted by (position, node name, i). ## Lookups 5. `owner(key)`: the node of the first point whose position is **>= H(key)**; if none, wrap around to the first point. An empty ring raises `LookupError`. 6. `replicas(key, n)`: start at the point `owner` would use and walk the ring clockwise (wrapping) over every point once. Look at each **distinct** node the first time it is met: if its zone is not yet used by a chosen node, choose it; otherwise remember it as skipped. Stop as soon as n nodes are chosen. If the walk ends with fewer than n chosen, append skipped nodes in the order they were first met until there are n (or none are left). `n < 1` gives `[]`. The owner is always first. 7. `moves(old, new, keys)`: for every key in `keys` whose owner differs, count the pair `(old_owner, new_owner)`; return a dict of these counts with keys in sorted order. 8. **Speed:** with 60 nodes at vnodes=128, building the ring, 60,000 `owner` calls and 10,000 `replicas(key, 3)` calls must take under 10 seconds in total on our shared test box. ## Scoring Hidden check groups (partial credit = fraction passed; all must pass): `hash_and_points` (1, 3, 4: owners for 2,000 keys on small rings), `weights_rounding` (3: e.g. weight 0.5/0.0078/0, exact owners), `wraparound_and_errors` (2, 5), `replicas_zones` (6), `replicas_fallback` (6: fewer zones than n, n larger than node count), `moves_counts` (7: adding, removing, reweighting a node), `speed` (8).

Exact owners for thousands of keys require the stated hash, point naming, rounding of weights (minimum one point) and >= with wraparound; the replica walk with zone skipping and ordered fallback, plus a speed check that rules out linear scans. 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. hash_and_points 2. weights_rounding 3. wraparound_and_errors 4. replicas_zones 5. replicas_fallback 6. moves_counts 7. speed (< 10 s)

Setup notes: Use Case Scout batch 8 case i09-consistent-hash-ring (offline fixture). Prepared with batch8/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`. Tools: Read/Write/Edit/Glob/Grep/Bash, web disabled. Single run, no retries. 1-min load average at scoring: 2.29. Claude Code's own reported cost for claude-sonnet-5.5: $0.066786. Score: 7/7. 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
19s
Human step-ins
0
Turns
4
Model usage
ModelInput tokOutput tokCost
AClaude Sonnet 5.5anthropic/claude-sonnet-5.5effort default (no --effort flag) · thinking on80,7052,392$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. hash_and_points 2. weights_rounding 3. wraparound_and_errors 4. replicas_zones 5. replicas_fallback 6. moves_counts 7. speed (< 10 s)
  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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Evidence log
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  "result": "I rewrote `ring.py` to the spec and ran a quick check of my own. I haven't run the hidden check groups.\n\n- **Speed:** with 60 nodes at vnodes=128 and 5 zones, building the ring, 60,000 `owner` calls and 10,000 `replicas(key, 3)` calls took about 0.09 s. The limit is 10 s.\n- **Replicas:** `replicas(\"a\", 3)` returned three nodes from three different zones. Asking for 100 returned all 60 nodes.\n- **Weights:** a node with weight 0.0078 got one point, and a weight-0 node was left out of the ring.\n\nWhere the spec was silent, I chose these behaviours:\n- **Empty ring:** `replicas` with `n < 1` returns `[]` even on an empty ring. With `n >= 1` it raises `LookupError`.\n- **`vnodes` check:** a `bool` is rejected as `vnodes`, along with non-ints and values below 1.\n\nI didn't test the exact owners, wraparound, error cases or `moves` counts.",
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]

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