Cloudflare
Cloudflare's Workers KV API is clean. One POST creates a namespace; a missing title is a typed 400 code 10019 'title is required'; and a chain works - create a namespace, PUT a value on a key, GET it back, list the keys. Writing to a namespace id that doesn't exist is a typed 404 code 10013 'namespace not found'. Idempotency is right on both levels: a KV PUT is idempotent by key (writing twice leaves one key, latest value), and namespace titles are unique (same title twice -> 400 code 10014 'already exists'). (Cloudflare's terms explicitly permit publishing reproducible benchmarks.)
Cross-validated onClaude Opus 4.8
The five tasks
Getting started from the docs aloneNailed it
Fixing its own mistake after an errorNailed it
Following a multi-step flowNailed it
Handling an unclear edge caseNailed it
Not double-charging on a retryNailed it
Here’s the receipt — what actually happened, not our summary of it.
Two independent guarantees. KV PUT is idempotent by key: PUT 'dupkey'=v1 then =v2 both 200, GET returns v2, and the key list contains exactly one 'dupkey' (overwrite, no duplicate). Namespace titles are a unique natural key: POST the same title 'agentcheck-idem' twice -> first returns an id, second returns 400 {code:10014, message:'a namespace with this account ID and title already exists'}. A dropped-response retry can't create a duplicate namespace or key.See everything the AI did (5 steps)
t1 Claude Opus 4.8 POST /storage/kv/namespaces (create) → 200 namespace id t2 Claude Opus 4.8 POST /storage/kv/namespaces (no title) → 400 code 10019 'title is required' t3 Claude Opus 4.8 POST ns -> PUT key -> GET key -> list keys → chain ok: value read back, key listed t4 Claude Opus 4.8 PUT value into non-existent namespace id → 404 code 10013 namespace not found t5 Claude Opus 4.8 PUT same key x2 ; POST same namespace title x2 → overwrite one key ; 400 code 10014 already exists
Tested 2026-07-04 with Claude Opus 4.8 agents · request a re-test