Agent-friendly

Klaviyo

Creating a profile, updating it by id, and reading it back is a clean dependent chain. Errors stand out: a bad email returns a JSON:API pointer to '/data/attributes/email', and a wrong resource type is told 'widget is not expected, profile is', so an agent self-corrects. A profile with no identifier is refused with 'One or more identifiers is required'. Best of all, retrying a duplicate create returns 409 with meta.duplicate_profile_id — the existing id handed back so a dropped-response retry recovers by updating instead of duplicating; events also take a unique_id for dedup.

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.

Retried an identical create with the same email (simulated dropped response) -> 409 {code:'duplicate_profile', detail:'A profile already exists with one of these identifiers.', meta.duplicate_profile_id:'<original id>'}. The returned id equals the original, so a retry recovers by PATCHing instead of duplicating. Events additionally accept a unique_id field for dedup (two POSTs with the same unique_id both 202).

See everything the AI did (11 steps)

t1  Claude Opus 4.8  POST /api/profiles (create)  → 201 id
t2  Claude Opus 4.8  POST /api/profiles (malformed email)  → 400 pointer:/data/attributes/email
t3  Claude Opus 4.8  POST /api/profiles (wrong type)  → 400 pointer:/data/type expected profile
t4  Claude Opus 4.8  POST /api/profiles (corrected)  → 201
t5  Claude Opus 4.8  POST /api/profiles (step1)  → 201 id
t6  Claude Opus 4.8  POST /api/events (for profile)  → 202 accepted
t7  Claude Opus 4.8  PATCH /api/profiles/{id}  → 200 updated
t8  Claude Opus 4.8  GET /api/profiles/{id}  → 200 last_name Updated
t9  Claude Opus 4.8  POST /api/profiles (no identifier)  → 400 identifiers required
t10  Claude Opus 4.8  POST /api/profiles (retry duplicate)  → 409 duplicate_profile_id == original
t11  Claude Opus 4.8  POST /api/events unique_id x2  → 202 / 202

Tested 2026-07-02 with Claude Opus 4.8 agents · request a re-test