event-contract-design

name	event-contract-design
description	Use when designing or reviewing asynchronous event contracts: producer/consumer ownership, event envelope, schema, topic/channel naming, ordering, idempotency, versioning, compatibility, replay, dead-letter behavior, and AsyncAPI/CloudEvents-style documentation. Do NOT use for domain-event discovery (use `event-storming`), broad interface contracts (use `system-interface-contracts`), inbound provider webhook mechanics (use `webhook-integration`), or HTTP endpoint design (use `api-design`).
license	MIT
compatibility	Portable async-event contract guidance for queues, streams, pub/sub, internal events, outbound webhooks, and documented event-driven APIs.
allowed-tools	Read Grep
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Event Contract Design

Coverage

Design asynchronous event contracts for producers and consumers. Covers event envelope, schema, event type, topic/channel naming, producer ownership, consumer expectations, required and optional fields, idempotency keys, ordering, causation and correlation IDs, schema evolution, replay, dead-letter behavior, compatibility, observability, and machine-readable documentation such as AsyncAPI or CloudEvents-style metadata.

Philosophy

An event is a public promise once another consumer depends on it. If the payload, ordering, retry, or compatibility rules are implicit, every consumer invents its own interpretation and the event stream becomes shared folklore.

Do not confuse event discovery with event contracts. Discovery asks what happened in the domain. Contract design asks what exactly will be published, consumed, replayed, and evolved.

Method

1. Name the producer, owner, intended consumers, and event purpose.
2. Separate business event type from transport topic or queue name.
3. Define envelope fields: id, type, source, time, subject, schema version, correlation, causation, tenant, and idempotency key.
4. Define payload schema with required, optional, nullable, and deprecated fields.
5. State ordering, delivery, retry, replay, and dead-letter expectations.
6. Define compatibility rules: additive fields, breaking changes, versioning, deprecation, and consumer migration.
7. Add observability fields needed to reconstruct publishing and consumption failures.
8. Provide at least one positive and one negative contract fixture.

Evals

This skill ships a comprehension-eval artifact at examples/evals/event-contract-design.json. The checklist below is the authoring gate for async event contracts; the eval file is the grader surface.

Verification

• Producer, owner, and consumers are named
• Event type, topic/channel, envelope, and payload are distinct
• Required, optional, nullable, and deprecated fields are explicit
• Idempotency, ordering, retry, replay, and dead-letter behavior are stated
• Compatibility rules distinguish additive from breaking changes
• Correlation and causation IDs cross async boundaries