Decision guide
OpenAPI vs AsyncAPI vs Protobuf
Which contract format should describe your API?
Side by side
| Criterion | OpenAPI | AsyncAPI | Protobuf (+ gRPC) |
|---|---|---|---|
| Protocol focus | HTTP | Message brokers / streams | gRPC / binary messages |
| Schema language | JSON Schema 2020-12 | JSON Schema / Avro / Protobuf | Protobuf IDL |
| Code generation | Clients, servers, SDKs | Docs, code, validators | Clients and servers (excellent) |
| Breaking-change tools | oasdiff | Schema registry compatibility | buf breaking |
| Mock/test tooling | Prism, Schemathesis | Microcks | grpcurl, buf, Microcks |
| Docs rendering | Redocly, Scalar, Swagger UI | AsyncAPI Studio/generator | buf.build, protoc-gen-doc |
When to choose each
OpenAPI
Describes HTTP APIs: paths, operations, JSON Schema payloads, security, webhooks.
Choose when
- REST/HTTP JSON APIs
- Public developer portals and SDKs
Avoid when
- Event streams
- gRPC services
AsyncAPI
Describes event-driven APIs: channels, operations, messages and protocol bindings.
Choose when
- Kafka/AMQP/MQTT/WebSocket events
- Documenting event contracts between teams
Avoid when
- Request/response HTTP APIs
Protobuf (+ gRPC)
IDL for binary messages and RPC services, with strict field numbering.
Choose when
- gRPC services
- High-volume events with schema registry
- Strong backward compatibility rules
Avoid when
- Public JSON APIs consumed by browsers
Verdict
Use the format that matches the interaction style — often all three in one company. What matters is contract-first discipline: specs in the repo, linted, diffed for breaking changes and used to generate code.