use-protobuf

name	use-protobuf
description	Design, change, and review Protobuf schemas used as gRPC contracts, streaming protocols, typed data models, grpc-gateway REST APIs, OpenAPI specifications, or generated SDK inputs. Use for .proto files, RPC request/response design, oneof stream events, validation annotations, API versioning, schema evolution, generated package layout, and avoiding weak Struct or Any payloads.

Use Protobuf

Treat Protobuf as the canonical typed contract for gRPC, REST gateway bindings, OpenAPI, generated SDKs, and shared data structures. Model the domain before optimizing generated code.

Workflow

1. Read repository instructions and inspect existing .proto files, packages, versions, imports, generation configuration, and generated output conventions.
2. Classify each service as gRPC-only or HTTP-exposed before adding annotations.
3. Model every RPC, stream packet, enum, error, and reusable message with concrete types.
4. Apply transport-specific validation and documentation rules.
5. Evaluate compatibility according to the package's release state and version.
6. Generate every configured SDK and OpenAPI artifact. Use $use-buf and $use-buf-plugins when available.
7. Run repository tests and inspect generated diffs; never hand-edit generated code.

Define RPC Contracts Explicitly

• Define a dedicated RPC method for every gRPC operation.
• Name unary messages MethodRequest and MethodResponse.
• Keep request and response messages distinct even when their current fields happen to match.
• Use TitleCase service, method, message, and enum names; use lower_snake_case fields and UPPER_SNAKE_CASE enum values.
• Prefer comments that explain semantics, units, identity, presence, ordering, and lifecycle rather than restating the field name.

rpc GetDocument(GetDocumentRequest) returns (GetDocumentResponse);

Model Streaming Without Base Payloads

Use the appropriate gRPC streaming form:

rpc ImportRecords(stream ImportRecordsStreamRequest) returns (ImportRecordsResponse);
rpc WatchJob(WatchJobRequest) returns (stream WatchJobStreamResponse);
rpc Transfer(stream TransferStreamRequest) returns (stream TransferStreamResponse);

MUST NOT create an abstract, generic, inherited, or catch-all base event/base payload for stream requests or responses. Define concrete stream event, step, or packet messages. When several variants may occur, use oneof whose alternatives are separate messages. Use one homogeneous stream message only when every packet genuinely has the same body and semantics, such as fixed-shape file chunks.

Read references/schema-design.md before designing or changing any streaming RPC.

Separate gRPC-Only and HTTP-Exposed Schemas

For gRPC-only services, do not add google.api.http, grpc-gateway, or grpc.gateway.protoc_gen_openapiv2 imports, method options, message options, enum options, or field options.

For every *Request exposed through grpc-gateway, regardless of GET, POST, body, path, or query placement:

• validate accepted input with buf.validate annotations;
• document every request field with appropriate openapiv2 information such as example, description, format, range, length, pattern, or collection bounds;
• keep runtime validation authoritative and make OpenAPI constraints agree with it;
• define the google.api.http binding and operation documentation intentionally.

Do not hardcode a Buf cache path when researching extension fields. Follow the export/query workflow in references/http-and-openapi.md.

Prefer Strong Domain Types

• Use an enum whenever the value belongs to a finite controlled vocabulary; do not disguise it as an unconstrained string.
• Give enums an explicit zero value and document what the enum classifies.
• Prefer dedicated messages, well-known types, repeated fields, maps, and oneof over google.protobuf.Struct or google.protobuf.Any.
• Use Struct or Any only for a real open-world boundary. Document why typed modeling is impossible and how consumers validate the payload.
• In Go, use protojson for Protobuf JSON and a registered type resolver for Any; do not route Protobuf through generic encoding/json. Research the canonical library for every other target language before implementing serialization.

Organize and Version Schemas

• Set go_package on first-party schemas that generate Go.
• Keep language-neutral source paths and verify Python output with protoc-gen-python plus protoc-gen-pyi. Do not assume a kebab-case Protobuf source/import path automatically makes Python output invalid, and do not rename the source tree merely out of fear that it might violate Python module conventions; generate and verify the selected plugins' actual module/import behavior.
• Put API versions in both package and directory identity, using forms such as v1, v2, v1alpha1, or v1beta1.
• Before release, while a feature is explicitly experimental or feature-flagged, allow deliberate breaking redesigns and field renumbering only when every producer, consumer, stored payload, and generated artifact can be rebuilt together.
• After release, never renumber or reuse field numbers. Reserve removed field numbers and names, and introduce a new version for incompatible redesigns.
• Keep generated Go packages, Python modules, TypeScript modules, and other SDK paths aligned with the source schema path. Preserve each language plugin's conventional leaf layout.
• Change output directories or flattening only when the user explicitly requests it or the repository already declares that convention in generation configuration.

Generate Complete SDK Surfaces

• Generate runtime code and the language's customary typing artifacts.
• For Python, generate .py plus .pyi when supported.
• For TypeScript, generate .ts or the ecosystem-appropriate .d.ts/.d.mts artifacts.
• Prefer the established generator for each language and OpenAPI ecosystem. Do not write a parallel custom generator or manually maintain generated output.

Read references/schema-design.md for complete modeling and evolution rules, references/http-and-openapi.md for gateway schemas, and references/api-design-sources.md when designing public resources, errors, pagination, or long-running operations.