protobuf

name	protobuf
description	Use when working with Protocol Buffer (.proto) files, buf.yaml, buf.gen.yaml, or buf.lock. Covers proto design, buf CLI, gRPC/Connect services, protovalidate constraints, schema evolution, and troubleshooting lint/breaking errors.
filePatterns	["**/*.proto","**/buf.yaml","**/buf.*.yaml","**/buf.gen.yaml","**/buf.gen.*.yaml","**/buf.lock"]

Protocol Buffers

When You Need This Skill

• Creating or editing .proto files
• Setting up buf.yaml or buf.gen.yaml
• Designing gRPC or Connect services
• Adding protovalidate constraints
• Troubleshooting buf lint or breaking change errors

Core Workflow

1. Match Project Style

Before writing proto code, review existing .proto files in the project. Match conventions for naming, field ordering, structural patterns, validation, and documentation style. If none exists, ask the user what style should be used or an existing library to emulate.

2. Write Proto Code

• Apply universal best practices from best_practices.md
• Add protovalidate constraints to every field—this is not optional for production APIs
• For service templates, see assets/

3. Verify Changes

Always run after making changes:

buf format -w && buf lint

Check for a Makefile first—many projects use make lint or make format.

Fix all errors before considering the change complete.

Quick Reference

Task	Reference
Field types, enums, oneofs, maps	quick_reference.md
Schema evolution, breaking changes	best_practices.md
Validation constraints	protovalidate.md
Complete service examples	examples.md, assets/
buf CLI, buf.yaml, buf.gen.yaml	buf_toolchain.md
Migrating from protoc	migration.md
Lint errors, common issues	troubleshooting.md
Proto API review checklist	review_checklist.md

Project Setup

New Project

1. Create directory structure:

   ├── buf.yaml
   ├── buf.gen.yaml
   └── proto/
       └── company/
           └── domain/
               └── v1/
                   └── service.proto
   

   buf.yaml sits above proto/: modules[].path resolves relative to the config's own directory, so a buf.yaml inside proto/ makes Buf look for proto/proto.

2. Use assets/buf.yaml as starting point

3. Add buf.build/bufbuild/protovalidate as a dependency in buf.yaml and run buf dep update

4. Use assets/buf.gen.*.yaml for code generation config

Code Generation Templates

Template	Use For
buf.gen.go.yaml	Go with gRPC
buf.gen.go-connect.yaml	Go with Connect
buf.gen.ts.yaml	TypeScript with Connect
buf.gen.python.yaml	Python with gRPC
buf.gen.java.yaml	Java with gRPC

Proto File Templates

Located in assets/proto/example/v1/:

Template	Description
book.proto	Entity message, BookRef oneof, enum
book_service.proto	Full CRUD with batch ops, pagination, ordering

Common Tasks

Add a new field

1. Use next sequential field number
2. Add protovalidate constraints: every field should have validation appropriate to its type (format validators, length bounds, numeric ranges, enum constraints, etc.)
3. Document the field
4. Run buf format -w && buf lint

Remove a field

1. Reserve the field number AND name:

   reserved 4;
   reserved "old_field_name";
   

2. Run buf breaking --against '.git#branch=main' to verify

Add protovalidate constraints

Every field in a production API should have appropriate validation. See protovalidate.md for the full reference.

Common constraints:

• String formats: .string.uuid, .string.email, .string.uri, .string.pattern
• String bounds: .string.min_len, .string.max_len
• Numeric bounds: .int32.gte, .uint32.lte
• Enum validation: .enum.defined_only, .enum.not_in = 0
• Repeated bounds: .repeated.min_items, .repeated.max_items
• Required fields: (buf.validate.field).required = true
• Oneof required: (buf.validate.oneof).required = true

Verification Checklist

After making changes:

• Every field has appropriate protovalidate constraints
• buf format -w (apply formatting)
• buf lint (check style rules)
• buf breaking --against '.git#branch=main' (if modifying existing schemas)