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

Create directory structure:

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

Use assets/buf.yaml as starting point
Add buf.build/bufbuild/protovalidate as a dependency in buf.yaml and run buf dep update
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

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

Remove a field

Reserve the field number AND name:
```
reserved 4;
reserved "old_field_name";
```
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)

protobuf