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 [bestpractices.md](references/bestpractices.md)
• Add [protovalidate](references/protovalidate.md) constraints to every field—this is not optional for production APIs
• For service templates, see [assets/](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

Project Setup

New Project

1. Create directory structure:

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

1. Use assets/buf.yaml as starting point
2. Add buf.build/bufbuild/protovalidate as a dependency in buf.yaml and run buf dep update
3. Use assets/buf.gen.*.yaml for code generation config

Code Generation Templates

Proto File Templates

Located in assets/proto/example/v1/:

Common Tasks

Add a new field

1. Use next sequential field number
2. Add [protovalidate](references/protovalidate.md) 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:

``protobuf reserved 4; reserved "oldfieldname"; ``

1. 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](references/protovalidate.md) for the full reference.

Common constraints:

• String formats: .string.uuid, .string.email, .string.uri, .string.pattern
• String bounds: .string.minlen, .string.maxlen
• Numeric bounds: .int32.gte, .uint32.lte
• Enum validation: .enum.definedonly, .enum.notin = 0
• Repeated bounds: .repeated.minitems, .repeated.maxitems
• 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)