protobuf-compat-checking

Overview

buf breaking is the canonical Protobuf compatibility checker from the Buf project (buf-breaking). It compares the current schema against a past version (BSR module, Git ref, tarball, or buf image) and reports changes that would break clients, servers, or generated code.

This is the Protobuf-specific counterpart to openapi-contract-diff (REST) and graphql-schema-regression (GraphQL).

When to use

• The repo has .proto files (gRPC service definitions, message-bus schemas, internal data contracts).
• The team uses Buf Schema Registry (BSR) or git-versioned Protobuf modules.
• A CI gate is needed on schema regressions before pushing a new version of generated client / server code.
• The team needs per-category strictness - e.g. FILE-level checks during early development, downgrade to WIRE for stable v1+ APIs where source-level changes are acceptable but wire-format breakage is not.

Breaking categories

buf breaking rules ship in four nested categories - stricter subsumes looser (buf-breaking):

Category	Detects
FILE	Source-code breakage per-file. Default; strictest.
PACKAGE	Source-code breakage per-package.
WIRE_JSON	Wire-format or JSON-encoding breakage.
WIRE	Wire-format breakage only. Loosest.

Choose FILE for new internal services where source compatibility matters; choose WIRE_JSON for stable APIs where source rearrangement (e.g. moving a message between files) is acceptable but JSON-mapping breakage is not; choose WIRE only when JSON encoding is out-of-scope.

Configuration

buf.yaml v2 form (buf-yaml):

version: v2
modules:
  - path: proto
    name: buf.build/acme/foo
breaking:
  use:
    - FILE
  except:
    - FILE_NO_DELETE          # allow file deletions despite using FILE
  ignore:
    - foo/legacy/             # exclude entire path from all breaking rules
  ignore_only:
    FILE_NO_DELETE:
      - foo/bar.proto         # exclude specific path from a specific rule
  ignore_unstable_packages: true   # ignore v1alpha1 / v1beta1 / *test* packages

version: v2 is required for the structure above; legacy v1 and v1beta1 files use a different shape (buf-yaml). When migrating, run buf migrate v1tov2 to convert.

ignore_unstable_packages: true is the canonical pattern - Protobuf convention is that v1alpha1, v1beta1, and *test* packages are unstable and explicitly NOT covered by compatibility guarantees.

Running

Against a Git reference

buf breaking --against '.git#branch=main'
buf breaking --against '.git#tag=v1.2.0'
buf breaking --against '.git#ref=HEAD~1'

(Per buf-breaking.)

The .git#... syntax tells buf to checkout the referenced ref into a temp directory and use it as the baseline. This is the most common CI pattern: compare PR against main.

Against a BSR module

buf breaking --against buf.build/acme/foo
buf breaking --against buf.build/acme/foo:<commit-sha>

Use the BSR baseline when the team treats BSR as the source of truth and a release labeled v1 is what production is consuming.

Against a tarball or buf image

buf breaking --against archive.tar.gz
buf breaking --against image.bin       # buf-image-format binary

Useful for hermetic builds where the baseline is published as an artifact rather than a git ref.

Reading the report

buf breaking exits non-zero on detected breaking changes. The default output prints one line per finding with the offending file, line, column, rule ID, and message:

proto/orders/v1/order.proto:42:3:Field "1" with name "amount" on message
"Order" has the same number as deleted field with name "total".
[ENUM_VALUE_NO_DELETE_RESERVED]

For machine consumption, supply --error-format json:

buf breaking --against '.git#branch=main' --error-format json

{
  "path": "proto/orders/v1/order.proto",
  "start_line": 42,
  "start_column": 3,
  "type": "ENUM_VALUE_NO_DELETE_RESERVED",
  "message": "Field 1 ..."
}

Pipe to jq for triage:

buf breaking --against '.git#branch=main' --error-format json | \
  jq -r '"\(.type) \(.path):\(.start_line) - \(.message)"'

CI integration

GitHub Actions

The Buf project ships an official Action (bufbuild/buf-action) that runs lint + breaking + push against BSR in one job. For self-hosted breaking-only:

# .github/workflows/buf-breaking.yml
name: buf-breaking

on:
  pull_request:
    paths:
      - '**/*.proto'
      - 'buf.yaml'
      - 'buf.lock'

jobs:
  breaking:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
        with:
          fetch-depth: 0   # need history for `.git#branch=main` baseline

      - uses: bufbuild/buf-setup-action@v1

      - name: buf breaking
        run: buf breaking --against '.git#branch=main,subdir=.'

fetch-depth: 0 is required so the .git#branch=main ref resolves - the default shallow clone doesn't have it.

For stricter pipelines, also gate on buf lint:

- name: buf lint
  run: buf lint
- name: buf breaking
  run: buf breaking --against '.git#branch=main,subdir=.'

Tag-based releases

For projects using git tags as release boundaries:

# Compare PR vs the latest released tag
LATEST_TAG=$(git describe --tags --abbrev=0)
buf breaking --against ".git#tag=${LATEST_TAG}"

This catches breaking changes vs production, not just vs the trunk HEAD - useful when main itself contains in-progress un-released work.

References

• buf-breaking - buf breaking semantics, categories, baseline reference formats.
• buf-yaml - v2 buf.yaml configuration; breaking.use, except, ignore, ignore_only, ignore_unstable_packages.
• openapi-contract-diff - REST counterpart.
• graphql-schema-regression - GraphQL counterpart.
• contract-compatibility-gate - gate skill aggregating breaking-change verdicts across protocols.