api-schema-diff 0.1.1

Detect breaking changes between API schemas (OpenAPI / JSON Schema)

api-schema-diff

Detect breaking changes between API schemas (OpenAPI / JSON Schema) in a deterministic, CI-friendly way.

api-schema-diff is a CLI tool designed to answer one question reliably:

"Will this schema change break existing clients?"

---

Why this exists

API changes often break clients silently.

Common causes:

• Removing endpoints
• Removing fields
• Changing parameter requirements
• Changing request/response schemas

api-schema-diff catches these issues before they reach production.

---

Features

Supported inputs

• OpenAPI 3.x (JSON / YAML)
• JSON Schema (generic structure diff)

Breaking changes detected

• Removed paths or operations
• Removed query / path / header parameters
• Parameters becoming required
• Parameter schema type changes
• Request body removed
• Request body becoming required
• Request body schema breaking changes
• Removed response status codes
• Response schema breaking changes
• Removed properties in schemas
• Property type changes
• Optional → required fields

Non-breaking changes detected

• Added paths or operations
• Added optional parameters
• Added optional request bodies
• Added response status codes
• Added optional properties

Designed for CI

• Deterministic output
• Stable exit codes
• JSON output for automation
• Works offline (no cloud / no LLM dependency)

---

Installation

pip install api-schema-diff

Or install from source:

git clone https://github.com/teolzr/schema-diff.git
cd schema-diff
pip install -e .

Requirements:

• Python 3.10 or higher
• typer>=0.12
• rich>=13.7

---

Usage

Basic usage

api-schema-diff old.json new.json

Exit codes:

• 0 → No breaking changes found
• 1 → Breaking changes detected

Output formats

Text output (default):

api-schema-diff old.json new.json

Output:

BREAKING CHANGES FOUND

┏━━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━┓
┃ Type         ┃ Path        ┃ Old Type┃ New Type┃ Message               ┃
┡━━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━╇━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━┩
│ removed_field│ User.email  │         │         │                       │
│ type_change  │ Order.amount│ <number>│ <string>│                       │
└──────────────┴─────────────┴─────────┴─────────┴───────────────────────┘

JSON output:

api-schema-diff old.json new.json --format json

Output:

{
  "breaking": [
    {
      "type": "removed_field",
      "severity": "breaking",
      "path": "User.email",
      "old_type": null,
      "new_type": null,
      "message": null
    },
    {
      "type": "type_change",
      "severity": "breaking",
      "path": "Order.amount",
      "old_type": "<number>",
      "new_type": "<string>",
      "message": null
    }
  ],
  "non_breaking": []
}

CLI Options

api-schema-diff [OPTIONS] OLD_FILE NEW_FILE

Arguments:

• OLD_FILE - Path to the old schema file (JSON or YAML)
• NEW_FILE - Path to the new schema file (JSON or YAML)

Options:

• --format [text|json] - Output format (default: text)
• --fail-on-breaking / --no-fail-on-breaking - Exit with code 1 when breaking changes are found (default: true)
• --help - Show help message

Report-only mode

Use --no-fail-on-breaking to always exit with code 0 (useful for reporting without failing CI):

api-schema-diff old.json new.json --no-fail-on-breaking

---

Examples

Example 1: Generic JSON diff

old.json:

{
  "User": {
    "email": "[e-mail]",
    "age": 30
  },
  "Order": {
    "amount": 12.5
  }
}

new.json:

{
  "User": {
    "age": "30"
  },
  "Order": {
    "amount": "12.5"
  },
  "NewThing": {}
}

api-schema-diff old.json new.json

Detected changes:

• Breaking: User.email field removed
• Breaking: User.age type changed from number to string
• Breaking: Order.amount type changed from number to string
• Non-breaking: NewThing object added

Example 2: OpenAPI diff

old-api.yaml:

openapi: 3.0.0
paths:
  /users:
    get:
      parameters:
        - name: limit
          in: query
          required: false
          schema:
            type: integer
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                type: object
                properties:
                  name:
                    type: string
                  email:
                    type: string

new-api.yaml:

openapi: 3.0.0
paths:
  /users:
    get:
      parameters:
        - name: limit
          in: query
          required: true  # Now required!
          schema:
            type: integer
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                type: object
                properties:
                  name:
                    type: string
                  # email removed!

api-schema-diff old-api.yaml new-api.yaml

Detected changes:

• Breaking: Query parameter limit became required
• Breaking: Response property email removed

---

CI/CD Integration

GitHub Actions

name: Schema Diff Check

on:
  pull_request:
    paths:
      - 'api/schema.yaml'

jobs:
  schema-diff:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0

      - name: Set up Python
        uses: actions/setup-python@v5
        with:
          python-version: '3.10'

      - name: Install api-schema-diff
        run: pip install api-schema-diff

      - name: Get old schema from main branch
        run: git show origin/main:api/schema.yaml > old-schema.yaml

      - name: Check for breaking changes
        run: api-schema-diff old-schema.yaml api/schema.yaml

GitLab CI

api-schema-diff:
  image: python:3.10
  before_script:
    - pip install api-schema-diff
  script:
    - git show origin/main:api/schema.yaml > old-schema.yaml
    - api-schema-diff old-schema.yaml api/schema.yaml
  only:
    changes:
      - api/schema.yaml

Pre-commit hook

Create .pre-commit-config.yaml:

repos:
  - repo: local
    hooks:
      - id: api-schema-diff
        name: Check API schema for breaking changes
        entry: bash -c 'git show HEAD:api/schema.yaml > /tmp/old-schema.yaml && api-schema-diff /tmp/old-schema.yaml api/schema.yaml'
        language: system
        files: 'api/schema.yaml'
        pass_filenames: false

---

Development

Setup

# Clone the repository
git clone https://github.com/teolzr/schema-diff.git
cd schema-diff

# Install in development mode
pip install -e .

# Install development dependencies
pip install pytest pytest-cov black ruff

Running tests

# Run all tests
pytest

# Run with coverage
pytest --cov=schema_diff --cov-report=html

# Run specific test file
pytest tests/test_openapi_diff.py

Note: The Python package is still named schema_diff internally, but the PyPI package and CLI command are api-schema-diff.

Code formatting

# Format code
black .

# Lint code
ruff check .

Project structure

schema-diff/
├── schema_diff/
│   ├── __init__.py
│   ├── cli.py          # CLI entry point
│   ├── diff.py         # Generic JSON diff logic
│   ├── loader.py       # Schema file loading
│   ├── models.py       # Data models
│   ├── rules.py        # Breaking change rules
│   └── openapi/
│       ├── __init__.py
│       ├── diff.py             # OpenAPI-specific diff
│       ├── json_schema_diff.py # JSON Schema diffing
│       ├── normalizer.py       # Schema normalization
│       └── resolver.py         # $ref resolution
├── tests/
├── pyproject.toml
└── README.md

---

How it works

1. Schema loading: Automatically detects schema type (OpenAPI vs generic JSON/YAML)
2. Normalization: Resolves $ref references and normalizes structure
3. Diffing: Compares schemas using rule-based detection
4. Classification: Categorizes changes as breaking or non-breaking
5. Reporting: Outputs results in human-readable or JSON format

Change detection logic

Breaking changes:

• Removing existing fields/paths → clients expect them
• Changing types → clients may send wrong data type
• Making optional fields required → clients may not send them
• Removing response fields → clients may depend on them

Non-breaking changes:

• Adding new fields/paths → clients can ignore them
• Making required fields optional → clients can still send them
• Adding new optional fields → backwards compatible

---

Roadmap

• Support for OpenAPI 2.0 (Swagger)
• GraphQL schema diffing
• Custom rule configuration
• HTML report generation
• API compatibility scoring
• Severity levels (error, warning, info)

---

Contributing

Contributions are welcome! Please:

1. Fork the repository
2. Create a feature branch (git checkout -b feature/amazing-feature)
3. Make your changes
4. Add tests for new functionality
5. Run tests and linting
6. Commit your changes (git commit -m 'Add amazing feature')
7. Push to the branch (git push origin feature/amazing-feature)
8. Open a Pull Request

---

License

MIT License - see LICENSE file for details

---

Credits

Built with:

• Typer - CLI framework
• Rich - Terminal formatting

---

Related Projects

• openapi-diff - OpenAPI comparison tool
• swagger-diff - Swagger API comparison
• json-schema-diff - JSON Schema comparison

---

Support

• 🐛 Issues: GitHub Issues
• 💬 Discussions: GitHub Discussions

Note: Repository name is schema-diff, but PyPI package name is api-schema-diff