🚀 Contract Drift Detection (CDD)

The ultimate API sidekick for Frontend and Backend teams.

Frontend developers are constantly blocked waiting for incomplete backend endpoints. When the backend is finally ready, the real responses often drift from the original OpenAPI contract, silently breaking the UI.

Contract Drift Detection is a lightweight developer CLI that solves both problems. It gives you a fully stateful mock API while the backend is being built, and acts as a Drift-Detecting Proxy to catch contract violations once the real backend is ready.

✨ Why this is awesome

1. 🏗️ Build Without Blockers (Stateful Mock Mode)

Stop using static, hardcoded JSON mocks. CDD reads your OpenAPI 3.x spec and spins up a local API that actually remembers state.

• How it works: You send a POST /users, and CDD saves it to a local .mock-db.json file. Your next GET /users returns that new user. All defined right inside your OpenAPI spec using x-mock-state vendor extensions. No backend code required.
• Validation default: Mock responses are schema-validated by default. Disable only when needed with --no-validate-mock.

2. 🚨 Catch Breaking Changes (Proxy Drift Mode)

When the real backend is ready, don't change your frontend code. Just start CDD in Proxy Mode.

• How it works: CDD intercepts frontend requests, forwards them to the real backend, and validates the response payload against your OpenAPI spec. If the backend team changed userId to user_id, CDD prints a massive red drift alert in your terminal before it breaks production.

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🆚 How is this different from other tools?

Frontend developers usually have to choose between stateless OpenAPI mocks or stateful JSON mocks. CDD gives you both, plus real-time drift analysis.

Feature	contract-drift-detection	Stoplight Prism	json-server
OpenAPI 3.x Support	✅ Yes	✅ Yes	❌ No
Stateful Mocks (CRUD)	✅ Yes (.mock-db.json)	❌ No (Stateless)	✅ Yes (db.json)
Custom Mock Logic	✅ Yes (x-mock-state)	❌ No	❌ No
Proxy & Drift Detection	✅ Yes (Console & CI Alerts)	✅ Yes (Validation Proxy)	❌ No

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⚡ Quickstart: Try it in 30 seconds

Don't have an OpenAPI spec yet? Run this command to generate a starter contract and boot up the stateful server immediately:

npx contract-drift-detection@latest quickstart

🛠️ Project Integration Guide

Here is how you actually integrate this into your React, Next.js, or Vue project workflow.

Step 1: Install as a Dev Dependency

npm install -D contract-drift-detection concurrently

Step 2: Update your package.json scripts

Add these scripts so your team can easily run the frontend and the mock server together.

{
  "scripts": {
    "dev": "next dev",
    "mock:api": "cdd --spec ./openapi.yaml --port 4010",
    "dev:mock": "concurrently \"npm run mock:api\" \"npm run dev\"",
    "proxy:api": "cdd --spec ./openapi.yaml --drift-check http://localhost:8080 --port 4010",
    "dev:proxy": "concurrently \"npm run proxy:api\" \"npm run dev\""
  }
}

Step 3: Point your Frontend to the CDD Port

Change your API base URL in your frontend .env file to point to CDD:

VITE_API_BASE_URL=http://localhost:4010

Now, your workflow is simple:

• Backend isn't ready? Run npm run dev:mock.
• Backend is running locally on 8080? Run npm run dev:proxy to start catching drift!

🤖 CI/CD Integration (GitHub Actions)

Don't let backend contract drifts get merged into main. You can use CDD in your CI pipeline to block Pull Requests if the backend responses diverge from the OpenAPI contract.

The workflow uses CI-friendly flags:

• --fail-on-drift to exit non-zero on mismatches.
• --reporter json --report-file drift-report.json for machine-readable artifacts.

(Check out our ready-to-use drift gate workflow example at .github/workflows/drift-check.yml in this repository).

✅ Support Matrix

Use this table for precise expectations in enterprise adoption:

Capability	Status	Notes
OpenAPI 3.x contract loading	✅ Supported	Local files + remote/discovered specs
REST route generation (GET/POST/PUT/PATCH/DELETE)	✅ Supported	Core stateful CRUD coverage
JSON request/response validation	✅ Supported	Primary validation path
Stateful behavior via x-mock-state	✅ Supported	create, update, delete, replace, append
Proxy-based drift detection	✅ Supported	Includes --strict, ignore rules, reporters
XML / multipart / binary-heavy payload parity	⚠️ Partial	Depends on endpoint shape and schema coverage
Streaming/SSE/WebSocket contracts	❌ Not targeted	Focus is OpenAPI REST request/response
GraphQL / gRPC / SOAP-native protocols	❌ Not supported	Outside OpenAPI REST scope

🏢 Production Adoption

• Team rollout guide: see docs/PRODUCTION_ADOPTION.md
• CI drift gate template: docs/GITHUB_ACTIONS_TEMPLATE.md
• Weekly canary workflow: .github/workflows/reliability-canary.yml

📖 Reference & Advanced Usage

OpenAPI workflow actions (x-mock-state)

To make your mocks stateful, add the x-mock-state extension directly under your path operations.

Supported actions: create, update, delete, replace, append.

Example:

paths:
  /tickets/{id}/resolve:
    post:
      x-mock-state:
        action: update
        target: tickets
        find_by: id
        set:
          status: resolved

Template values are supported in assign and set:

• {{body.field}}
• {{params.id}}
• {{query.key}}
• {{headers.authorization}}

CLI Usage & Options

serve subcommand is optional. Root command accepts serve options directly.

cdd [--spec <path> | --spec-url <url> | --discover <backend-url>] 
    [--port 4010] 
    [--host 0.0.0.0] 
    [--db .mock-db.json] 
    [--cors-origin *] 
    [--drift-check <url>] 
    [--no-validate-mock]
    [--fallback-to-mock] 
    [--verbose]

cdd init [--spec openapi.yaml] [--template rest-crud|none]

cdd quickstart [--spec openapi.yaml] [--port 4010]

Spec resolution priority:

1. --spec (Local file)
2. --spec-url (Explicit remote URL)
3. --discover (Auto-scans common backend paths)

Spec Discovery

If you don't have the spec locally, use --discover http://localhost:8080. CDD will automatically check common paths including:

• /openapi.json
• /swagger.json
• /v3/api-docs
• etc.

CORS Notes

Default CORS is enabled for all origins (*) for fast local onboarding. For stricter browser setup:

npx contract-drift-detection@latest --spec ./openapi.yaml --cors-origin http://localhost:5173

Runtime diagnostic endpoints

When the server is running, you can access:

• GET /__health → health check
• GET /__spec → resolved OpenAPI in memory
• GET /__routes → generated route summary

Mock validation mode

• Default behavior validates mock-mode responses against your OpenAPI success schemas.
• To disable this check temporarily, pass --no-validate-mock.
• Proxy drift detection (--drift-check) remains available for validating real backend responses.

.driftignore syntax

Create a .driftignore file in your project root to suppress specific drift findings.

Each line format:

[METHOD] [PATH_PATTERN] [JSON_PATH]

Examples:

* * body.trace_id
GET /users body.metadata.*
POST /orders/* body.items.*.debug

Notes:

• * wildcard is supported in all three fields.
• JSON_PATH is matched as dot-path form rooted at body (not data).
• Use body to ignore root-level errors (for example AJV instance path /).
• Use body.* to ignore object-level item paths (for example AJV paths like /0, /1).
• Lines starting with # are comments.

You can verify .driftignore load status in startup logs:

• Ignore rules: cli-paths=..., file-rules=<count>

🐛 Troubleshooting

• Could not discover an OpenAPI spec: Try an explicit endpoint with --spec-url, or use quickstart if the backend doesn't expose OpenAPI yet.
• EADDRINUSE: address already in use: Port 4010 is taken. Pass --port 4020.
• Debugging: Need full stack traces? Run CDD_SHOW_STACK=1 npx contract-drift-detection@latest --spec ./openapi.yaml

📄 License

MIT

📦 Binaries

contract-drift-detection, cdd

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