SchemGuard

API Schema Drift Monitor — detect breaking changes in OpenAPI specs before they break your consumers.

Why

APIs break silently. A field gets renamed, an endpoint gets removed, an enum value disappears — and downstream consumers break in production. SchemGuard catches these before deploy.

Install

npm install -g schemaguard

Usage

Diff two specs

schemaguard diff old-api.yaml new-api.yaml

Output:

Found 11 change(s):

❌ BREAKING CHANGES (9):
──────────────────────────────────────────────────
  ⛔ [endpoint-removed]
     Endpoint removed: DELETE /pets/{petId}
     at: DELETE /pets/{petId}

  ⛔ [field-type-changed]
     Parameter type changed: petId (string → integer)
     at: GET /pets/{petId} > param petId
  ...

🚨 9 breaking change(s) detected — deployment blocked.

CI mode

schemaguard ci --spec ./openapi.yaml --baseline ./main-openapi.yaml

• Exit 0 = no breaking changes, safe to deploy
• Exit 1 = breaking changes detected, blocks the pipeline
• Exit 2 = error (invalid spec, file not found)

Lint a spec

schemaguard lint ./openapi.yaml

Checks for missing operationId, missing descriptions, no security schemes, etc.

JSON output

schemaguard diff old.yaml new.yaml --format json

Returns structured JSON for programmatic consumption by agents and CI tools.

What it detects

Breaking changes (exit code 1)

| Rule | Description | |------|-------------| | endpoint-removed | An endpoint was deleted | | method-removed | An HTTP method was removed from a path | | required-param-added | A new required parameter was added | | param-removed | An existing parameter was removed | | request-field-made-required | A request field became required | | field-type-changed | A field's type was changed | | response-field-removed | A response field was removed | | enum-value-removed | An enum value was narrowed | | auth-requirement-changed | Security schemes were modified | | response-code-removed | A response status code was removed |

Non-breaking changes (info only)

| Rule | Description | |------|-------------| | endpoint-added | A new endpoint was added | | optional-param-added | A new optional parameter was added | | response-field-added | A new response field was added | | enum-value-added | An enum value was widened | | description-changed | Description or summary text changed | | deprecated | An endpoint was marked as deprecated |

GitHub Actions

- name: Check API compatibility
  run: npx schemaguard ci --spec ./openapi.yaml --baseline ./baseline.yaml

Programmatic API

import { parseSpec, diffSpecs, formatDiff } from 'schemaguard';

const oldSpec = parseSpec('./v1.yaml');
const newSpec = parseSpec('./v2.yaml');
const result = diffSpecs(oldSpec, newSpec);

if (result.hasBreakingChanges) {
  console.log(`${result.breaking.length} breaking changes found`);
}

License

MIT