SchemaLens API

🚀 Quick Start

Send two SQL schemas to the endpoint and get back a full diff, migration SQL, and breaking change analysis.

curl -X POST https://schemalens.tech/api/diff \
  -H "Content-Type: application/json" \
  -d '{
    "schemaA": "CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(255));",
    "schemaB": "CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(255), email VARCHAR(255));",
    "dialect": "postgres",
    "format": "json"
  }'

📋 Parameters

Parameter	Type	Required	Description
schemaA	string	Yes	The old (base) schema SQL.
schemaB	string	Yes	The new (target) schema SQL.
dialect	string	No	postgres (default), mysql, sqlite, mssql
format	string	No	json (default) or markdown

📤 Response (JSON)

{
  "diff": { /* full diff object */ },
  "migration": "ALTER TABLE \"users\" ADD \"email\" VARCHAR ( 255 );",
  "breakingChanges": [],
  "summary": {
    "tablesAdded": 0,
    "tablesRemoved": 0,
    "tablesModified": 1,
    "enumsAdded": 0,
    "enumsRemoved": 0,
    "triggersAdded": 0,
    "triggersRemoved": 0,
    "triggersModified": 0,
    "viewsAdded": 0,
    "viewsRemoved": 0,
    "viewsModified": 0,
    "breakingChangeCount": 0
  }
}

📝 Response (Markdown)

Set format: "markdown" to receive a full markdown diff report suitable for PR comments, Slack messages, or documentation.

curl -X POST https://schemalens.tech/api/diff \
  -H "Content-Type: application/json" \
  -d '{
    "schemaA": "...",
    "schemaB": "...",
    "dialect": "postgres",
    "format": "markdown"
  }'

Response:

{
  "markdown": "# Schema Diff Report\n\n**Dialect:** PostgreSQL..."
}

🔥 Breaking Change Detection

The API automatically detects dangerous schema changes and returns them in the breakingChanges array:

• critical DROP_TABLE — Table removal
• critical DROP_COLUMN — Column removal
• critical ADD_NOT_NULL_NO_DEFAULT — NOT NULL column added without default
• critical DROP_CONSTRAINT — PRIMARY KEY, UNIQUE, or CHECK constraint dropped
• warning NARROW_TYPE — Column type narrowed (e.g., VARCHAR(255) → VARCHAR(100))
• warning ADD_FK_NO_INDEX — Foreign key added without supporting index

🛠️ Supported Objects

• CREATE TABLE (columns, constraints, indexes)
• CREATE INDEX / UNIQUE INDEX
• CREATE TYPE ... AS ENUM (PostgreSQL)
• CREATE TRIGGER (PostgreSQL)
• CREATE VIEW

🔒 Privacy

The API is stateless. Schema data is processed in-memory and never stored. No API key is required for the free tier. Rate limits are enforced at the edge by Vercel.

💻 CLI Alternative

For local use or CI/CD pipelines, use the SchemaLens CLI — a zero-dependency Node.js script that runs offline.

node ci/schemalens-diff.js schemaA.sql schemaB.sql --dialect=postgres --format=json