> ## Documentation Index
> Fetch the complete documentation index at: https://docs.lettr.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Auth Check

> Validate the provided API key and return associated team information. Use this endpoint to verify API key validity in client libraries.

Validate your API key and confirm it is active. Returns the team ID associated with the key along with an ISO 8601 timestamp.

Use this endpoint to verify API key validity in client libraries or during initial setup before making other API calls.


## OpenAPI

````yaml GET /auth/check
openapi: 3.1.0
info:
  title: Lettr API
  version: 1.0.0
  description: >-
    Lettr Email API - Send transactional emails with tracking, attachments, and
    personalization.
  contact:
    name: Lettr Support
    url: https://lettr.com
  license:
    name: Proprietary
    url: https://lettr.com/terms
servers:
  - url: https://app.lettr.com/api
    description: Production
security: []
tags:
  - name: Emails
    description: Email sending operations
  - name: Templates
    description: Email template management operations
  - name: Domains
    description: Domain management operations
  - name: Webhooks
    description: Webhook management operations
  - name: Audience
    description: 'Audience management: lists, contacts, topics, properties, and segments'
  - name: Campaigns
    description: >-
      Campaign operations: listing, stats, engagement events, and
      dispatch/scheduling
paths:
  /auth/check:
    get:
      tags:
        - System
      summary: Validate API Key
      description: >-
        Validate the provided API key and return associated team information.
        Use this endpoint to verify API key validity in client libraries.
      operationId: authCheck
      responses:
        '200':
          description: API key is valid
          content:
            application/json:
              schema:
                type: object
                required:
                  - message
                  - data
                properties:
                  message:
                    type: string
                    example: API key is valid.
                  data:
                    type: object
                    required:
                      - team_id
                      - timestamp
                    properties:
                      team_id:
                        type: integer
                        description: The team ID associated with the API key
                        example: 123
                      timestamp:
                        type: string
                        format: date-time
                        example: '2024-01-15T10:30:00.000Z'
              example:
                message: API key is valid.
                data:
                  team_id: 123
                  timestamp: '2024-01-15T10:30:00.000Z'
        '401':
          $ref: '#/components/responses/UnauthorizedError'
      security:
        - bearerAuth: []
components:
  responses:
    UnauthorizedError:
      description: Unauthorized - Missing or invalid API key
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/UnauthorizedResponse'
          examples:
            missing_key:
              summary: Missing API key
              value:
                message: API key is required.
            invalid_key:
              summary: Invalid API key
              value:
                message: Invalid API key.
  schemas:
    UnauthorizedResponse:
      type: object
      required:
        - message
      description: Error response for authentication failures (401).
      properties:
        message:
          type: string
          description: Human-readable error message
          example: API key is required.
  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer
      description: API key for authentication

````