Skip to main content
The generate letter endpoint creates professional letters using artificial intelligence based on your specified parameters. You can customize the letter type, tone, language, and provide details to generate personalized content.

​
Endpoint

POST https://api.lettergenerator.co/v1/generate

​
Request parameters

​
type
string
required
The type of letter to generate. Choose from 25+ available letter types.Available types:
  • cover_letter - Professional cover letter for job applications
  • recommendation_letter - Letter of recommendation
  • resignation_letter - Formal resignation letter
  • thank_you_letter - Thank you or appreciation letter
  • apology_letter - Formal or personal apology
  • complaint_letter - Complaint or grievance letter
  • love_letter - Romantic or affectionate letter
  • invitation_letter - Event or occasion invitation
  • business_letter - General business correspondence
  • formal_letter - Formal correspondence
  • And 15+ more types
​
tone
string
required
The tone or style of the letter. Choose from 17 available tones.Available tones:
  • professional - Business-appropriate and formal
  • friendly - Warm and approachable
  • persuasive - Convincing and compelling
  • romantic - Affectionate and heartfelt
  • formal - Very formal and traditional
  • casual - Relaxed and informal
  • apologetic - Sincere and remorseful
  • enthusiastic - Energetic and positive
  • empathetic - Understanding and compassionate
  • And 8+ more tones
​
recipientName
string
required
The name of the person or organization receiving the letter.Example: "Sarah Johnson", "TechCorp Hiring Team"
​
purpose
string
required
A brief description of the letter’s main purpose or objective.Example: "Applying for Senior Software Engineer position", "Requesting refund for defective product"
​
details
string
required
Additional context, background information, or specific points to include in the letter. This helps the AI generate more personalized and relevant content.Example: "I have 5 years of experience in full-stack development with React and Node.js. Led a team of 3 developers on the XYZ project."
​
language
string
default:"en"
The language code for the generated letter. Supports multiple languages for global communication.Supported languages:
  • en - English (default)
  • es - Spanish
  • fr - French
  • de - German
  • it - Italian
  • pt - Portuguese
  • zh - Chinese
  • ja - Japanese
  • And more
​
senderName
string
The name of the person sending the letter. If provided, it will be used in the letter signature.Example: "John Smith"
​
length
string
default:"medium"
The desired length of the generated letter.Options:
  • short - Brief and concise (100-200 words)
  • medium - Standard length (200-400 words)
  • long - Detailed and comprehensive (400-600 words)

​
Request example

curl https://api.lettergenerator.co/v1/generate \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -X POST \
  -d '{
    "type": "cover_letter",
    "tone": "professional",
    "recipientName": "Sarah Johnson",
    "purpose": "Applying for Senior Software Engineer position at TechCorp",
    "details": "I have 5 years of experience in full-stack development, specializing in React, Node.js, and cloud infrastructure. I led the development of a microservices platform that improved system performance by 40%. I am passionate about building scalable applications and mentoring junior developers.",
    "senderName": "Michael Chen",
    "language": "en",
    "length": "medium"
  }'

​
Response

A successful request returns a JSON object with the generated letter content.
​
id
string
Unique identifier for the generated letter.
​
letter
string
The complete generated letter content in plain text format.
​
wordCount
integer
The number of words in the generated letter.
​
type
string
The letter type that was requested.
​
tone
string
The tone that was applied to the letter.
​
language
string
The language code of the generated letter.
​
createdAt
string
ISO 8601 timestamp of when the letter was generated.
​
creditsUsed
integer
Number of API credits consumed by this request.

​
Response example

{
  "id": "let_9j4k2m1n8p7q6r5s",
  "letter": "Dear Sarah Johnson,\n\nI am writing to express my strong interest in the Senior Software Engineer position at TechCorp. With five years of hands-on experience in full-stack development and a proven track record of delivering scalable solutions, I am confident that I would be a valuable addition to your team.\n\nThroughout my career, I have specialized in React, Node.js, and cloud infrastructure, consistently pushing the boundaries of what's possible in modern web development. Most notably, I led the development of a comprehensive microservices platform that improved overall system performance by 40%, demonstrating my ability to architect solutions that drive tangible business results.\n\nBeyond technical expertise, I am deeply passionate about building scalable applications and mentoring junior developers. I believe that great engineering is not just about writing code, but about fostering a collaborative environment where innovation thrives and team members grow together.\n\nI would welcome the opportunity to discuss how my experience and vision align with TechCorp's goals. Thank you for considering my application.\n\nSincerely,\nMichael Chen",
  "wordCount": 187,
  "type": "cover_letter",
  "tone": "professional",
  "language": "en",
  "createdAt": "2026-03-03T14:30:00Z",
  "creditsUsed": 1
}

​
Error responses

The API returns standard HTTP status codes to indicate success or failure.

​
400 Bad Request

Returned when the request is malformed or missing required parameters. 
{
  "error": {
    "code": "invalid_request",
    "message": "Missing required parameter: type",
    "type": "validation_error",
    "param": "type"
  }
}

​
401 Unauthorized

Returned when the API key is missing or invalid. 
{
  "error": {
    "code": "invalid_api_key",
    "message": "The API key provided is invalid or has been revoked",
    "type": "authentication_error"
  }
}

​
429 Too Many Requests

Returned when you exceed your rate limit. 
{
  "error": {
    "code": "rate_limit_exceeded",
    "message": "You have exceeded your daily rate limit of 100 requests. Limit resets at 2026-03-04T00:00:00Z",
    "type": "rate_limit_error",
    "retryAfter": 34560
  }
}

​
500 Internal Server Error

Returned when an unexpected error occurs on the server. 
{
  "error": {
    "code": "internal_error",
    "message": "An unexpected error occurred. Please try again later.",
    "type": "server_error"
  }
}

​
503 Service Unavailable

Returned when the service is temporarily unavailable due to maintenance or high load. 
{
  "error": {
    "code": "service_unavailable",
    "message": "The service is temporarily unavailable. Please try again in a few moments.",
    "type": "server_error",
    "retryAfter": 60
  }
}

​
Error codes

​
invalid_request
validation_error
The request is malformed, missing required parameters, or contains invalid values.
​
invalid_api_key
authentication_error
The API key is missing, invalid, expired, or has been revoked.
​
insufficient_credits
payment_error
Your account has run out of API credits. Upgrade your plan or wait for the next billing cycle.
​
rate_limit_exceeded
rate_limit_error
You have exceeded your plan’s rate limit. Wait for the limit to reset or upgrade your plan.
​
invalid_type
validation_error
The specified letter type is not supported. Check the available types in the parameters section.
​
invalid_tone
validation_error
The specified tone is not supported. Check the available tones in the parameters section.
​
invalid_language
validation_error
The specified language code is not supported.
​
content_policy_violation
validation_error
The request was rejected because it violates our content policy. Ensure your inputs are appropriate and professional.
​
internal_error
server_error
An unexpected error occurred on the server. Please try again later.
​
service_unavailable
server_error
The service is temporarily unavailable. Check our status page for updates.

​
Tips for best results

Provide detailed and specific information in the details parameter. The more context you provide, the more personalized and relevant your generated letter will be.
Choose the appropriate tone for your use case. A cover letter should use “professional” or “enthusiastic”, while a love letter might use “romantic” or “heartfelt”.
For multilingual letters, ensure that the purpose and details parameters are provided in the same language as the language parameter for best results.
Always review and edit generated letters before sending them. While our AI produces high-quality content, you should personalize it further to match your specific situation.

Build docs developers (and LLMs) love

Get started for freeTalk to us