▣ OUTPUT KIT

Generate Letter Document from JSON API

What it does

The Letter Generator API turns letter content in JSON into a formatted business letter. Send the sender, recipient, date and paragraphs, and get back a PDF, HTML or DOCX file.

Put the content in data. Required fields are the sender and recipient (each with a name and address lines), the date and the paragraphs. Optional fields add a subject, salutation, closing, signature and reference. Use page to set the paper size, margin and orientation.

Set format to pdf (default), html, docx or json. With json, the response includes the letter as a PDF data_uri plus metadata. Set download to true to get the file itself. The metadata reports the bytes, the pages count for PDF output, any render warnings, and timing in queued_ms and render_ms.

Teams use the Letter Generator API to send customer notices, engagement letters, internal approvals and mail merge exports from application data. Each letter comes out with the same layout, so printed and PDF letters stay consistent.

For your own letter design, fill a DOCX or HTML template with the Validate & Generate Documents from Templates API. For other document types, use the Generate Report Document from JSON API.

POST

Generate a preset document from JSON (letter)

https://api.eu.apyhub.com/callable-labs/generate-letter-document-from-json-api/v1/documents/preset/letter

QUICKSTART

GUIDE

Quickstart

Render a letter with the required sender, recipient, date, and body paragraphs.

curl -X POST "https://api.eu.apyhub.com/callable-labs/generate-letter-document-from-json-api/v1/documents/preset/letter" \
  -H "apy-token: $APY_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "data": {
      "date": "18 September 2026",
      "sender": {
        "name": "Ada Lovelace"
      },
      "recipient": {
        "name": "Charles Babbage"
      },
      "paragraphs": [
        "Thank you for the drawings you sent last week.",
        "I believe the engine might act upon other things besides number."
      ]
    }
  }'

What you'll get back

Returns a JSON object with bytes and queued_ms/render_ms timing fields, plus cache, engine, format, data_uri, warnings, media_type, and engine_version. pages is an integer when the output is a PDF, or null for DOCX/HTML.

{
  "bytes": 12345,
  "cache": "miss",
  "pages": 2,
  "engine": "string",
  "format": "pdf",
  "data_uri": "data:application/pdf;base64,....",
  "warnings": [],
  "queued_ms": 12,
  "render_ms": 345,
  "media_type": "application/pdf",
  "engine_version": "string"
}

TRY ITLIVE · 200 ATOMS

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About this endpoint

What it does

Generates a letter document from JSON input. You provide the letter content plus optional page and output settings, and the endpoint returns rendering metadata and, for document outputs, the generated payload details.

Request Body

Parameter	Type	Description
data	Object	Letter content. Includes `sender`, `recipient`, `date`, and `paragraphs`.
data.date	String	Date string. Max length 32.
data.sender	Object	Sender details. Includes `name` and optional contact/address fields.
data.sender.name	String	Sender name. Max length 200.
data.sender.email	String	Sender email. Max length 200. Default: null.
data.sender.phone	String	Sender phone. Max length 64. Default: null.
data.sender.tax_id	String	VAT / tax registration number. Max length 64. Default: null.
data.sender.address	String Array	Address lines. Max items: 8.
data.closing	String	Letter closing. Default: `Kind regards,`. Max length 200.
data.subject	String	Letter subject. Max length 200. Default: null.
data.recipient	Object	Recipient details. Includes `name` and optional contact/address fields.
data.recipient.name	String	Recipient name. Max length 200.
data.recipient.email	String	Recipient email. Max length 200. Default: null.
data.recipient.phone	String	Recipient phone. Max length 64. Default: null.
data.recipient.tax_id	String	VAT / tax registration number. Max length 64. Default: null.
data.recipient.address	String Array	Address lines. Max items: 8.
data.reference	String	Reference. Max length 64. Default: null.
data.signature	String	Signature name or text. Max length 200. Default: null.
data.paragraphs	String Array	Body paragraphs. Min items: 1, max items: 50.
data.salutation	String	Letter salutation. Default: `Dear Sir or Madam,`. Max length 200.
page	Object	Paper size, margin, and orientation for PDF output.
page.size	ENUM	Allowed values: `A4`, `Letter`, `Legal`, `A3`, `A5`. Default: `A4`.
page.margin_mm	Number	Uniform page margin in millimetres. Minimum 0, maximum 50. Default: 18.
page.orientation	ENUM	Allowed values: `portrait`, `landscape`. Default: `portrait`.
format	ENUM	Allowed values: `json`, `html`, `pdf`, `docx`. Default: `json`. `pdf`, `html` or `docx` return the bytes; `json` returns a PDF data URI plus metadata.
download	Boolean	Send `Content-Disposition: attachment` so browsers save the file. Default: false.

Response

Returns a JSON object with bytes (integer), cache (string), pages (integer, nullable), engine (string), format (string), data_uri (string), warnings (array of strings), queued_ms (integer), render_ms (integer), media_type (string), and engine_version (string) fields. The format field is limited to docx, pdf, or html; pages is null for DOCX/HTML.

Replace the Response table with this, removing the Status Code column:

Attribute	Type	Description
bytes	Integer	Number of bytes in the generated output.
cache	ENUM	Allowed values: `hit`, `miss`.
pages	Integer	PDF page count; null for DOCX/HTML.
engine	String	Rendering engine name.
format	ENUM	Allowed values: `docx`, `pdf`, `html`.
data_uri	String	Data URI in the form `data:<media_type>;base64,…`.
warnings	String Array	Warnings emitted during rendering.
queued_ms	Integer	Time spent queued, in milliseconds.
render_ms	Integer	Time spent rendering, in milliseconds.
media_type	String	MIME type of the generated output.
engine_version	String	Rendering engine version.

Notes

When format is json, the response includes a PDF data URI plus metadata instead of raw bytes. The pages field can be null when the generated output is DOCX or HTML, so do not rely on it for those formats.

▣ COMMON ERRORS

Errors any endpoint can return

400bad_request

Required parameter missing or malformed body.

401unauthorized

API key missing, revoked, or not authorized for this service.

429rate_limited

Your plan's per-second rate exceeded. Retry with exponential backoff.

503upstream_busy

Backend temporarily unavailable. Try again in a few seconds.

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From the blog

Reads that touch this API.

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