Generate Letter Document from JSON API

What it does

Letter Document Generator turns structured letter content into a rendered document. Send a body object with data, plus optional page and output settings, and get back a letter in pdf, html, or docx form. If you choose json, the response includes a PDF data URI and render metadata.

Use it when you need to create formal correspondence from application data: customer notices, engagement letters, internal approvals, or mail-merge style exports. The letter payload accepts the sender and recipient parties, date, paragraphs, and optional fields such as subject, salutation, closing, signature, and reference. You can also control paper size, margin, and orientation through the page object.

The response includes the generated file’s format, media_type, bytes, engine, engine_version, cache, and timing fields such as queued_ms and render_ms. For PDF output, it can also return pages; if rendering produces issues or notes, they appear in warnings.

Use Letter Document Generator when your backend needs a consistent, template-free way to produce printable business letters from JSON.

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"
}

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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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