curl -X POST 'https://api.resend.com/contact-properties' \
  -H "Authorization: Bearer $RESEND_API_KEY" \
  -H 'User-Agent: my-app/1.0' \
  -H 'Content-Type: application/json' \
  -d '{
  "key": "string",
  "type": "string",
  "fallback_value": "string"
}'

# POST /contact-properties (Resend API)
Base URL: https://api.resend.com
Auth: Authorization: Bearer <RESEND_API_KEY>
Last verified: 2026-09-03

> Create a new contact property

## Critical Gotchas
* All requests must include a `User-Agent` header (e.g. `User-Agent: my-app/1.0`) or the API returns HTTP 403.
* The `to` field is an array of strings, even when sending to a single recipient.

## cURL
curl -X POST 'https://api.resend.com/contact-properties' \
  -H "Authorization: Bearer $RESEND_API_KEY" \
  -H 'User-Agent: my-app/1.0' \
  -H 'Content-Type: application/json' \
  -d '{"key":"string","type":"string","fallback_value":"string"}'

## Parameters
| Name | In | Type | Required | Description |
| :--- | :--- | :--- | :--- | :--- |
| key | body | string | Yes | The property key. Max length is 50 characters. Only alpha... |
| type | body | string | Yes | The property type. |
| fallback_value | body | string | No | The default value to use when the property is not set for... |

## Response (200 OK)
{"id":"78261eea-8f8b-4381-83c6-79fa7120f1cf","object":"contact_property"}

---
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POST

/contact-properties

Resend API

Create a new contact property

Base URL: https://api.resend.com
Auth: Authorization: Bearer <RESEND_API_KEY>
Last verified: 2026-09-03 · upstream hash matched

Actions

Agents: curl -H "Accept: text/markdown" this URL
→ 224 tokens · Vary: Accept

Critical gotchas

All requests must include a User-Agent header (e.g. User-Agent: my-app/1.0) or the API returns HTTP 403.

The to field is an array of strings, even when sending to a single recipient.

cURL

curl -X POST 'https://api.resend.com/contact-properties' \  -H "Authorization: Bearer $RESEND_API_KEY" \  -H 'User-Agent: my-app/1.0' \  -H 'Content-Type: application/json' \  -d '{  "key": "string",  "type": "string",  "fallback_value": "string"}'

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Parameters

Name	In	Type	Required	Description
key	body	string	Yes	The property key. Max length is 50 characters. Only alphanumeric characters and underscores are allowed.
type	body	string	Yes	The property type.
fallback_value	body	string	No	The default value to use when the property is not set for a contact. Must match the type specified in the type field.

Response 200 OK

{
  "id": "78261eea-8f8b-4381-83c6-79fa7120f1cf",
  "object": "contact_property"
}