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Welcome

The Assembly API lets you manage clients, companies, messaging, files, billing, and more programmatically. The endpoints in this reference are generated directly from the API’s OpenAPI specification, so they always reflect the live contract.

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Authentication

All requests are authenticated with an API key sent in the X-API-KEY header.
Requests without a valid key return 401 Unauthorized; a key lacking permission for the requested action returns 403 Forbidden.