Create a one-time send

Create a one-time send

Creates a one-time send (a newsletter) for a filter over segments and attributes or a CSV of recipients, as a draft, scheduled, or sent right away.

Note: Stays a draft unless you set send_now to true or scheduled_at to a future Unix timestamp. The name and audience can't change after creation, and a workspace with a subscription center requires subscription_topic_id.

Set up

Use the first option your agent supports.

MCP (official, remote)

Add this server to your agent's MCP settings, then sign in when asked.

{ "mcpServers": { "customer-io": { "url": "https://mcp.customer.io/mcp" } } }

Call the MCP tool cio_write_api.

Server URL: https://mcp.customer.io/mcp

API (official)

• Base URL: https://api.customer.io
• Endpoint: POST /v1/newsletters
• Auth: send the header Authorization: Bearer $CUSTOMERIO_API_KEY
• Get a key: https://fly.customer.io/settings/api_credentials?keyType=app
• Docs: https://docs.customer.io/integrations/api/app/

Before anything else, confirm access with the cheapest read-only call, like a list or a search. Never send or change anything to test access.

Rules

• Ask the user before anything that sends messages, costs money, or changes data, and say how many records it touches. One approval covers a batch the user has seen.
• Never print API keys.