Writing a clear, well-structured email takes time, and it's the kind of task an LLM is genuinely good at. But wiring up your own prompt-to-email pipeline means picking a model, threading the original message in as context, handling streaming, and keeping it all behind your API keys. The Nylas Smart Compose endpoints do that for you: send a natural-language prompt, get back a written message body, and the reply variant pulls in the original email as context automatically.
This post walks through Smart Compose from two angles: the HTTP API for your backend, and the nylas CLI for the terminal. I work on the CLI, so the terminal commands below are the ones I reach for when I'm testing a prompt.
How Smart Compose works
Smart Compose is two endpoints that turn a prompt into a message body. You send a natural-language prompt, and the response comes back with a suggestion field holding the generated text. There's a POST /messages/smart-compose for writing a brand-new message, and a POST /messages/{message_id}/smart-compose for writing a reply, where the original message is folded into the context so the response actually answers it.
The key thing to understand is that Smart Compose generates text, it doesn't send anything. The suggestion it returns is a message body you do something with: pass it straight to the Send Message endpoint, or pre-fill it into a draft for a human to review and edit first. That separation is deliberate, since it lets you put a person between the AI's output and the recipient, which is usually what you want for anything an LLM wrote.
