# Message headers on Send response and new include_basic_headers option

Source: https://developer.nylas.com/docs/changelogs/2026-05-20-message-headers/

The Send Message endpoint now returns message headers on the response, and a new lightweight `include_basic_headers` option returns only the three RFC threading headers — `Message-ID`, `In-Reply-To`, and `References`. Together these eliminate the second `GET /messages/{id}` round-trip that customers previously needed to pull threading data after a send.

## Added

- **Headers on the [Send Message](/docs/reference/api/messages/send-message/) response** — Pass `fields=include_headers` or `fields=include_basic_headers` on `POST /v3/grants/{grant_id}/messages/send` and Nylas returns the message's `headers` array on the response. This works for **synchronous** send only; the parameter has no effect when you set the `send_at` field. Supported across Google, Microsoft, EAS, EWS, and IMAP, with the same provider caveats that apply to `include_headers` on Get Message today.

- **`fields=include_basic_headers` option** — Returns only `Message-ID`, `In-Reply-To`, and `References` in the `headers` array. The new value is available on:
  - [`POST /v3/grants/{grant_id}/messages/send`](/docs/reference/api/messages/send-message/)
  - [`GET /v3/grants/{grant_id}/messages`](/docs/reference/api/messages/get-messages/)
  - [`GET /v3/grants/{grant_id}/messages/{message_id}`](/docs/reference/api/messages/get-messages-id/)

  Use this option when you only need to track message identity and thread relationships. The response is significantly smaller than `include_headers` (full headers can be larger than the message body itself), and EWS returns the threading headers reliably with `include_basic_headers` — whereas `include_headers` on EWS only returns headers Nylas generates for MIME.

- **[Using email headers and MIME data](/docs/v3/email/headers-mime-data/)** — Updated with the new option, a provider compatibility matrix, examples for both Get Message and Send Message flows, and a note that the Send response support is sync-only.