description: "The raw HTTP endpoints behind a session's .in and .out streams: append records, read them over SSE, and drain them non-streaming."
---
Every session has two durable streams: `.in` carries records from your clients to the task, `.out` carries records from the task back to your clients. The [`sessions` SDK](/ai-chat/sessions) wraps these as `session.in.*` and `session.out.*`. This page documents the underlying HTTP endpoints for callers that aren't using the TypeScript SDK.
All channel endpoints live under `/realtime/v1/sessions/{session}/{io}`, where:
- `{session}` is the session's friendly ID (`session_…`) or your `externalId`. One token authorizes both forms.
- `{io}` is either `in` or `out`.
Authorize requests with a secret key or a [session public token](/management/authentication#session-scopes). The token's scopes decide what you can do — see [Authorization](#authorization) below.
## Append a record
Append a single record to a channel.
```bash Append to .in
curl -X POST "https://api.trigger.dev/realtime/v1/sessions/{session}/in/append" \
-H "Authorization: Bearer $TRIGGER_TOKEN" \
-H "Content-Type: application/json" \
-H "X-Part-Id: 0f8c2b1e-..." \
--data '{"type":"user-message","text":"hello"}'
```
The body is the raw record — any text up to 1MiB (records over the per-record cap return `413`). The response is `{ "ok": true }`.
Set the `X-Part-Id` header to a unique value per record to make the append idempotent: replaying the same `X-Part-Id` does not duplicate the record. Appending to a closed or expired session returns `400`.
<Warning>
Appending to `.out` requires a **secret key**. A session public token (even one with
`write:sessions`) can only append to `.in` — appending to `.out` with a public token returns
`403`. The `.out` stream is the task's to write.
</Warning>
## Read a channel over SSE
Subscribe to a channel as a Server-Sent Events stream. New records are delivered as they arrive.
| `Last-Event-ID` | request | Resume after this sequence number. Set it to the last `id:` you received to pick up exactly where you left off after a disconnect. |
| `Timeout-Seconds` | request | How long the server holds the stream open with no new records before closing, `1`–`600`. |
Each SSE event carries:
- `id:` — the record's sequence number. Use the most recent one as `Last-Event-ID` to resume.
- `data:` — a JSON record `{ "data": <record>, "id": <id> }`. For `.out` on a `chat.agent` session, `data` is a UI message chunk (text, reasoning, tool call, or a custom data part).
Some `.out` events are **control records** rather than data. A control record has an empty body and carries a `trigger-control` header naming its subtype:
| Subtype | Meaning |
| --- | --- |
| `turn-complete` | The current turn finished. Carries sibling headers `public-access-token` (a refreshed session token), `session-in-event-id`, and `last-event-id`. |
| `upgrade-required` | The session needs to hand off to a run on a newer deployed version. |
Route control records by their subtype instead of treating them as message content. The TypeScript SDK does this for you — `session.out.read` filters control records out of the chunk stream and surfaces them through `onControl`.
## Drain records non-streaming
Fetch a batch of records without holding an SSE connection open. Useful for polling or for reading a tail at startup.
Each record carries `data`, `id`, `seqNum`, and an optional `headers` array (present on control records). Page forward by passing the highest `seqNum` you received as the next `afterEventId`.
## Authorization
The action you can take depends on your token and the channel:
| Action | Endpoint | Required authorization |
| --- | --- | --- |
| Subscribe (SSE) | `GET .../{io}` | `read:sessions:{id}` — works on both `.in` and `.out` |
| Drain records | `GET .../{io}/records` | `read:sessions:{id}` — works on both `.in` and `.out` |
| Append to `.in` | `POST .../in/append` | `write:sessions:{id}` |
| Append to `.out` | `POST .../out/append` | Secret key only |
Reads work in both directions for a `read:sessions` token. Writes split by direction: a `write:sessions` token can append to `.in`, but `.out` is reserved for the task and requires a secret key. See [session scopes](/management/authentication#session-scopes) for how to mint a token.
## Using the SDK instead
If you're writing TypeScript, the [`sessions` SDK](/ai-chat/sessions) is the ergonomic path. `sessions.open(idOrExternalId)` returns a `SessionHandle` whose `session.in` and `session.out` channels call these endpoints for you, with auto-retry, `Last-Event-ID` resume, and control-record routing built in:
See [`session.in`](/ai-chat/sessions#session-in-—-clients-→-task) and [`session.out`](/ai-chat/sessions#session-out-—-task-→-clients) for the full handle API.
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