---
title: "Send a single user message"
method: POST
path: "/assistant/chat/message"
tags: ["Chat"]
---

# Send a single user message

`POST /assistant/chat/message`

## Request

- Base URL: `https://api.mobilerun.ai`
- URL: `https://api.mobilerun.ai/assistant/chat/message`
- Auth: HTTP bearer

Send a single user message. The response format follows the Accept header: `text/event-stream` for SSE, `application/json` for a buffered assistant reply. Interactive HITL requires `Accept: text/event-stream`: the stream can emit a `tool-hitl-approval` event, whose decision must be delivered to `/assistant/chat/permission`. Buffered JSON responses do not provide an interactive HITL continuation contract. `sessionId` targets a concrete active chat. The resolved chat session ID is returned as `chatSessionId` in the JSON body and as the `X-Chat-Session-Id` response header on the SSE response.

## Request body

- object
  - `message` string, required
  - `agent` string
  - `model` string
  - `sessionId` string, uuid, required

## Response `200`

Streamed events (SSE) or buffered assistant reply (JSON), selected by Accept.

- object
  - `assistantText` string, required
  - `errorText` string
  - `chatSessionId` string, uuid, required

## Other responses

- `400`: Bad Request
- `401`: Unauthorized
- `402`: Insufficient credits — caller must top up before retrying
- `404`: Unknown or archived sessionId
- `409`: Another prompt is already in-flight for this user, or the assistant is at its parallel-turn limit (retryable), or this turn id was already used / the turn was rejected (`code: conflict`, `x-should-retry: false`)
- `410`: The session can no longer accept messages because its agent environment is no longer active.
- `413`: Request body exceeds the 256 KB limit.
- `500`: Internal Server Error
- `503`: Billing service temporarily unavailable; response includes a `Retry-After` header (seconds) indicating when to retry.
- `504`: JSON response timed out after 110s; partial assistantText is returned.

## Changes

- **2026-10-29** `ce2372de8503`: 1 info
  - added the new optional request property `model`
- **2026-10-16** `4ad548c528a8`: 1 info
  - endpoint added

[Operation history](https://skmtc.dev/droidrun/apis/droidrun-cloud/changes/assistant/chat/message/post.md)

---

[API](https://skmtc.dev/droidrun/apis/droidrun-cloud.md) - [All operations](https://skmtc.dev/droidrun/apis/droidrun-cloud/llms.txt) - [OpenAPI document](https://skmtc.dev/droidrun/apis/droidrun-cloud/revisions/1cc95586c933?raw)