---
title: "Append Messages"
method: POST
path: "/mle-agent/sessions/{session_id}/messages"
tags: ["agent-chat-sessions"]
---

# Append Messages

`POST /mle-agent/sessions/{session_id}/messages`

Append messages to an existing session.

Args:
    request: FastAPI request used by the rate-limiter.
    session_id: Chat session primary key.
    body: Messages to append.
    auth: Authenticated caller.
    service: Chat-session service.

Returns:
    ChatSessionMessagesAppendResponse with messages_added count.

Raises:
    HTTPException: 404 when the session is missing or not owned.

## Path parameters

- `session_id` string, required

## Request body

- ChatSessionMessagesAppend — Request model for appending messages to a session.
  - `messages` ChatMessageCreate[], required — Messages to append
    - `role` string, required — Message role: 'user', 'assistant', or 'tool'
    - `content` string, required — Message content
    - `tool_call_id` string, nullable — Tool call ID for tool messages
    - `tool_calls` object[], nullable — Tool calls made by assistant

## Response `200`

Successful Response

- ChatSessionMessagesAppendResponse — Response model for appending messages.
  - `success` boolean
  - `session_id` string, required
  - `messages_added` integer, required

## Other responses

- `422` — Validation Error

---

[API](https://skmtc.net/pioneer/apis/brain-api.md) · [All operations](https://skmtc.net/pioneer/apis/brain-api/llms.txt) · [OpenAPI document](https://skmtc-service-staging.skmtc.workers.dev/v1/apis/pioneer/brain-api/versions/31dfe831e079/schema)