> ## Documentation Index > Fetch the complete documentation index at: https://docs.gumloop.com/llms.txt > Use this file to discover all available pages before exploring further. # Send message > Append a user message to an existing session and resume the agent. The session must be in a terminal state (`idle`, `completed`, or `failed`); sending to a session that is `processing` or `queued` returns `409 interaction_not_in_terminal_state`. ### Streaming the response `api.gumloop.com` only serves the non-streaming response above. To stream agent output as it's produced, send the same request body (with `stream: true`) to the streaming host instead: ``` POST https://ws.gumloop.com/api/v1/sessions/{session_id}/messages ``` The response is `text/event-stream` (Server-Sent Events). With the Python SDK, `client.sessions.stream_message(session_id, input="...")` routes to `ws.gumloop.com` automatically and yields parsed `StreamEvent` objects. If you send `stream: true` to `api.gumloop.com` by mistake, the response is a `400` whose body contains the correct streaming host so you can retry against it. ## OpenAPI ````yaml post /sessions/{session_id}/messages openapi: 3.0.0 info: title: Public API version: 1.0.0 servers: - url: https://api.gumloop.com/api/v1 security: [] paths: /sessions/{session_id}/messages: post: tags: - Sessions summary: Send message description: > Append a user message to an existing session and resume the agent. The session must be in a terminal state (`idle`, `completed`, or `failed`); sending to a session that is `processing` or `queued` returns `409 interaction_not_in_terminal_state`. ### Streaming the response `api.gumloop.com` only serves the non-streaming response above. To stream agent output as it's produced, send the same request body (with `stream: true`) to the streaming host instead: ``` POST https://ws.gumloop.com/api/v1/sessions/{session_id}/messages ``` The response is `text/event-stream` (Server-Sent Events). With the Python SDK, `client.sessions.stream_message(session_id, input="...")` routes to `ws.gumloop.com` automatically and yields parsed `StreamEvent` objects. If you send `stream: true` to `api.gumloop.com` by mistake, the response is a `400` whose body contains the correct streaming host so you can retry against it. operationId: sendMessage parameters: - in: path name: session_id required: true schema: type: string description: ID of the session to continue. requestBody: required: true content: application/json: schema: type: object properties: input: type: string description: >- The next user message. Required. Also accepted as `message` for backwards compatibility. example: Now write a follow-up email. stream: type: boolean default: false description: >- Must be `false` (or omitted) when calling `api.gumloop.com`. Set to `true` only when calling `ws.gumloop.com` (see the streaming section above). required: - input responses: '202': description: >- Message was enqueued. `queue_position` is set only when the session was queued behind concurrent runs. content: application/json: schema: type: object properties: session: type: object properties: id: type: string example: sess_xYz789AbCd agent_id: type: string example: abc123DEFghiJKL messages: type: array items: type: object properties: id: type: string nullable: true example: msg_g7h8i9 role: type: string nullable: true example: user content: type: string nullable: true example: Now write a follow-up email. created_at: type: string format: date-time nullable: true example: '2026-05-15T14:35:00Z' creator_id: type: string nullable: true example: user_2b9d71f0 parts: type: array nullable: true items: type: object created_at: type: string format: date-time nullable: true example: '2026-05-15T14:32:00Z' state: type: string nullable: true enum: - processing - completed - failed - queued - idle example: processing agent_name: type: string nullable: true example: Sales research agent agent_team_id: type: string nullable: true example: team_4f8c92ab agent_creator_user_id: type: string nullable: true example: user_2b9d71f0 agent_icon_url: type: string nullable: true example: null agent_tools: type: array description: >- Tools available to the agent. Secret references are stripped before being returned. items: type: object participants: type: object description: Map of participant user IDs to participant metadata. creator: type: object nullable: true description: >- Creator of the session. `null` when no creator is recorded. properties: id: type: string nullable: true first_name: type: string nullable: true last_name: type: string nullable: true email: type: string nullable: true profile_picture: type: string nullable: true queue_position: type: integer nullable: true description: >- Position in the per-agent queue. Populated only when the session was queued; otherwise `null`. example: null examples: processing: summary: Continuation enqueued and processing value: session: id: sess_xYz789AbCd agent_id: abc123DEFghiJKL messages: - id: msg_a1b2c3 role: user content: Research Acme Corp and draft a brief. created_at: '2026-05-15T14:32:00Z' creator_id: user_2b9d71f0 parts: null - id: msg_d4e5f6 role: assistant content: Here is what I found about Acme Corp... created_at: '2026-05-15T14:32:09Z' creator_id: null parts: null - id: msg_g7h8i9 role: user content: Now write a follow-up email. created_at: '2026-05-15T14:35:00Z' creator_id: user_2b9d71f0 parts: null created_at: '2026-05-15T14:32:00Z' state: processing agent_name: Sales research agent agent_team_id: team_4f8c92ab agent_creator_user_id: user_2b9d71f0 agent_icon_url: null agent_tools: [] participants: user_2b9d71f0: first_name: Ada last_name: Lovelace creator: id: user_2b9d71f0 first_name: Ada last_name: Lovelace email: ada@example.com profile_picture: null queue_position: null '400': description: >- Bad request — missing `input`, or `stream: true` was set on this host (use the streaming host). '401': description: Unauthorized — missing or invalid API key. '403': description: Forbidden — the caller does not have update access on the session. '404': description: Session not found. '409': description: >- Conflict — the session is not in a terminal state (`interaction_not_in_terminal_state`). '429': description: Rate limited — the agent's concurrency limit was exceeded. '500': description: Internal server error. security: - bearerAuth: [] x-codeSamples: - lang: bash label: cURL source: > curl -X POST 'https://api.gumloop.com/api/v1/sessions/sess_xYz789AbCd/messages' \ -H 'Authorization: Bearer YOUR_ACCESS_TOKEN' \ -H 'Content-Type: application/json' \ -d '{"input": "Now write a follow-up email."}' - lang: python label: Python source: | from gumloop import Gumloop client = Gumloop(access_token="YOUR_ACCESS_TOKEN") response = client.sessions.send( "sess_xYz789AbCd", input="Now write a follow-up email.", ) print(response.session.state) components: securitySchemes: bearerAuth: type: http scheme: bearer description: >- A personal API key or an [OAuth 2.0](/api-reference/oauth) access token. Personal API keys also require the `x-auth-key` header with your user ID. ````