> ## Documentation Index
> Fetch the complete documentation index at: https://docs.meetdarwin.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Send the visitor's message and get the agent's reply

> Requires the `chat:write` scope.

With `stream: true` the answer is `text/event-stream` in the AI SDK UI message stream format: `data:` lines with chunks such as `{"type":"text-delta","delta":"…"}`, ending with `data: [DONE]`. A streamed answer is never stored for Idempotency-Key replay.



## OpenAPI

````yaml /api-reference/openapi.json post /chat/sessions/{id}/messages
openapi: 3.1.0
info:
  title: Darwin API
  version: '2026-10-12'
  description: >-
    Read and act on your Darwin workspace: conversations, contacts, meetings,
    analytics, knowledge, the agent and its opening tests, with signed webhooks
    for what happens.


    Authenticate with a secret key from Integrations → API: `Authorization:
    Bearer dk_live_…`. Keys carry scopes; personal data needs `contacts:read`.

    Rate limits are per key per minute: origin 120, adapt 120, evolve 600, apex
    1200; heavy calls (exports, crawls, model work) have a smaller budget. Every
    response carries RateLimit-* headers.

    Lists are cursor-paginated (`limit`, `starting_after`, `next_cursor`).
    Writes accept `Idempotency-Key`. Errors are RFC 9457 problem documents with
    a stable `code` and a `request_id`.
  contact:
    name: Darwin
    url: https://docs.meetdarwin.ai/api-reference/introduction
servers:
  - url: https://api.meetdarwin.ai/v1
security:
  - bearer: []
tags:
  - name: Workspace
  - name: Conversations
  - name: Contacts
  - name: Meetings
  - name: Analytics
  - name: Events
  - name: Webhooks
  - name: Knowledge
  - name: Agent
  - name: Experiments
  - name: Privacy
  - name: Chat
paths:
  /chat/sessions/{id}/messages:
    post:
      tags:
        - Chat
      summary: Send the visitor's message and get the agent's reply
      description: >-
        Requires the `chat:write` scope.


        With `stream: true` the answer is `text/event-stream` in the AI SDK UI
        message stream format: `data:` lines with chunks such as
        `{"type":"text-delta","delta":"…"}`, ending with `data: [DONE]`. A
        streamed answer is never stored for Idempotency-Key replay.
      operationId: sendChatMessage
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
        - name: Idempotency-Key
          in: header
          required: false
          description: >-
            Retry safely: the first response is replayed for 24 hours (not for
            streamed answers).
          schema:
            type: string
            maxLength: 255
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/ChatMessageCreate'
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ChatReply'
        '400':
          description: Invalid request (`invalid_request`, `unsupported_version`).
          content:
            application/problem+json:
              schema:
                $ref: '#/components/schemas/Problem'
        '401':
          description: >-
            Missing or invalid key (`authentication_required`,
            `invalid_api_key`).
          content:
            application/problem+json:
              schema:
                $ref: '#/components/schemas/Problem'
        '402':
          description: >-
            The workspace must add a card before the API can be used
            (`payment_required`).
          content:
            application/problem+json:
              schema:
                $ref: '#/components/schemas/Problem'
        '403':
          description: The key lacks the scope (`insufficient_scope`).
          content:
            application/problem+json:
              schema:
                $ref: '#/components/schemas/Problem'
        '404':
          description: No such object in this workspace (`not_found`).
          content:
            application/problem+json:
              schema:
                $ref: '#/components/schemas/Problem'
        '409':
          description: >-
            Conflicts with the current state (`conflict`,
            `idempotency_key_in_use`).
          content:
            application/problem+json:
              schema:
                $ref: '#/components/schemas/Problem'
        '422':
          description: Refused by a rule (`unprocessable`, `idempotency_key_reused`).
          content:
            application/problem+json:
              schema:
                $ref: '#/components/schemas/Problem'
        '429':
          description: >-
            Rate limited (`rate_limited`) or a monthly allowance used
            (`quota_exceeded`); see Retry-After.
          content:
            application/problem+json:
              schema:
                $ref: '#/components/schemas/Problem'
components:
  schemas:
    ChatMessageCreate:
      type: object
      properties:
        text:
          type: string
          minLength: 1
          maxLength: 4000
        stream:
          description: 'true: answer as server-sent events while the agent writes.'
          type: boolean
      required:
        - text
      additionalProperties: false
    ChatReply:
      type: object
      properties:
        object:
          type: string
          const: chat_reply
        session:
          type: string
        message:
          type: object
          properties:
            id:
              type: string
            role:
              type: string
              enum:
                - user
                - assistant
            text:
              type: string
            created_at:
              type:
                - string
                - 'null'
          required:
            - id
            - role
            - text
            - created_at
        actions:
          type: array
          items:
            type: object
            properties:
              type:
                type: string
                enum:
                  - booking_offered
                  - handoff_requested
                  - searched_knowledge
              booking_url:
                type:
                  - string
                  - 'null'
              reason:
                type:
                  - string
                  - 'null'
            required:
              - type
          description: >-
            What the agent did besides answering, so your interface can show a
            booking button or tell a person.
      required:
        - object
        - session
        - message
        - actions
    Problem:
      type: object
      properties:
        type:
          type: string
          example: https://docs.meetdarwin.ai/api-reference/errors#insufficient_scope
        title:
          type: string
        status:
          type: integer
          minimum: -9007199254740991
          maximum: 9007199254740991
        code:
          type: string
          example: insufficient_scope
        detail:
          type: string
        param:
          type: string
        request_id:
          type: string
          example: req_8m2k4p0q1x7z9c3v5b6n
      required:
        - type
        - title
        - status
        - code
        - detail
        - request_id
  securitySchemes:
    bearer:
      type: http
      scheme: bearer
      description: >-
        A secret key (dk_live_…). Scopes: conversations:read, contacts:read,
        contacts:write, meetings:read, analytics:read, events:read,
        webhooks:write, knowledge:read, knowledge:write, agent:read,
        agent:write, experiments:read, experiments:write, privacy:write,
        chat:write.

````

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