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

# Create chat completion

> Send a user message into an existing chat and receive the assistant response messages.



## OpenAPI

````yaml /openapi.json post /api/chats/{chatId}/completions
openapi: 3.1.0
info:
  title: UponAI Platform API
  version: '2026-04-20'
  description: >-
    Programmatic control plane for configuring UponAI agents, LLM
    configurations, conversation flows, knowledge bases, calls, chats, voices,
    and test tooling. All endpoints require a bearer token obtained from your
    profile settings.
servers:
  - url: https://api.upon-ai.com
    description: Production
security:
  - bearerAuth: []
tags:
  - name: Calls
    description: Initiate and manage UponAI powered voice and web calls.
  - name: Agents
    description: Configure agents that power calls and web sessions.
  - name: LLM Models
    description: Manage reusable large language model configurations for agents.
  - name: Chat Agents
    description: Configure agents for browser and messaging chat sessions.
  - name: Chats
    description: Start and manage live chat sessions.
  - name: Conversation Flows
    description: Build and maintain reusable dialog graphs for agents.
  - name: Knowledge Bases
    description: Store and curate grounded knowledge sources.
  - name: Voices
    description: Discover, search, add, and clone speech synthesis voices.
  - name: Test Case Definitions
    description: Define simulation test cases for response engines.
  - name: Test Runs
    description: Retrieve results for individual and batch test runs.
  - name: Account
    description: Account level utilities and limits.
  - name: Phone Numbers
    description: List and assign phone numbers to agents.
paths:
  /api/chats/{chatId}/completions:
    parameters:
      - $ref: '#/components/parameters/ChatIdPath'
    post:
      tags:
        - Chats
      summary: Create chat completion
      description: >-
        Send a user message into an existing chat and receive the assistant
        response messages.
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/ChatCompletionRequest'
            examples:
              default:
                value:
                  workspaceId: 42
                  content: Can you send me pricing for the enterprise plan?
      responses:
        '201':
          description: Chat completion messages.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ChatCompletionResponse'
components:
  parameters:
    ChatIdPath:
      name: chatId
      in: path
      required: true
      schema:
        type: string
      description: Chat identifier.
  schemas:
    ChatCompletionRequest:
      type: object
      required:
        - workspaceId
        - content
      properties:
        workspaceId:
          type: integer
        content:
          type: string
    ChatCompletionResponse:
      type: object
      properties:
        messages:
          type: array
          items:
            $ref: '#/components/schemas/Message'
    Message:
      type: object
      properties:
        role:
          type: string
          enum:
            - assistant
            - user
        content:
          type: string
        name:
          type: string
          nullable: true
        tool_calls:
          type: array
          items:
            type: object
      required:
        - role
        - content
  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer
      description: Generate tokens from your profile settings at app.uponai.com.

````