Vapi Create Function Tool and Attach to Assistant

arazzo: 1.0.1
info:
  title: Vapi Create Function Tool and Attach to Assistant
  summary: Create a custom function tool and attach it to a freshly created assistant.
  description: >-
    The custom-capability flow. It creates a custom function tool the assistant
    can invoke, creates a bare assistant, and then patches the assistant's model
    to attach the tool by id so the assistant can call it during a conversation.
    Every step spells out its request inline so the flow can be read and executed
    without opening the underlying OpenAPI description.
  version: 1.0.0
sourceDescriptions:
- name: assistantsApi
  url: ../openapi/vapi-ai-assistants-api-openapi.yml
  type: openapi
- name: toolsApi
  url: ../openapi/vapi-ai-tools-api-openapi.yml
  type: openapi
workflows:
- workflowId: create-tool-attach-assistant
  summary: Create a function tool and bind it to an assistant via a model update.
  description: >-
    Creates a custom function tool, creates an assistant, then updates the
    assistant so its model references the tool id.
  inputs:
    type: object
    required:
    - apiToken
    - functionName
    - functionDescription
    - name
    - firstMessage
    - systemPrompt
    properties:
      apiToken:
        type: string
        description: Vapi private API key used as a Bearer token.
      functionName:
        type: string
        description: The name of the function the assistant can invoke.
      functionDescription:
        type: string
        description: A description telling the model when to call the function.
      name:
        type: string
        description: A human-readable name for the assistant.
      firstMessage:
        type: string
        description: The opening message the assistant speaks.
      systemPrompt:
        type: string
        description: The system prompt that shapes the assistant's behavior.
  steps:
  - stepId: createTool
    description: >-
      Create a custom function tool with a name, description, and a simple
      object parameter schema the assistant can fill in.
    operationId: ToolController_create
    parameters:
    - name: Authorization
      in: header
      value: Bearer $inputs.apiToken
    requestBody:
      contentType: application/json
      payload:
        type: function
        function:
          name: $inputs.functionName
          description: $inputs.functionDescription
          parameters:
            type: object
            properties:
              query:
                type: string
                description: The user request to pass to the function.
            required:
            - query
    successCriteria:
    - condition: $statusCode == 201
    outputs:
      toolId: $response.body#/id
  - stepId: createAssistant
    description: Create the assistant that will later be given access to the tool.
    operationId: AssistantController_create
    parameters:
    - name: Authorization
      in: header
      value: Bearer $inputs.apiToken
    requestBody:
      contentType: application/json
      payload:
        name: $inputs.name
        firstMessage: $inputs.firstMessage
        model:
          provider: openai
          model: gpt-5.4
          messages:
          - role: system
            content: $inputs.systemPrompt
        voice:
          provider: azure
          voiceId: andrew
    successCriteria:
    - condition: $statusCode == 201
    outputs:
      assistantId: $response.body#/id
  - stepId: attachTool
    description: >-
      Update the assistant so its model references the function tool by id,
      attaching the capability to the existing assistant.
    operationId: AssistantController_update
    parameters:
    - name: Authorization
      in: header
      value: Bearer $inputs.apiToken
    - name: id
      in: path
      value: $steps.createAssistant.outputs.assistantId
    requestBody:
      contentType: application/json
      payload:
        model:
          provider: openai
          model: gpt-5.4
          messages:
          - role: system
            content: $inputs.systemPrompt
          toolIds:
          - $steps.createTool.outputs.toolId
    successCriteria:
    - condition: $statusCode == 200
    outputs:
      assistantId: $response.body#/id
  outputs:
    toolId: $steps.createTool.outputs.toolId
    assistantId: $steps.attachTool.outputs.assistantId