Tool Schema to OpenAPI 3.1 Spec Generator

Transform OpenAI and Anthropic function calling JSON schemas into complete OpenAPI 3.1 documentation.

API Title:

Version:

Presets:

OpenAI / Anthropic Tool Schema JSONArray or single tool object

[
  {
    "type": "function",
    "function": {
      "name": "get_weather_forecast",
      "description": "Retrieve real-time weather and forecast data for a specified geographic location.",
      "parameters": {
        "type": "object",
        "properties": {
          "location": {
            "type": "string",
            "description": "City and state or country (e.g. San Francisco, CA)"
          },
          "days": {
            "type": "integer",
            "description": "Number of days for forecast (1-7)",
            "minimum": 1,
            "maximum": 7
          },
          "unit": {
            "type": "string",
            "enum": [
              "celsius",
              "fahrenheit"
            ]
          }
        },
        "required": [
          "location"
        ]
      }
    }
  },
  {
    "type": "function",
    "function": {
      "name": "search_flight_tickets",
      "description": "Search available direct and connecting airline flights.",
      "parameters": {
        "type": "object",
        "properties": {
          "origin": {
            "type": "string",
            "description": "3-letter IATA departure airport code (e.g. SFO)"
          },
          "destination": {
            "type": "string",
            "description": "3-letter IATA destination code (e.g. JFK)"
          },
          "departureDate": {
            "type": "string",
            "format": "date",
            "description": "Departure date YYYY-MM-DD"
          },
          "passengers": {
            "type": "integer",
            "minimum": 1,
            "maximum": 9
          }
        },
        "required": [
          "origin",
          "destination",
          "departureDate"
        ]
      }
    }
  }
]

OpenAPI 3.1 Output

openapi: 3.1.0
info:
  title: LLM Tool Calling API
  version: 1.0.0
  description: OpenAPI 3.1 specification generated automatically from LLM
    function/tool calling schemas.
paths:
  /tools/get_weather_forecast:
    post:
      operationId: get_weather_forecast
      summary: Retrieve real-time weather and forecast data for a specified geographic
        location
      description: Retrieve real-time weather and forecast data for a specified
        geographic location.
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/get_weather_forecast_Request"
      responses:
        "200":
          description: Tool execution result output
          content:
            application/json:
              schema:
                type: object
                properties:
                  result:
                    type: string
                    description: JSON string or structured output returned by tool execution
                  success:
                    type: boolean
        "400":
          description: Invalid parameter arguments passed to tool
  /tools/search_flight_tickets:
    post:
      operationId: search_flight_tickets
      summary: Search available direct and connecting airline flights.
      description: Search available direct and connecting airline flights.
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/search_flight_tickets_Request"
      responses:
        "200":
          description: Tool execution result output
          content:
            application/json:
              schema:
                type: object
                properties:
                  result:
                    type: string
                    description: JSON string or structured output returned by tool execution
                  success:
                    type: boolean
        "400":
          description: Invalid parameter arguments passed to tool
components:
  schemas:
    get_weather_forecast_Request:
      type: object
      properties:
        location:
          type: string
          description: City and state or country (e.g. San Francisco, CA)
        days:
          type: integer
          description: Number of days for forecast (1-7)
          minimum: 1
          maximum: 7
        unit:
          type: string
          enum:
            - celsius
            - fahrenheit
      required:
        - location
    search_flight_tickets_Request:
      type: object
      properties:
        origin:
          type: string
          description: 3-letter IATA departure airport code (e.g. SFO)
        destination:
          type: string
          description: 3-letter IATA destination code (e.g. JFK)
        departureDate:
          type: string
          format: date
          description: Departure date YYYY-MM-DD
        passengers:
          type: integer
          minimum: 1
          maximum: 9
      required:
        - origin
        - destination
        - departureDate

Bridging AI Tool Calling and Formal API Contracts

When building autonomous AI agents with tools (like OpenAI Assistants API, LangChain, or Claude Tool Use), developers define function parameters using JSON Schema objects. Converting these tool arrays into standard OpenAPI 3.1 specifications enables automatic Swagger UI documentation, client SDK generation, and contract-first API testing.

Key Benefits of OpenAPI 3.1 Standardization

Full JSON Schema Alignment: Unlike OpenAPI 3.0, OpenAPI 3.1 is 100% compatible with the latest JSON Schema standard (Draft 2020-12), meaning complex union types, enums, and nested arrays translate with zero syntax divergence.
Automated Mock Servers: Load the generated OpenAPI specification into mock servers (like Prism or Postman) to simulate backend tool executions during agent development.
Swagger & ReDoc Documentation: Share interactive API references with frontend and QA teams detailing every tool parameter and constraint.