OpenAPI Tool: Dynamic Tool Generation from REST APIs for Agent Invocation

Many internal services have REST APIs but no MCP interface. OpenAPI Tool lets you dynamically generate Tools from OpenAPI specs—as long as the API has documentation, it can connect to an Agent.

Use Cases

  • Internal REST API, no MCP support
  • Third-party REST API with OpenAPI spec
  • Need to quickly connect an API to Agent without hand-writing Tools

Quick Start

1. Prepare OpenAPI spec

Say there’s a weather API, OpenAPI spec looks roughly like this:

openapi: 3.0.0
info:
  title: Weather API
  version: 1.0.0
paths:
  /weather:
    get:
      operationId: getWeather
      parameters:
        - name: city
          in: query
          required: true
          schema:
            type: string
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: object
                properties:
                  temp:
                    type: number
                  condition:
                    type: string

Save as weather-api.yaml.

2. Generate Tool

import "google.golang.org/adk/tool/openapi"

openapiTool, err := openapi.NewOpenAPITool(ctx,
    openapi.WithSpecFile("weather-api.yaml"),
)

3. Register to Agent

agent, _ := llmagent.New(llmagent.Config{
    Name:        "weather_assistant",
    Model:       model,
    Instruction: "你是天气助手，帮我查询天气信息。",
    Tools: []tool.Tool{
        openapiTool,  // Exposes getWeather Tool
    },
})

Load spec from URL

Can also load from URL:

openapiTool, err := openapi.NewOpenAPITool(ctx,
    openapi.WithSpecURL("https://api.example.com/openapi.yaml"),
)

Auth Configuration

If API needs auth:

openapiTool, err := openapi.NewOpenAPITool(ctx,
    openapi.WithSpecFile("internal-api.yaml"),
    openapi.WithAuth(openapi.APIKeyAuth("X-API-Key", "your-key")),
)

Supported methods:

  • API Key (header or query)
  • Bearer Token
  • Basic Auth

Common Issues

Q: Spec too complex, Tool generation errors A: Simplify spec, only expose necessary endpoints. OpenAPI Tool may have incomplete support for some spec features.

Q: API needs POST body but Tool call fails A: Check if content/application/json’s schema definition in spec is complete.

Next Steps

Finally, look at Tool Confirmation—when Agent needs to do risky operations (delete data, transfer money), how to let user confirm before executing.

← MCP Server Integration | Tool Confirmation & Security →

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