Tonic is an OpenAPI 3.2 doc generator for Go frameworks. Unlike tools like Swaggo that rely on code comments, Tonic uses reflection to inspect docs straight from your routes and struct binding tags, both request and response. It currently ships adapters for Chi, Echo, Fiber, and Gin.

In the Go world, people tend to go with the Design-First approach (I am not sure why). Swaggo applies this approach, but it doesn't always work. In reality, things change. I started with a clean API spec, but once business needs shift, that spec becomes outdated fast. I got tired of updating comments every time an endpoint changed, so tedious and error-prone tasks.

Following the Code-First approach, your docs evolve with your code. So I bring Tonic to save time and keep things in sync with the actual code.

Usage flow of swaggo combo:

graph TD
    A[Initialize project via CLI]
    B[Generate Swagger files from code comments]
    C[Serve generated spec files]

    A --> B
    B --> C

Meanwhile, Tonic just reflects the code and generates the swagger documentation directly from the code itself.

Using reflect, Tonic reads struct's metadata like JSON tag, data type ... and generate an object schema for the struct. For example:

type ArticleDTO struct {
    ID 		    int 	`json:"id"`
    Title 	    string	`json:"title" binding:"required,min=4,max=255"`
    Content 	string	`json:"content" binding:"required,min=20"`
}

Will be generated to:

{
    "id": {
	    "type": "integer"
    },
    "title": {
        "type": "string",
        "minLength": 4,
        "maxLength": 255
    },
    "content": {
        "type": "string",
        "minLength": 20
    }
}

Combine with route definitions, Tonic constructs an object to contain API documentation data in runtime then hosts a Swagger UI for it.

Tonic supports multiple UI providers for the generated OpenAPI document. See examples for more detail.

ReDoc	Swagger UI
Scalar	RapiDoc