Golang Swagger

Persona: You are a Go API documentation engineer. You treat docs as a contract — accurate, complete annotations prevent integration bugs and make the Swagger UI the source of truth for API consumers.

Modes:

Setup

Three steps to get Swagger UI running:

swag init                        # generates docs/ with docs.go, swagger.json, swagger.yaml
swag init -g cmd/api/main.go     # if general info is not in main.go
swag fmt                         # format annotation comments (like go fmt)

Import the docs package to register the spec. Use a blank import when only wiring the UI; use a named import when you also need to override docs.SwaggerInfo at runtime:

import _ "yourmodule/docs"          // blank: registers spec, no identifier
import docs "yourmodule/docs"       // named: use when overriding SwaggerInfo

Wire the UI endpoint — pick your framework:

// Gin
r.GET("/swagger/*any", ginSwagger.WrapHandler(swaggerFiles.Handler))
// Echo
e.GET("/swagger/*", echoSwagger.WrapHandler)
// Fiber
app.Get("/swagger/*", fiberSwagger.WrapHandler(swaggerFiles.Handler))
// net/http
mux.Handle("/swagger/", httpSwagger.Handler(swaggerFiles.Handler))
// Chi
r.Get("/swagger/*", httpSwagger.Handler(swaggerFiles.Handler))

Access the UI at /swagger/index.html.

For dynamic host/basepath (multi-environment), use a named import and override before serving:

import docs "yourmodule/docs"
docs.SwaggerInfo.Host     = os.Getenv("API_HOST")
docs.SwaggerInfo.BasePath = "/api/v1"

Full CLI reference

General API Info

Place in main.go (or the file passed via -g). These annotations define the top-level spec:

// @title           My API
// @version         1.0
// @description     Short description of the API.
// @host            localhost:8080
// @BasePath        /api/v1
// @schemes         http https
// @contact.name    API Support
// @contact.email   support@example.com
// @license.name    Apache 2.0
// @securityDefinitions.apikey Bearer
// @in header
// @name Authorization
// @description Type "Bearer" followed by a space and the JWT token.

Operation Annotations

Annotate each handler function. The standard doc comment (// FuncName godoc) must precede swag annotations — it anchors indentation for swag fmt.

// ShowAccount godoc
// @Summary      Get account by ID
// @Description  Returns account details for the given ID.
// @Tags         accounts
// @Accept       json
// @Produce      json
// @Param        id      path  int  true  "Account ID"
// @Param        filter  query string false "Optional search filter"
// @Success      200  {object}  model.Account
// @Success      204  "No content"
// @Failure      400  {object}  api.ErrorResponse
// @Failure      404  {object}  api.ErrorResponse
// @Router       /accounts/{id} [get]
// @Security     Bearer
func ShowAccount(c *gin.Context) {}

@Param format: @Param "" [attributes]

| | Usage | | ---------- | ------------------------------------ | | path | URL path segment (/users/{id}) | | query | URL query string (?filter=x) | | body | Request body — type must be a struct | | header | HTTP header | | formData | Multipart/form field |

Optional attributes on @Param: default(v), minimum(n), maximum(n), minLength(n), maxLength(n), Enums(a,b,c), example(v), collectionFormat(multi).

@Success/@Failure format: @Success {} ""

| | When | | -------------------- | ---------------- | | {object} | Single struct | | {array} | Slice of structs | | string / integer | Primitive |

Generics (swag v2): @Success 200 {object} api.Response[model.User]

Nested composition: @Success 200 {object} api.Response{data=model.User}

Security Definitions

Define once at the API level (in main.go), apply per endpoint with @Security.

// Bearer / JWT
// @securityDefinitions.apikey Bearer
// @in header
// @name Authorization
// API key in header
// @securityDefinitions.apikey ApiKeyAuth
// @in header
// @name X-API-Key
// Basic auth
// @securityDefinitions.basic BasicAuth
// OAuth2 authorization code
// @securityDefinitions.oauth2.authorizationCode OAuth2
// @authorizationUrl https://example.com/oauth/authorize
// @tokenUrl https://example.com/oauth/token
// @scope.read Read access
// @scope.write Write access

Apply to an endpoint:

// @Security Bearer
// @Security OAuth2[read, write]
// @Security BasicAuth && ApiKeyAuth   // AND — both required

Struct Tags

Enrich models without changing their Go type:

type CreateUserRequest struct {
    Name   string json:"name" example:"Jane Doe" minLength:"2" maxLength:"100"
    Role   string json:"role" enums:"admin,user,guest" example:"user"
    Age    int    json:"age" minimum:"18" maximum:"120"
    Avatar []byte json:"avatar" swaggertype:"string" format:"base64"
    Secret string json:"-" swaggerignore:"true"  // excluded from docs
}

| Tag | Purpose | | --- | --- | | example | Example value shown in Swagger UI | | enums | Comma-separated allowed values | | swaggertype | Override detected type (e.g., "primitive,integer" for time.Time) | | swaggerignore:"true" | Exclude field from the generated schema | | extensions | Add OpenAPI extensions: extensions:"x-nullable,x-deprecated=true" |

Common Mistakes

| Mistake | Why it breaks | Fix | | --- | --- | --- | | Missing _ "yourmodule/docs" import | Schema not registered; UI loads empty | Add blank import in main.go or server init | | Stale docs/ after code changes | Docs diverge from implementation; consumers get wrong schema | Re-run swag init after every annotation change | | @Param body with primitive type | swag cannot derive schema from string; generation fails | Always use a named struct for body params | | No @Security on protected routes | Swagger UI shows no lock icon; testers send unauthenticated requests | Apply @Security to every authenticated endpoint | | General info annotations in the wrong file | swag silently skips them; spec has no title/host | Use -g flag or move annotations to main.go | | Using {object} with a map type | swag cannot generate a schema for map[string]any without help | Use a named struct or annotate with swaggertype | | Multi-word @Tags without quotes | Tags split on spaces, producing malformed grouping | Quote tags with spaces: @Tags "user accounts" |

Cross-References

Three steps to get Swagger UI running:

swag init                        # generates docs/ with docs.go, swagger.json, swagger.yaml
swag init -g cmd/api/main.go     # if general info is not in main.go
swag fmt                         # format annotation comments (like go fmt)

Import the docs package to register the spec. Use a blank import when only wiring the UI; use a named import when you also need to override docs.SwaggerInfo at runtime:

import _ "yourmodule/docs"          // blank: registers spec, no identifier
import docs "yourmodule/docs"       // named: use when overriding SwaggerInfo

Wire the UI endpoint — pick your framework:

// Gin
r.GET("/swagger/*any", ginSwagger.WrapHandler(swaggerFiles.Handler))
// Echo
e.GET("/swagger/*", echoSwagger.WrapHandler)
// Fiber
app.Get("/swagger/*", fiberSwagger.WrapHandler(swaggerFiles.Handler))
// net/http
mux.Handle("/swagger/", httpSwagger.Handler(swaggerFiles.Handler))
// Chi
r.Get("/swagger/*", httpSwagger.Handler(swaggerFiles.Handler))

Access the UI at /swagger/index.html.

For dynamic host/basepath (multi-environment), use a named import and override before serving:

import docs "yourmodule/docs"
docs.SwaggerInfo.Host     = os.Getenv("API_HOST")
docs.SwaggerInfo.BasePath = "/api/v1"

Full CLI reference