oapixconstgen

A Go tool that extracts typed constants from OpenAPI specification files and generates Go code.

Overview

When defining OpenAPI specs, you often need to reuse constant values (like pagination limits, timeouts, or configuration values) across multiple places using YAML anchors. oapixconstgen reads these constants from the x-constants extension and generates a Go file with properly typed constants, keeping your API spec and Go code in sync.

Installation

go install github.com/psyb0t/oapixconstgen@latest

Or add as a project tool (Go 1.24+):

go get -tool github.com/psyb0t/oapixconstgen@latest

This adds it to your go.mod under the tool directive, making it a versioned project dependency.

Usage

oapixconstgen -spec=path/to/api.yaml [-out=output/dir] [-pkg=packagename]

Or if installed as a project tool:

go tool oapixconstgen -spec=path/to/api.yaml [-out=output/dir] [-pkg=packagename]

Flags

Flag	Default	Description
-spec	(required)	Path to the OpenAPI spec file
-out	.	Output directory for generated file
-pkg	api	Package name for generated Go file

Example

Given an OpenAPI spec with x-constants:

openapi: "3.0.0"

x-constants:
  int:
    defaultPageSize: &defaultPageSize 20
    maxPageSize: &maxPageSize 100
  uint:
    minPasswordLength: &minPasswordLength 8
  float64:
    requestTimeout: &requestTimeout 30.5
  string:
    defaultLocale: &defaultLocale "en-US"
  bool:
    enableCache: &enableCache true

info:
  version: 1.0.0
  title: My API

paths:
  /items:
    get:
      parameters:
        - name: limit
          in: query
          schema:
            type: integer
            default: *defaultPageSize
            maximum: *maxPageSize

Run the generator:

oapixconstgen -spec=api.yaml -out=./internal/api -pkg=api

This generates internal/api/constants.gen.go:

// Code generated by oapixconstgen. DO NOT EDIT.

package api

const (
	DefaultLocale     string  = "en-US"
	DefaultPageSize   int     = 20
	EnableCache       bool    = true
	MaxPageSize       int     = 100
	MinPasswordLength uint    = 8
	RequestTimeout    float64 = 30.5
)

Supported Types

Type Key	Go Type
int	int
int8	int8
int16	int16
int32	int32
int64	int64
uint	uint
uint8	uint8
uint16	uint16
uint32	uint32
uint64	uint64
float32	float32
float64	float64
string	string
bool	bool

Naming Convention

Constant names are converted from snake_case or camelCase to Go exported names:

OpenAPI Name	Go Constant
defaultLimit	DefaultLimit
max_page_size	MaxPageSize

Use with go:generate

Add a generate directive to automate constant generation:

//go:generate oapixconstgen -spec=../../api/openapi.yaml -out=. -pkg=api

Or if using as a project tool:

//go:generate go tool oapixconstgen -spec=../../api/openapi.yaml -out=. -pkg=api

Then run:

go generate ./...

License

MIT