oapixconstgen

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

An OpenAPI spec often repeats the same values: a page size used as a default and a maximum, a password length used as minLength in several schemas. YAML anchors let the spec define each value once. oapixconstgen reads those values from the x-constants extension at the top of the spec and writes them to a Go file as typed constants, so the handlers, validation and tests use the numbers the spec declares instead of copies that drift.

Install

go install github.com/psyb0t/oapixconstgen@latest

Or add it as a versioned tool of your own module (Go 1.24 or newer):

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

That records it under the tool directive of your go.mod, and you run it with go tool oapixconstgen. The module itself declares go 1.26 in its go.mod, so building it needs Go 1.26 or newer.

Quick look

Define the values under x-constants, grouped by Go type, and reference them through anchors:

openapi: "3.0.0"

x-constants:
  int:
    defaultPageSize: &defaultPageSize 20
    maxPageSize: &maxPageSize 100

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

It writes internal/api/constants.gen.go:

// Code generated by oapixconstgen. DO NOT EDIT.

package api

const (
	DefaultPageSize int = 20
	MaxPageSize     int = 100
)

Where to go next

You want to	Read
Learn every flag, the exit codes and the log lines, or wire it into go generate	Command line
Write the x-constants block: type keys, values, names and ordering	The x-constants block
Build, lint and test the tool itself, or see how CI and releases run	Development

The source is on GitHub, and the release history is in the CHANGELOG.