Command line

oapixconstgen is a single command with four flags. It reads one spec, writes one Go file, and exits. All of it lives in main.go.

oapixconstgen \
  -spec=api/openapi.yaml \
  -out=internal/api \
  -file=constants.gen.go \
  -pkg=api

Only -spec is required. Installed as a project tool, the same flags follow go tool:

go tool oapixconstgen -spec=api/openapi.yaml -out=internal/api

Flags

Flag	Default	Description
`-spec`	(required)	Path to the OpenAPI spec file.
`-out`	`.`	Directory the generated file is written to. It must already exist; the tool does not create it.
`-file`	`constants.gen.go`	Name of the generated file inside `-out`.
`-pkg`	`api`	Package name written in the generated file’s `package` clause.

Flags use the Go flag package syntax, so -spec=api.yaml, -spec api.yaml and --spec=api.yaml all work. -h prints the flag list.

What a run does

Reads the file at -spec and parses it as YAML. JSON is valid YAML, so a JSON spec works too.
Takes the top-level x-constants map and ignores everything else in the document. The rest of the spec is not validated. See The x-constants block for the format.
Builds one const block, sorted by the names as written in the spec, and runs it through go/format.
Writes it to -out/-file, replacing any file already there. The file is created with mode 0600, readable and writable by its owner only.

If the spec has no x-constants, or none of its type keys are supported, the tool writes nothing, leaves any existing output file alone, and exits 0.

Exit codes

Code	When
`0`	The file was written, or there were no constants to write.
`1`	A flag could not be parsed (including `-h`), `-spec` is missing, the spec cannot be read or parsed, the generated code is not valid Go syntax, or the output file cannot be written.

Log output

Every message goes to stderr through log/slog in its text format. stdout stays empty.

Level	Message	Meaning
`INFO`	`constants generated`	Success. The `path` field holds the file that was written.
`INFO`	`no x-constants found in spec, skipping generation`	Nothing to write. Exit code 0.
`ERROR`	`spec path is required`	`-spec` was not given.
`ERROR`	`failed to read spec file`	The spec path does not exist or cannot be read.
`ERROR`	`failed to parse spec file`	The file is not valid YAML, or `x-constants` is not a map of maps.
`ERROR`	`failed to generate code`	The generated source does not parse as Go. The usual causes are an invalid `-pkg`, a constant name that is not a Go identifier, or a value that does not fit its type key. The `error` field holds the parser message.
`ERROR`	`failed to write output file`	The `-out` directory does not exist or is not writable.

A successful run looks like this:

time=2026-01-28T10:00:00.000Z level=INFO msg="constants generated" path=internal/api/constants.gen.go

Use with go generate

Put a directive in a Go file of the package that should hold the constants. go generate runs it from that package’s directory, so -spec is relative to it:

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

As a project tool:

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

Then regenerate everything:

go generate ./...

The generated file starts with // Code generated by oapixconstgen. DO NOT EDIT., the marker Go tooling and linters use to recognize generated code. Edit the spec and run the generator again rather than changing the file by hand.