golang/go · error

expecting a Go version like

Error message

expecting a Go version like %q

What it means

Thrown by goVersionFlag.Set when the provided Go version string does not match `modfile.GoVersionRE`. This regex validates the structural format of a Go version (e.g., '1.22', '1.22.1'). The flag is used by `go mod tidy -go` to set or update the go directive in go.mod. An invalid format like 'v1.22' or '1.x' or 'go1.22' fails the regex.

Solutions

1. Use bare major.minor format without 'v' or 'go' prefix: `go mod tidy -go=1.22`
2. Include patch version if needed: `go mod tidy -go=1.22.1`
3. Check the latest supported version with `go version` and use that as a reference

Example fix

// before
go mod tidy -go=v1.22
// after
go mod tidy -go=1.22

import (
    "regexp"
    "golang.org/x/mod/modfile"
)

func validateGoVersionFlag(s string) error {
    if !modfile.GoVersionRE.MatchString(s) {
        return fmt.Errorf("%q is not a valid Go version (use format like 1.22)", s)
    }
    return nil
}

When it happens

Trigger: Running `go mod tidy -go=v1.22` (with a 'v' prefix), `go mod tidy -go=1` (too short), `go mod tidy -go=go1.22` (with 'go' prefix), or `go mod tidy -go=latest` (non-numeric). The regex GoVersionRE expects a pattern like `\d\.\d\d?(\.\d\d?)?`.

Common situations: Confusing module version syntax (v1.2.3) with Go toolchain version syntax (1.22); adding a 'v' prefix by habit; adding 'go' prefix because that's how it appears in go.mod; using a major.minor only like '1' instead of '1.22'.

Related errors

• invalid version interval
• invalid version
• maximum supported Go version is
• non-canonical module version
• non-semver module version

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/bda9c4017f423813. Report an issue: GitHub.

Appendix: source

Thrown at src/cmd/go/internal/modcmd/tidy.go:96

	base.AddModCommonFlags(&cmdTidy.Flag)
}

// A goVersionFlag is a flag.Value representing a supported Go version.
//
// (Note that the -go argument to 'go mod edit' is *not* a goVersionFlag.
// It intentionally allows newer-than-supported versions as arguments.)
type goVersionFlag struct {
	v string
}

func (f *goVersionFlag) String() string { return f.v }
func (f *goVersionFlag) Get() any       { return f.v }

func (f *goVersionFlag) Set(s string) error {
	if s != "" {
		latest := gover.Local()
		if !modfile.GoVersionRE.MatchString(s) {
			return fmt.Errorf("expecting a Go version like %q", latest)
		}
		if gover.Compare(s, latest) > 0 {
			return fmt.Errorf("maximum supported Go version is %s", latest)
		}
	}

	f.v = s
	return nil
}

func runTidy(ctx context.Context, cmd *base.Command, args []string) {
	moduleLoader := modload.NewLoader()
	if len(args) > 0 {
		base.Fatalf("go: 'go mod tidy' accepts no arguments")
	}

	// Tidy aims to make 'go test' reproducible for any package in 'all', so we
	// need to include test dependencies. For modules that specify go 1.15 or

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