What causes "unknown vcs: %s %s" in golang/go?

Inspect the repository's go-import meta tag and correct the vcs attribute to one of git, hg, svn, fossil.

unknown vcs: %s %s

Error message

unknown vcs: %s %s

What it means

newVCSRepo looks up the requested VCS name in the vcsCmds map (which only contains 'hg', 'svn', 'fossil' — git is handled earlier by newGitRepo). Any other name yields this error. The two %s are the unrecognised vcs and the remote URL.

Source

Thrown at src/cmd/go/internal/modfetch/codehost/vcs.go:103

	fetchOnce sync.Once
	fetchErr  error
	fetched   atomic.Bool

	repoSumOnce sync.Once
	repoSum     string

	statCache     par.ErrCache[string, *RevInfo]  // cache key is revision
	readFileCache par.ErrCache[[2]string, []byte] // cache key is revision and file path
}

func newVCSRepo(ctx context.Context, vcs, remote string, local bool) (Repo, error) {
	if vcs == "git" {
		return newGitRepo(ctx, remote, local)
	}
	r := &vcsRepo{remote: remote, local: local}
	cmd := vcsCmds[vcs]
	if cmd == nil {
		return nil, fmt.Errorf("unknown vcs: %s %s", vcs, remote)
	}
	r.cmd = cmd
	if local {
		info, err := os.Stat(remote)
		if err != nil {
			return nil, err
		}
		if !info.IsDir() {
			return nil, fmt.Errorf("%s exists but is not a directory", remote)
		}
		r.dir = remote
		r.mu.Path = r.dir + ".lock"
		return r, nil
	}
	if !strings.Contains(remote, "://") {
		return nil, fmt.Errorf("invalid vcs remote: %s %s", vcs, remote)
	}
	var err error

View on GitHub (pinned to b6b368adc5)

Solutions

Inspect the repository's go-import meta tag and correct the vcs attribute to one of git, hg, svn, fossil.
If the upstream truly uses an unsupported VCS, mirror it to git and update the import path or GOPROXY accordingly.
Fetch the module through the module proxy (GOPROXY=https://proxy.golang.org) which serves a zip and never invokes the VCS code path.

Example fix

<!-- before -->
<meta name="go-import" content="example.org/m vcs=bzr https://bzr.example.org/m">

<!-- after -->
<meta name="go-import" content="example.org/m vcs=git https://git.example.org/m">

Defensive patterns

Strategy: validation

Validate before calling

var supportedVCS = map[string]bool{"git": true, "hg": true, "svn": true, "fossil": true}

func vcsSupported(vcs string) bool { return supportedVCS[vcs] }

Prevention

Validate go-import meta tags against the supported VCS set before relying on them.
Prefer git for new vanity imports — it has the broadest support and is the most tested path.
Use GOPROXY to avoid exercising the direct-VCS code path for exotic repos.

When it happens

Trigger: codehost.newVCSRepo or a caller passes a vcs string that is not 'git', 'hg', 'svn', or 'fossil'. Typically the result of a malformed vanity import meta tag or a corrupted repo metadata record.

Common situations: A go-import meta tag advertises `vcs=bzr` or `vcs=cvs`; a manually crafted module cache record names an unsupported VCS; future/typo'd VCS strings like 'github' or 'https'.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/555072f1d280d08d. Report an issue: GitHub.