golang/go · error

error loading go.mod: %s:%d: %v

Error message

error loading go.mod:
%s:%d: %v

What it means

Outside workspace mode, a 'godebug' directive in a go.mod failed CheckGodebug. Reported with the go.mod path and the directive's line number; multiple bad godebug lines accumulate into errs. This is the go.mod analog of error 1066 (which is for go.work).

Source

Thrown at src/cmd/go/internal/modload/init.go:1066

			continue
		}
		if ld.inWorkspaceMode() && !strings.HasPrefix(cfg.CmdName, "work ") {
			// Refuse to use workspace if its go version is too old.
			// Disable this check if we are a workspace command like work use or work sync,
			// which will fix the problem.
			mv := gover.FromGoMod(f)
			wv := gover.FromGoWork(workFile)
			if gover.Compare(mv, wv) > 0 && gover.Compare(mv, gover.GoStrictVersion) >= 0 {
				errs = append(errs, errWorkTooOld(gomod, workFile, mv))
				continue
			}
		}

		if !ld.inWorkspaceMode() {
			ok := true
			for _, g := range f.Godebug {
				if err := CheckGodebug("godebug", g.Key, g.Value); err != nil {
					errs = append(errs, fmt.Errorf("error loading go.mod:\n%s:%d: %v", base.ShortPath(gomod), g.Syntax.Start.Line, err))
					ok = false
				}
			}
			if !ok {
				continue
			}
		}

		modFiles = append(modFiles, f)
		mainModule := f.Module.Mod
		mainModules = append(mainModules, mainModule)
		indices = append(indices, indexModFile(data, f, mainModule, fixed))

		if err := module.CheckImportPath(f.Module.Mod.Path); err != nil {
			if pathErr, ok := err.(*module.InvalidPathError); ok {
				pathErr.Kind = "module"
			}
			errs = append(errs, err)

View on GitHub (pinned to b6b368adc5)

Solutions

1. Edit go.mod at the reported line and fix the key=value pair (no spaces/commas, known key).
2. Remove the godebug directive if the setting is no longer relevant.
3. Upgrade the toolchain to a version that still recognizes the key (if it was removed).
4. Re-run after editing; further godebug lines may be reported in subsequent batches.

Example fix

// before (go.mod line 12)
godebug unknownkey=1   // unknown godebug "unknownkey"

// after
godebug panicnil=0      // a real, known godebug key

// Validate godebug directives in go.mod.
data, _ := os.ReadFile("go.mod")
f, _ := modfile.Parse("go.mod", data, nil)
for _, g := range f.Godebug {
    if err := modload.CheckGodebug("godebug", g.Key, g.Value); err != nil {
        return fmt.Errorf("go.mod godebug %s=%s: %w", g.Key, g.Value, err)
    }
}

out, err := exec.Command("go", "build", "./...").CombinedOutput()
if err != nil && bytes.Contains(out, []byte("error loading go.mod")) && bytes.Contains(out, []byte("godebug")) {
    return fmt.Errorf("invalid godebug in go.mod; fix at reported line: %s", out)
}
return err

When it happens

Trigger: A module's go.mod has a 'godebug' line whose key/value is malformed or references an unknown/removed setting; ld is NOT in workspace mode so this branch (not the go.work one) runs.

Common situations: Module authored against a different Go version that knew a now-removed godebug key; typo in the directive; GODEBUG env value mistakenly committed into go.mod with the wrong format.

Related errors

• key contains space
• value contains space
• key contains comma
• value contains comma
• value for default= must be goVERSION

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/875d0760cd9d5318. Report an issue: GitHub.