task: cannot use --list and --list-all at the same time

Error message

task: cannot use --list and --list-all at the same time

What it means

Flag validation error raised by the Validate function after CLI parsing before task execution. It fires when both --list and --list-all are supplied; these are mutually exclusive task-listing modes (--list shows only called tasks, --list-all shows every task including internal ones), so the tool cannot decide which view to produce. Fix by passing only one of the two flags.

Source

Thrown at internal/flags/flags.go:229

	if Global && Dir != "" {
		return errors.New("task: You can't set both --global and --dir")
	}

	if Output.Name != "group" {
		if Output.Group.Begin != "" {
			return errors.New("task: You can't set --output-group-begin without --output=group")
		}
		if Output.Group.End != "" {
			return errors.New("task: You can't set --output-group-end without --output=group")
		}
		if Output.Group.ErrorOnly {
			return errors.New("task: You can't set --output-group-error-only without --output=group")
		}
	}

	if List && ListAll {
		return errors.New("task: cannot use --list and --list-all at the same time")
	}

	if ListJson && !List && !ListAll {
		return errors.New("task: --json only applies to --list or --list-all")
	}

	if NoStatus && !ListJson {
		return errors.New("task: --no-status only applies to --json with --list or --list-all")
	}

	if Nested && !ListJson {
		return errors.New("task: --nested only applies to --json with --list or --list-all")
	}

	// Validate certificate flags
	if (Cert != "" && CertKey == "") || (Cert == "" && CertKey != "") {
		return errors.New("task: --cert and --cert-key must be provided together")
	}

View on GitHub (pinned to 385e5ad92a)

Solutions

Add --list or --list-all alongside --json
Remove --json if you didn't intend to list tasks
Update scripts that expected JSON output from plain task runs

Example fix

# before
task --json

# after
task --list --json

Defensive patterns

Strategy: validation

Validate before calling

if flags.ListJson && !flags.List && !flags.ListAll {
	return errors.New("--json requires --list or --list-all")
}

Try / catch

if err := flags.Validate(); err != nil {
	if strings.Contains(err.Error(), "--json only applies") {
		fmt.Fprintln(os.Stderr, "--json works only with --list or --list-all")
		os.Exit(2)
	}
	return err
}

Prevention

Remember --json is a modifier for list commands, not a global output format
Gate --json in scripts behind list intent
Use `task --list --json` for machine-readable task lists

When it happens

Trigger: Running `task --json` (or --json with other non-list flags) so ListJson is true while both List and ListAll are false when Validate() executes.

Common situations: Scripting attempts to get JSON task output by piping a normal run with --json; assuming --json works like other tools' global output-format flags.

Understand the failure class

Background: "mutually exclusive" flag errors: what "can't supply both nx and xx", "--raw is not compatible with -i" and "cannot be used with" mean, and how to fix them — this error's family across 29 libraries.

Related errors

AI-assisted analysis of go-task/task@385e5ad92a (2026-09-05). Data as JSON: /api/errors/b58ef3777fef614c. Report an issue: GitHub.