nektos/act · error

unknown server OS

Error message

unknown server OS: %s

What it means

parseDevice maps a --device flag string into a container.DeviceMapping and dispatches on the Docker server OS reported by the daemon. Only 'linux' and 'windows' are recognized; any other server OS string produces this error. The OS value comes from the daemon's system info, so it reflects what the connected Docker daemon reports.

Solutions

1. Verify what the daemon reports: docker info --format '{{.OperatingSystem}}' / check OSType
2. Connect to a Docker daemon that reports linux (or windows) — act only supports those for device mapping
3. If invoking the API from code, pass the OS from host info's OSType field verbatim
4. Update act to a newer version in case support for your daemon was added

// Go: check daemon OS before touching device APIs
info, err := container.GetHostInfo(ctx)
if err != nil { return err }
if info.OSType != "linux" && info.OSType != "windows" {
    return fmt.Errorf("unsupported daemon OS %q for device mapping", info.OSType)
}

func isSupportedServerOS(os string) bool { return os == "linux" || os == "windows" }

When it happens

Trigger: Calling the device parsing path when the connected Docker daemon reports an operating system other than linux or windows (for example a future or unusual daemon), or when the serverOS argument was passed an empty/garbage string by a caller.

Common situations: Running act against an exotic or misconfigured Docker endpoint, a daemon whose .ServerInfo returns an unexpected OSType, or a custom build of the container package invoked with an unchecked OS string.

Related errors

• invalid device specification
• bad format for path
• bad mode specified
• Cannot process container options
• invalid device cgroup format

AI-assisted analysis of nektos/act@4f41128141 (2026-08-15). Data as JSON: /api/errors/161b16009ab7d419. Report an issue: GitHub.

Appendix: source

Thrown at pkg/container/docker_cli.go:1010

		k, v, ok := strings.Cut(option, "=")
		if !ok {
			return nil, errors.New("invalid storage option")
		}
		m[k] = v
	}
	return m, nil
}

// parseDevice parses a device mapping string to a container.DeviceMapping struct
func parseDevice(device, serverOS string) (container.DeviceMapping, error) {
	switch serverOS {
	case "linux":
		return parseLinuxDevice(device)
	case "windows":
		// Windows doesn't support mapping, so passing the given value as-is.
		return container.DeviceMapping{PathOnHost: device}, nil
	default:
		return container.DeviceMapping{}, fmt.Errorf("unknown server OS: %s", serverOS)
	}
}

// parseLinuxDevice parses a device mapping string to a container.DeviceMapping struct
// knowing that the target is a Linux daemon
func parseLinuxDevice(device string) (container.DeviceMapping, error) {
	var src, dst string
	permissions := "rwm"
	// We expect 3 parts at maximum; limit to 4 parts to detect invalid options.
	arr := strings.SplitN(device, ":", 4)
	switch len(arr) {
	case 3:
		permissions = arr[2]
		fallthrough
	case 2:
		if validDeviceMode(arr[1]) {
			permissions = arr[1]
		} else {

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