diff --git a/cmd/cue/cmd/cmd.go b/cmd/cue/cmd/cmd.go
index 001af58..ad466c5 100644
--- a/cmd/cue/cmd/cmd.go
+++ b/cmd/cue/cmd/cmd.go
@@ -16,6 +16,7 @@
 
 import (
 	"fmt"
+	"os"
 
 	"github.com/spf13/cobra"
 )
@@ -210,7 +211,13 @@
 
 `,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			fmt.Println("cmd run but shouldn't")
+			if len(args) == 0 {
+				fmt.Println("cmd must be run as one of its subcommands")
+			} else {
+				fmt.Printf("cmd must be run as one of its subcommands: unknown subcommand %q\n", args[0])
+			}
+			fmt.Println("Run 'cue help cmd' for known subcommands.")
+			os.Exit(1) // TODO: get rid of this
 			return nil
 		},
 	}
diff --git a/cmd/cue/cmd/cmd_test.go b/cmd/cue/cmd/cmd_test.go
index 0ad23ff..c24e638 100644
--- a/cmd/cue/cmd/cmd_test.go
+++ b/cmd/cue/cmd/cmd_test.go
@@ -43,7 +43,7 @@
 			stdout = cmd.OutOrStdout()
 			stderr = cmd.OutOrStderr()
 
-			tools := buildTools(rootCmd, args)
+			tools, _ := buildTools(rootCmd, args)
 			cmd, err := addCustom(rootCmd, "command", name, tools)
 			if err != nil {
 				return err
diff --git a/cmd/cue/cmd/common.go b/cmd/cue/cmd/common.go
index a41c350..992164d 100644
--- a/cmd/cue/cmd/common.go
+++ b/cmd/cue/cmd/common.go
@@ -111,10 +111,10 @@
 	return instances
 }
 
-func buildTools(cmd *cobra.Command, args []string) *cue.Instance {
+func buildTools(cmd *cobra.Command, args []string) (*cue.Instance, error) {
 	binst := loadFromArgs(cmd, args)
 	if len(binst) == 0 {
-		return nil
+		return nil, nil
 	}
 
 	included := map[string]bool{}
@@ -130,6 +130,5 @@
 	}
 
 	inst := cue.Merge(buildInstances(cmd, binst)...).Build(ti)
-	exitIfErr(cmd, inst, inst.Err, true)
-	return inst
+	return inst, inst.Err
 }
diff --git a/cmd/cue/cmd/get.go b/cmd/cue/cmd/get.go
index a0603ef..6cf6049 100644
--- a/cmd/cue/cmd/get.go
+++ b/cmd/cue/cmd/get.go
@@ -16,6 +16,7 @@
 
 import (
 	"fmt"
+	"os"
 
 	"github.com/spf13/cobra"
 )
@@ -35,7 +36,13 @@
 per language and are documented in the respective subcommands.
 `,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			fmt.Println("get must be run as one of its subcommands")
+			if len(args) == 0 {
+				fmt.Println("get must be run as one of its subcommands")
+			} else {
+				fmt.Printf("get must be run as one of its subcommands: unknown subcommand %q\n", args[0])
+			}
+			fmt.Println("Run 'cue help get' for known subcommands.")
+			os.Exit(1) // TODO: get rid of this
 			return nil
 		},
 	}
diff --git a/cmd/cue/cmd/root.go b/cmd/cue/cmd/root.go
index 7dbf6e7..671da30 100644
--- a/cmd/cue/cmd/root.go
+++ b/cmd/cue/cmd/root.go
@@ -16,11 +16,14 @@
 
 import (
 	"context"
-	"errors"
+	"fmt"
 	"io"
 	logger "log"
 	"os"
+	"strings"
 
+	"cuelang.org/go/cue/errors"
+	"cuelang.org/go/cue/token"
 	"github.com/spf13/cobra"
 )
 
@@ -147,50 +150,93 @@
 	cmd = newRootCmd()
 	rootCmd := cmd.root
 	rootCmd.SetArgs(args)
-	if len(args) >= 1 && args[0] != "help" {
-		// TODO: for now we only allow one instance. Eventually, we can allow
-		// more if they all belong to the same package and we merge them
-		// before computing commands.
-		if cmd, _, err := rootCmd.Find(args); err != nil || cmd == nil {
-			tools := buildTools(rootCmd, args[1:])
-			addCustom(rootCmd, commandSection, args[0], tools)
-		}
+	if len(args) == 0 {
+		return cmd, nil
+	}
 
-		type subSpec struct {
-			name string
-			cmd  *cobra.Command
-		}
-		sub := map[string]subSpec{
-			"cmd": {commandSection, cmd.cmd},
-			// "serve": {"server", nil},
-			// "fix":   {"fix", nil},
-		}
-		if sub, ok := sub[args[0]]; ok && len(args) >= 2 {
-			args = args[1:]
-			if len(args) == 0 {
-				tools := buildTools(rootCmd, args)
-				// list available commands
-				commands := tools.Lookup(sub.name)
-				i, err := commands.Fields()
-				if err != nil {
-					return nil, err
-				}
-				for i.Next() {
-					_, _ = addCustom(sub.cmd, sub.name, i.Label(), tools)
-				}
-				return cmd, nil
-			}
-			tools := buildTools(rootCmd, args[1:])
-			_, err := addCustom(sub.cmd, sub.name, args[0], tools)
-			if err != nil {
-				log.Printf("%s %q is not defined", sub.name, args[0])
-				exit()
-			}
-		}
+	var sub = map[string]*subSpec{
+		"cmd": {commandSection, cmd.cmd},
+		// "serve": {"server", nil},
+		// "fix":   {"fix", nil},
+	}
+
+	if args[0] == "help" {
+		return cmd, addSubcommands(cmd, sub, args[1:])
+	}
+
+	if _, ok := sub[args[0]]; ok {
+		return cmd, addSubcommands(cmd, sub, args)
+	}
+
+	if c, _, err := rootCmd.Find(args); err == nil && c != nil {
+		return cmd, nil
+	}
+
+	if !isCommandName(args[0]) {
+		return cmd, nil // Forces unknown command message from Cobra.
+	}
+
+	tools, err := buildTools(rootCmd, args[1:])
+	if err != nil {
+		return cmd, err
+	}
+	_, err = addCustom(rootCmd, commandSection, args[0], tools)
+	if err != nil {
+		fmt.Printf("command %s %q is not defined\n", commandSection, args[0])
+		fmt.Println("Run 'cue help' to show available commands.")
+		os.Exit(1)
 	}
 	return cmd, nil
 }
 
+type subSpec struct {
+	name string
+	cmd  *cobra.Command
+}
+
+func addSubcommands(cmd *Command, sub map[string]*subSpec, args []string) error {
+	if len(args) == 0 {
+		return nil
+	}
+
+	if _, ok := sub[args[0]]; ok {
+		args = args[1:]
+	}
+
+	if len(args) > 0 {
+		if !isCommandName(args[0]) {
+			return nil // Forces unknown command message from Cobra.
+		}
+		args = args[1:]
+	}
+
+	tools, err := buildTools(cmd.root, args)
+	if err != nil {
+		return err
+	}
+
+	// TODO: for now we only allow one instance. Eventually, we can allow
+	// more if they all belong to the same package and we merge them
+	// before computing commands.
+	for _, spec := range sub {
+		commands := tools.Lookup(spec.name)
+		i, err := commands.Fields()
+		if err != nil {
+			return errors.Newf(token.NoPos, "could not create command definitions: %v", err)
+		}
+		for i.Next() {
+			_, _ = addCustom(spec.cmd, spec.name, i.Label(), tools)
+		}
+	}
+	return nil
+}
+
+func isCommandName(s string) bool {
+	return !strings.Contains(s, `/\`) &&
+		!strings.HasPrefix(s, ".") &&
+		!strings.HasSuffix(s, ".cue")
+}
+
 type panicError struct {
 	Err error
 }