diff --git a/cli/cmd/args_test.go b/cli/cmd/args_test.go
new file mode 100644
index 00000000..4dd5fa9b
--- /dev/null
+++ b/cli/cmd/args_test.go
@@ -0,0 +1,106 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cmd
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/spf13/cobra"
+	"github.com/spf13/pflag"
+)
+
+// execute drives the CLI through rootCmd.Execute. Calling ValidateArgs
+// directly is not enough: it passes on the non-runnable plugin parent
+// while the real CLI exits 0 (#345).
+func execute(t *testing.T, args ...string) error {
+	t.Helper()
+	out := &bytes.Buffer{}
+	rootCmd.SetOut(out)
+	rootCmd.SetErr(out)
+	rootCmd.SetArgs(args)
+	err := rootCmd.Execute()
+	resetFlags(rootCmd)
+	return err
+}
+
+// Flag values persist across Execute calls, so reset them between cases.
+func resetFlags(cmd *cobra.Command) {
+	cmd.Flags().Visit(func(f *pflag.Flag) {
+		_ = f.Value.Set(f.DefValue)
+		f.Changed = false
+	})
+	for _, sub := range cmd.Commands() {
+		resetFlags(sub)
+	}
+}
+
+func TestArgumentValidation(t *testing.T) {
+	t.Setenv("HOME", t.TempDir())
+
+	tests := []struct {
+		name    string
+		args    []string
+		wantErr bool
+	}{
+		// The five reproductions from #345.
+		{name: "plugin unknown subcommand", args: []string{"plugin", "bogus"}, wantErr: true},
+		{name: "plugin help routed as args", args: []string{"plugin", "help", "list"}, wantErr: true},
+		{name: "plugin install extra args", args: []string{"plugin", "install", "a", "b", "c"}, wantErr: true},
+		{name: "plugin install no name no --all", args: []string{"plugin", "install"}, wantErr: true},
+		{name: "convert trailing arg", args: []string{"convert", "--from", "x", "--input", "y", "extra"}, wantErr: true},
+		{name: "plugin bare", args: []string{"plugin"}, wantErr: true},
+		{name: "plugin install --all with name", args: []string{"plugin", "install", "--all", "foo"}, wantErr: true},
+		// Valid invocations keep working.
+		{name: "plugin list", args: []string{"plugin", "list"}},
+		{name: "plugin install by name", args: []string{"plugin", "install", "foo"}},
+		{name: "plugin install --all", args: []string{"plugin", "install", "--all"}},
+		{name: "convert flags only", args: []string{"convert", "--from", "x", "--input", "y"}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := execute(t, tt.args...)
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("Execute(%q) error = %v, wantErr %v", tt.args, err, tt.wantErr)
+			}
+		})
+	}
+}
+
+// Every command needs an Args validator, and every parent needs to be
+// runnable, or cobra silently accepts arbitrary arguments (#345).
+func TestEveryCommandValidatesArgs(t *testing.T) {
+	var walk func(cmd *cobra.Command)
+	walk = func(cmd *cobra.Command) {
+		for _, sub := range cmd.Commands() {
+			if sub.Name() == "help" || sub.Name() == "completion" {
+				continue
+			}
+			if sub.HasSubCommands() {
+				if !sub.Runnable() {
+					t.Errorf("%s: parent is not runnable, unknown subcommands would exit 0", sub.CommandPath())
+				}
+				walk(sub)
+			}
+			if sub.Args == nil {
+				t.Errorf("%s: no Args validator", sub.CommandPath())
+			}
+		}
+	}
+	walk(rootCmd)
+}
diff --git a/cli/cmd/convert.go b/cli/cmd/convert.go
index 1d0734fc..1d20d78f 100644
--- a/cli/cmd/convert.go
+++ b/cli/cmd/convert.go
@@ -25,6 +25,7 @@ import (
 var convertCmd = &cobra.Command{
 	Use:   "convert --from <platform> --input <path> | --to <platform> --input <path>",
 	Short: "Convert a semantic model between Ossie and a platform format",
+	Args:  cobra.NoArgs,
 	RunE:  runConvert,
 }
 
diff --git a/cli/cmd/plugin/install.go b/cli/cmd/plugin/install.go
index 7a77f235..38476898 100644
--- a/cli/cmd/plugin/install.go
+++ b/cli/cmd/plugin/install.go
@@ -17,6 +17,7 @@
 package plugin
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/spf13/cobra"
@@ -25,6 +26,7 @@ import (
 var installCmd = &cobra.Command{
 	Use:   "install [name[@version] | url]",
 	Short: "Install a plugin from the registry or a URL",
+	Args:  cobra.MaximumNArgs(1),
 	RunE:  runPluginInstall,
 }
 
@@ -33,6 +35,17 @@ func init() {
 }
 
 func runPluginInstall(cmd *cobra.Command, args []string) error {
+	all, err := cmd.Flags().GetBool("all")
+	if err != nil {
+		return err
+	}
+	hasName := len(args) == 1
+	if all && hasName {
+		return errors.New("--all cannot be combined with a plugin name")
+	}
+	if !all && !hasName {
+		return errors.New("requires a plugin name or --all")
+	}
 	fmt.Fprintln(cmd.OutOrStdout(), "not yet implemented")
 	return nil
 }
diff --git a/cli/cmd/plugin/plugin.go b/cli/cmd/plugin/plugin.go
index b1f217bc..647e0c82 100644
--- a/cli/cmd/plugin/plugin.go
+++ b/cli/cmd/plugin/plugin.go
@@ -16,13 +16,25 @@
 
 package plugin
 
-import "github.com/spf13/cobra"
+import (
+	"errors"
+
+	"github.com/spf13/cobra"
+)
 
 // Cmd is the parent "ossie plugin" command. It is exported so cmd/root.go can
-// register it. Invoking it bare prints help.
+// register it.
+//
+// The RunE matters: cobra never reaches ValidateArgs on a non-runnable
+// command, so without it a bare "ossie plugin" or an unknown subcommand
+// prints help and exits 0 (#345).
 var Cmd = &cobra.Command{
 	Use:   "plugin",
 	Short: "Manage Ossie plugins",
+	Args:  cobra.NoArgs,
+	RunE: func(cmd *cobra.Command, args []string) error {
+		return errors.New("a subcommand is required")
+	},
 }
 
 func init() {