diff --git a/src/cli/ModernMT.CLI/CliCompletion.cs b/src/cli/ModernMT.CLI/CliCompletion.cs
new file mode 100644
index 0000000..fc64b73
--- /dev/null
+++ b/src/cli/ModernMT.CLI/CliCompletion.cs
@@ -0,0 +1,246 @@
+#nullable enable
+
+using System.CommandLine;
+using System.Text;
+
+namespace ModernMT.CLI;
+
+/// <summary>
+/// Generates shell completion scripts from the command tree this CLI actually exposes.
+/// </summary>
+internal static class CliCompletion
+{
+    /// <summary>Shells a script can be emitted for.</summary>
+    public static string[] SupportedShells { get; } = ["bash", "zsh", "fish"];
+
+    /// <summary>
+    /// Builds the `completion &lt;shell&gt;` command. Pass the root command so the script describes
+    /// the tree as assembled, including any hand-written commands added alongside the generated ones.
+    /// </summary>
+    public static Command CreateCommand(Command root, string toolName)
+    {
+        var shell = new Argument<string>("shell")
+        {
+            Description = "Shell to emit a completion script for: bash, zsh, or fish.",
+        };
+
+        var command = new Command(
+            "completion",
+            "Print a shell completion script on stdout, generated from this CLI's command tree.")
+        {
+            shell,
+        };
+
+        command.SetAction(parseResult =>
+        {
+            var requested = (parseResult.GetValue(shell) ?? string.Empty).Trim().ToLowerInvariant();
+            if (!SupportedShells.Contains(requested, StringComparer.Ordinal))
+            {
+                Console.Error.WriteLine(
+                    "Unsupported shell '" + requested + "'. Accepted values: " + string.Join(", ", SupportedShells) + ".");
+                return 2;
+            }
+
+            Console.WriteLine(Generate(requested, root, toolName));
+            return 0;
+        });
+
+        return command;
+    }
+
+    public static string Generate(string shell, Command root, string toolName) => shell switch
+    {
+        "bash" => GenerateBash(root, toolName),
+        "zsh" => GenerateZsh(root, toolName),
+        "fish" => GenerateFish(root, toolName),
+        _ => throw new ArgumentException("Unsupported shell '" + shell + "'.", nameof(shell)),
+    };
+
+    private static IEnumerable<Command> VisibleSubcommands(Command command) =>
+        command.Subcommands.Where(x => !x.Hidden).OrderBy(x => x.Name, StringComparer.Ordinal);
+
+    private static List<string> OptionNames(Command command) =>
+        [.. command.Options
+            .Where(option => !option.Hidden)
+            .SelectMany(option => new[] { option.Name }.Concat(option.Aliases))
+            .Distinct(StringComparer.Ordinal)
+            .OrderBy(name => name, StringComparer.Ordinal)];
+
+    /// <summary>
+    /// Enum-typed options complete their own values, read from the option's value type rather than
+    /// from the spec, so hand-added options are covered too.
+    /// </summary>
+    private static List<(string Option, string Values)> EnumOptions(Command command) =>
+        [.. command.Options
+            .Where(option => !option.Hidden)
+            .Select(option => (option.Name, Type: Nullable.GetUnderlyingType(option.ValueType) ?? option.ValueType))
+            .Where(x => x.Type.IsEnum)
+            .Select(x => (x.Name, string.Join(" ", Enum.GetNames(x.Type))))];
+
+    private static IEnumerable<(string Top, Command Command)> Walk(Command command, string top)
+    {
+        foreach (var child in VisibleSubcommands(command))
+        {
+            var childTop = string.IsNullOrEmpty(top) ? child.Name : top;
+            yield return (childTop, child);
+            foreach (var descendant in Walk(child, childTop))
+            {
+                yield return descendant;
+            }
+        }
+    }
+
+    private static string GenerateBash(Command root, string toolName)
+    {
+        var id = Identifier(toolName);
+        var builder = new StringBuilder();
+        builder.AppendLine("# " + toolName + " completion (bash), generated from the CLI command tree.");
+        builder.AppendLine("_" + id + "_completions() {");
+        builder.AppendLine("  local cur prev sub i");
+        builder.AppendLine("  COMPREPLY=()");
+        builder.AppendLine("  cur=\"${COMP_WORDS[COMP_CWORD]}\"");
+        builder.AppendLine("  prev=\"${COMP_WORDS[COMP_CWORD-1]}\"");
+        builder.AppendLine("  sub=\"\"");
+        builder.AppendLine("  for ((i=1; i<COMP_CWORD; i++)); do");
+        builder.AppendLine("    case \"${COMP_WORDS[i]}\" in");
+        builder.AppendLine("      -*) ;;");
+        builder.AppendLine("      *) if [[ -z \"$sub\" ]]; then sub=\"${COMP_WORDS[i]}\"; fi ;;");
+        builder.AppendLine("    esac");
+        builder.AppendLine("  done");
+        builder.AppendLine();
+
+        foreach (var (top, command) in Walk(root, string.Empty))
+        {
+            foreach (var (option, values) in EnumOptions(command))
+            {
+                builder.AppendLine("  if [[ \"$prev\" == \"" + option + "\" && \"$sub\" == \"" + top + "\" ]]; then");
+                builder.AppendLine("    COMPREPLY=( $(compgen -W \"" + values + "\" -- \"$cur\") ); return 0");
+                builder.AppendLine("  fi");
+            }
+        }
+
+        builder.AppendLine("  if [[ \"$cur\" == -* ]]; then");
+        builder.AppendLine("    local options=\"" + string.Join(" ", OptionNames(root)) + "\"");
+        builder.AppendLine("    case \"$sub\" in");
+        foreach (var command in VisibleSubcommands(root))
+        {
+            var options = OptionNames(command);
+            if (options.Count > 0)
+            {
+                builder.AppendLine("      " + command.Name + ") options=\"$options " + string.Join(" ", options) + "\" ;;");
+            }
+        }
+
+        builder.AppendLine("    esac");
+        builder.AppendLine("    COMPREPLY=( $(compgen -W \"$options\" -- \"$cur\") ); return 0");
+        builder.AppendLine("  fi");
+        builder.AppendLine();
+        builder.AppendLine("  if [[ -z \"$sub\" ]]; then");
+        builder.AppendLine("    COMPREPLY=( $(compgen -W \"" + string.Join(" ", VisibleSubcommands(root).Select(x => x.Name)) + "\" -- \"$cur\") ); return 0");
+        builder.AppendLine("  fi");
+        builder.AppendLine();
+        builder.AppendLine("  case \"$sub\" in");
+        foreach (var command in VisibleSubcommands(root))
+        {
+            var children = VisibleSubcommands(command).Select(x => x.Name).ToList();
+            if (children.Count > 0)
+            {
+                builder.AppendLine("    " + command.Name + ") COMPREPLY=( $(compgen -W \"" + string.Join(" ", children) + "\" -- \"$cur\") ) ;;");
+            }
+        }
+
+        builder.AppendLine("  esac");
+        builder.AppendLine("}");
+        builder.AppendLine("complete -F _" + id + "_completions " + toolName);
+        return builder.ToString();
+    }
+
+    private static string GenerateZsh(Command root, string toolName)
+    {
+        var id = Identifier(toolName);
+        var builder = new StringBuilder();
+        builder.AppendLine("#compdef " + toolName);
+        builder.AppendLine("# " + toolName + " completion (zsh), generated from the CLI command tree.");
+        builder.AppendLine("_" + id + "() {");
+        builder.AppendLine("  local -a commands");
+        builder.Append("  commands=(");
+        foreach (var command in VisibleSubcommands(root))
+        {
+            builder.Append(" '" + command.Name + ":" + Describe(command.Description) + "'");
+        }
+
+        builder.AppendLine(" )");
+        builder.AppendLine("  if (( CURRENT == 2 )); then");
+        builder.AppendLine("    _describe -t commands 'command' commands && return");
+        builder.AppendLine("  fi");
+        builder.AppendLine("  local sub=\"${words[2]}\"");
+        builder.AppendLine("  local previous=\"${words[CURRENT-1]}\"");
+
+        foreach (var (top, command) in Walk(root, string.Empty))
+        {
+            foreach (var (option, values) in EnumOptions(command))
+            {
+                builder.AppendLine("  if [[ \"$previous\" == \"" + option + "\" && \"$sub\" == \"" + top + "\" ]]; then");
+                builder.AppendLine("    _values '" + option.TrimStart('-') + "' " + values + " && return");
+                builder.AppendLine("  fi");
+            }
+        }
+
+        builder.AppendLine("  case \"$sub\" in");
+        foreach (var command in VisibleSubcommands(root))
+        {
+            var words = VisibleSubcommands(command).Select(x => x.Name)
+                .Concat(OptionNames(command))
+                .Concat(OptionNames(root))
+                .Distinct(StringComparer.Ordinal)
+                .Select(x => "'" + x + "'");
+            builder.AppendLine("    " + command.Name + ") _values 'argument' " + string.Join(" ", words) + " ;;");
+        }
+
+        builder.AppendLine("  esac");
+        builder.AppendLine("}");
+        builder.AppendLine("_" + id + " \"$@\"");
+        return builder.ToString();
+    }
+
+    private static string GenerateFish(Command root, string toolName)
+    {
+        var builder = new StringBuilder();
+        builder.AppendLine("# " + toolName + " completion (fish), generated from the CLI command tree.");
+        builder.AppendLine("complete -c " + toolName + " -f");
+        foreach (var command in VisibleSubcommands(root))
+        {
+            builder.AppendLine("complete -c " + toolName + " -n '__fish_use_subcommand' -a '" + command.Name + "' -d '" + Describe(command.Description) + "'");
+            foreach (var child in VisibleSubcommands(command))
+            {
+                builder.AppendLine("complete -c " + toolName + " -n '__fish_seen_subcommand_from " + command.Name + "' -a '" + child.Name + "'");
+            }
+
+            foreach (var option in OptionNames(command).Where(x => x.StartsWith("--", StringComparison.Ordinal)))
+            {
+                builder.AppendLine("complete -c " + toolName + " -n '__fish_seen_subcommand_from " + command.Name + "' -l '" + option.Substring(2) + "'");
+            }
+        }
+
+        foreach (var option in OptionNames(root).Where(x => x.StartsWith("--", StringComparison.Ordinal)))
+        {
+            builder.AppendLine("complete -c " + toolName + " -l '" + option.Substring(2) + "'");
+        }
+
+        foreach (var (top, command) in Walk(root, string.Empty))
+        {
+            foreach (var (option, values) in EnumOptions(command))
+            {
+                builder.AppendLine("complete -c " + toolName + " -n '__fish_seen_subcommand_from " + top + "' -l " + option.TrimStart('-') + " -x -a '" + values + "'");
+            }
+        }
+
+        return builder.ToString();
+    }
+
+    private static string Describe(string? description) =>
+        (description ?? string.Empty).Replace("'", " ", StringComparison.Ordinal).Replace("\n", " ", StringComparison.Ordinal);
+
+    private static string Identifier(string toolName) =>
+        new([.. toolName.Select(c => char.IsLetterOrDigit(c) ? c : '_')]);
+}
\ No newline at end of file
diff --git a/src/cli/ModernMT.CLI/Program.cs b/src/cli/ModernMT.CLI/Program.cs
index db50078..2c591cd 100644
--- a/src/cli/ModernMT.CLI/Program.cs
+++ b/src/cli/ModernMT.CLI/Program.cs
@@ -12,5 +12,6 @@
 rootCommand.Options.Add(CliOptions.OutputDirectory);
 rootCommand.Subcommands.Add(AuthCommand.Create());
 rootCommand.Subcommands.Add(DefaultApiGroupCommand.Create());
+rootCommand.Subcommands.Add(CliCompletion.CreateCommand(rootCommand, @"modern-mt"));
 
 return await rootCommand.Parse(args).InvokeAsync().ConfigureAwait(false);
\ No newline at end of file
diff --git a/src/cli/ModernMT.CLI/README.md b/src/cli/ModernMT.CLI/README.md
index e9a5706..1061a83 100644
--- a/src/cli/ModernMT.CLI/README.md
+++ b/src/cli/ModernMT.CLI/README.md
@@ -15,6 +15,18 @@ modern-mt --help
 modern-mt default --help
 ```
 
+## Shell completion
+
+The `completion` command prints a script built from the live command tree, so
+subcommands, option names and the values of enum-typed options stay correct after the
+spec is refreshed and the CLI regenerated — there is no word list to maintain.
+
+```bash
+modern-mt completion bash > /usr/local/etc/bash_completion.d/modern-mt
+modern-mt completion zsh  > _modern-mt   # then put it on your zsh fpath
+modern-mt completion fish > ~/.config/fish/completions/modern-mt.fish
+```
+
 ## Customization
 
 Generated operation, tag, and API group command classes are partial. Implement