diff --git a/command.go b/command.go
index 8b99e26..2a089ba 100644
--- a/command.go
+++ b/command.go
@@ -59,6 +59,12 @@ type Command struct {
 	Middleware  MiddlewareFunc
 	Handler     HandlerFunc
 	HelpHandler HandlerFunc
+	// CompletionHandler is called when the command is run in completion
+	// mode. If nil, only the default completion handler is used.
+	//
+	// Flag and option parsing is best-effort in this mode, so even if an Option
+	// is "required" it may not be set.
+	CompletionHandler CompletionHandlerFunc
 }
 
 // AddSubcommands adds the given subcommands, setting their
@@ -193,15 +199,22 @@ type Invocation struct {
 	ctx         context.Context
 	Command     *Command
 	parsedFlags *pflag.FlagSet
-	Args        []string
+
+	// Args is reduced into the remaining arguments after parsing flags
+	// during Run.
+	Args []string
+
 	// Environ is a list of environment variables. Use EnvsWithPrefix to parse
 	// os.Environ.
 	Environ Environ
 	Stdout  io.Writer
 	Stderr  io.Writer
 	Stdin   io.Reader
-	Logger  slog.Logger
-	Net     Net
+
+	// Deprecated
+	Logger slog.Logger
+	// Deprecated
+	Net Net
 
 	// testing
 	signalNotifyContext func(parent context.Context, signals ...os.Signal) (ctx context.Context, stop context.CancelFunc)
@@ -282,6 +295,17 @@ func copyFlagSetWithout(fs *pflag.FlagSet, without string) *pflag.FlagSet {
 	return fs2
 }
 
+func (inv *Invocation) CurWords() (prev string, cur string) {
+	if len(inv.Args) == 1 {
+		cur = inv.Args[0]
+		prev = ""
+	} else {
+		cur = inv.Args[len(inv.Args)-1]
+		prev = inv.Args[len(inv.Args)-2]
+	}
+	return
+}
+
 // run recursively executes the command and its children.
 // allArgs is wired through the stack so that global flags can be accepted
 // anywhere in the command invocation.
@@ -378,8 +402,19 @@ func (inv *Invocation) run(state *runState) error {
 		}
 	}
 
+	// Outputted completions are not filtered based on the word under the cursor, as every shell we support does this already.
+	// We only look at the current word to figure out handler to run, or what directory to inspect.
+	if inv.IsCompletionMode() {
+		for _, e := range inv.complete() {
+			fmt.Fprintln(inv.Stdout, e)
+		}
+		return nil
+	}
+
+	ignoreFlagParseErrors := inv.Command.RawArgs
+
 	// Flag parse errors are irrelevant for raw args commands.
-	if !inv.Command.RawArgs && state.flagParseErr != nil && !errors.Is(state.flagParseErr, pflag.ErrHelp) {
+	if !ignoreFlagParseErrors && state.flagParseErr != nil && !errors.Is(state.flagParseErr, pflag.ErrHelp) {
 		return xerrors.Errorf(
 			"parsing flags (%v) for %q: %w",
 			state.allArgs,
@@ -396,7 +431,7 @@ func (inv *Invocation) run(state *runState) error {
 		}
 	}
 	// Don't error for missing flags if `--help` was supplied.
-	if len(missing) > 0 && !errors.Is(state.flagParseErr, pflag.ErrHelp) {
+	if len(missing) > 0 && !inv.IsCompletionMode() && !errors.Is(state.flagParseErr, pflag.ErrHelp) {
 		return xerrors.Errorf("Missing values for the required flags: %s", strings.Join(missing, ", "))
 	}
 
@@ -553,6 +588,65 @@ func (inv *Invocation) with(fn func(*Invocation)) *Invocation {
 	return &i2
 }
 
+func (inv *Invocation) complete() []string {
+	prev, cur := inv.CurWords()
+
+	// If the current word is a flag
+	if strings.HasPrefix(cur, "--") {
+		flagParts := strings.Split(cur, "=")
+		flagName := flagParts[0][2:]
+		// If it's an equals flag
+		if len(flagParts) == 2 {
+			if out := inv.completeFlag(flagName); out != nil {
+				for i, o := range out {
+					out[i] = fmt.Sprintf("--%s=%s", flagName, o)
+				}
+				return out
+			}
+		} else if out := inv.Command.Options.ByFlag(flagName); out != nil {
+			// If the current word is a valid flag, auto-complete it so the
+			// shell moves the cursor
+			return []string{cur}
+		}
+	}
+	// If the previous word is a flag, then we're writing it's value
+	// and we should check it's handler
+	if strings.HasPrefix(prev, "--") {
+		word := prev[2:]
+		if out := inv.completeFlag(word); out != nil {
+			return out
+		}
+	}
+	// If the current word is the command, move the shell cursor
+	if inv.Command.Name() == cur {
+		return []string{inv.Command.Name()}
+	}
+	var completions []string
+
+	if inv.Command.CompletionHandler != nil {
+		completions = append(completions, inv.Command.CompletionHandler(inv)...)
+	}
+
+	completions = append(completions, DefaultCompletionHandler(inv)...)
+
+	return completions
+}
+
+func (inv *Invocation) completeFlag(word string) []string {
+	opt := inv.Command.Options.ByFlag(word)
+	if opt == nil {
+		return nil
+	}
+	if opt.CompletionHandler != nil {
+		return opt.CompletionHandler(inv)
+	}
+	val, ok := opt.Value.(*Enum)
+	if ok {
+		return val.Choices
+	}
+	return nil
+}
+
 // MiddlewareFunc returns the next handler in the chain,
 // or nil if there are no more.
 type MiddlewareFunc func(next HandlerFunc) HandlerFunc
@@ -637,3 +731,5 @@ func RequireRangeArgs(start, end int) MiddlewareFunc {
 
 // HandlerFunc handles an Invocation of a command.
 type HandlerFunc func(i *Invocation) error
+
+type CompletionHandlerFunc func(i *Invocation) []string
diff --git a/command_test.go b/command_test.go
index f6a20a2..de6c12d 100644
--- a/command_test.go
+++ b/command_test.go
@@ -12,6 +12,7 @@ import (
 	"golang.org/x/xerrors"
 
 	serpent "github.com/coder/serpent"
+	"github.com/coder/serpent/completion"
 )
 
 // ioBufs is the standard input, output, and error for a command.
@@ -30,100 +31,147 @@ func fakeIO(i *serpent.Invocation) *ioBufs {
 	return &b
 }
 
-func TestCommand(t *testing.T) {
-	t.Parallel()
-
-	cmd := func() *serpent.Command {
-		var (
-			verbose bool
-			lower   bool
-			prefix  string
-			reqBool bool
-			reqStr  string
-		)
-		return &serpent.Command{
-			Use: "root [subcommand]",
-			Options: serpent.OptionSet{
-				serpent.Option{
-					Name:  "verbose",
-					Flag:  "verbose",
-					Value: serpent.BoolOf(&verbose),
-				},
-				serpent.Option{
-					Name:  "prefix",
-					Flag:  "prefix",
-					Value: serpent.StringOf(&prefix),
-				},
+func sampleCommand(t *testing.T) *serpent.Command {
+	t.Helper()
+	var (
+		verbose bool
+		lower   bool
+		prefix  string
+		reqBool bool
+		reqStr  string
+		reqArr  []string
+		fileArr []string
+		enumStr string
+	)
+	enumChoices := []string{"foo", "bar", "qux"}
+	return &serpent.Command{
+		Use: "root [subcommand]",
+		Options: serpent.OptionSet{
+			serpent.Option{
+				Name:  "verbose",
+				Flag:  "verbose",
+				Value: serpent.BoolOf(&verbose),
 			},
-			Children: []*serpent.Command{
-				{
-					Use:   "required-flag --req-bool=true --req-string=foo",
-					Short: "Example with required flags",
-					Options: serpent.OptionSet{
-						serpent.Option{
-							Name:     "req-bool",
-							Flag:     "req-bool",
-							Value:    serpent.BoolOf(&reqBool),
-							Required: true,
-						},
-						serpent.Option{
-							Name: "req-string",
-							Flag: "req-string",
-							Value: serpent.Validate(serpent.StringOf(&reqStr), func(value *serpent.String) error {
-								ok := strings.Contains(value.String(), " ")
-								if !ok {
-									return xerrors.Errorf("string must contain a space")
-								}
-								return nil
-							}),
-							Required: true,
-						},
+			serpent.Option{
+				Name:  "prefix",
+				Flag:  "prefix",
+				Value: serpent.StringOf(&prefix),
+			},
+		},
+		Children: []*serpent.Command{
+			{
+				Use:   "required-flag --req-bool=true --req-string=foo",
+				Short: "Example with required flags",
+				Options: serpent.OptionSet{
+					serpent.Option{
+						Name:          "req-bool",
+						Flag:          "req-bool",
+						FlagShorthand: "b",
+						Value:         serpent.BoolOf(&reqBool),
+						Required:      true,
 					},
-					HelpHandler: func(i *serpent.Invocation) error {
-						_, _ = i.Stdout.Write([]byte("help text.png"))
-						return nil
+					serpent.Option{
+						Name:          "req-string",
+						Flag:          "req-string",
+						FlagShorthand: "s",
+						Value: serpent.Validate(serpent.StringOf(&reqStr), func(value *serpent.String) error {
+							ok := strings.Contains(value.String(), " ")
+							if !ok {
+								return xerrors.Errorf("string must contain a space")
+							}
+							return nil
+						}),
+						Required: true,
 					},
-					Handler: func(i *serpent.Invocation) error {
-						_, _ = i.Stdout.Write([]byte(fmt.Sprintf("%s-%t", reqStr, reqBool)))
-						return nil
+					serpent.Option{
+						Name:  "req-enum",
+						Flag:  "req-enum",
+						Value: serpent.EnumOf(&enumStr, enumChoices...),
+					},
+					serpent.Option{
+						Name:          "req-array",
+						Flag:          "req-array",
+						FlagShorthand: "a",
+						Value:         serpent.StringArrayOf(&reqArr),
 					},
 				},
-				{
-					Use:   "toupper [word]",
-					Short: "Converts a word to upper case",
-					Middleware: serpent.Chain(
-						serpent.RequireNArgs(1),
-					),
-					Aliases: []string{"up"},
-					Options: serpent.OptionSet{
-						serpent.Option{
-							Name:  "lower",
-							Flag:  "lower",
-							Value: serpent.BoolOf(&lower),
-						},
+				HelpHandler: func(i *serpent.Invocation) error {
+					_, _ = i.Stdout.Write([]byte("help text.png"))
+					return nil
+				},
+				Handler: func(i *serpent.Invocation) error {
+					_, _ = i.Stdout.Write([]byte(fmt.Sprintf("%s-%t", reqStr, reqBool)))
+					return nil
+				},
+			},
+			{
+				Use:   "toupper [word]",
+				Short: "Converts a word to upper case",
+				Middleware: serpent.Chain(
+					serpent.RequireNArgs(1),
+				),
+				Aliases: []string{"up"},
+				Options: serpent.OptionSet{
+					serpent.Option{
+						Name:  "lower",
+						Flag:  "lower",
+						Value: serpent.BoolOf(&lower),
 					},
-					Handler: func(i *serpent.Invocation) error {
-						_, _ = i.Stdout.Write([]byte(prefix))
-						w := i.Args[0]
-						if lower {
-							w = strings.ToLower(w)
-						} else {
-							w = strings.ToUpper(w)
-						}
-						_, _ = i.Stdout.Write(
-							[]byte(
-								w,
-							),
-						)
-						if verbose {
-							_, _ = i.Stdout.Write([]byte("!!!"))
-						}
-						return nil
+				},
+				Handler: func(i *serpent.Invocation) error {
+					_, _ = i.Stdout.Write([]byte(prefix))
+					w := i.Args[0]
+					if lower {
+						w = strings.ToLower(w)
+					} else {
+						w = strings.ToUpper(w)
+					}
+					_, _ = i.Stdout.Write(
+						[]byte(
+							w,
+						),
+					)
+					if verbose {
+						_, _ = i.Stdout.Write([]byte("!!!"))
+					}
+					return nil
+				},
+			},
+			{
+				Use: "file <file>",
+				Handler: func(inv *serpent.Invocation) error {
+					return nil
+				},
+				CompletionHandler: completion.FileHandler(func(info os.FileInfo) bool {
+					return true
+				}),
+				Middleware: serpent.RequireNArgs(1),
+			},
+			{
+				Use: "altfile",
+				Handler: func(inv *serpent.Invocation) error {
+					return nil
+				},
+				Options: serpent.OptionSet{
+					{
+						Name:        "extra",
+						Flag:        "extra",
+						Description: "Extra files.",
+						Value:       serpent.StringArrayOf(&fileArr),
 					},
 				},
+				CompletionHandler: func(i *serpent.Invocation) []string {
+					return []string{"doesntexist.go"}
+				},
 			},
-		}
+		},
 	}
+}
+
+func TestCommand(t *testing.T) {
+	t.Parallel()
+
+	cmd := func() *serpent.Command { return sampleCommand(t) }
 
 	t.Run("SimpleOK", func(t *testing.T) {
 		t.Parallel()
diff --git a/completion.go b/completion.go
new file mode 100644
index 0000000..a0fb779
--- /dev/null
+++ b/completion.go
@@ -0,0 +1,26 @@
+package serpent
+
+// CompletionModeEnv is a special environment variable that is
+// set when the command is being run in completion mode.
+const CompletionModeEnv = "COMPLETION_MODE"
+
+// IsCompletionMode returns true if the command is being run in completion mode.
+func (inv *Invocation) IsCompletionMode() bool {
+	_, ok := inv.Environ.Lookup(CompletionModeEnv)
+	return ok
+}
+
+// DefaultCompletionHandler returns a handler that prints all
+// known flags and subcommands that haven't already been set to valid values.
+func DefaultCompletionHandler(inv *Invocation) []string {
+	var allResps []string
+	for _, cmd := range inv.Command.Children {
+		allResps = append(allResps, cmd.Name())
+	}
+	for _, opt := range inv.Command.Options {
+		if opt.ValueSource == ValueSourceNone || opt.ValueSource == ValueSourceDefault || opt.Value.Type() == "string-array" {
+			allResps = append(allResps, "--"+opt.Flag)
+		}
+	}
+	return allResps
+}
diff --git a/completion/README.md b/completion/README.md
new file mode 100644
index 0000000..d7021ed
--- /dev/null
+++ b/completion/README.md
@@ -0,0 +1,11 @@
+# completion
+
+The `completion` package extends `serpent` to allow applications to generate rich auto-completions.
+
+
+## Protocol
+
+The completion scripts call out to the serpent command to generate
+completions. The convention is to pass the exact args and flags (or
+cmdline) of the in-progress command with a `COMPLETION_MODE=1` environment variable. That environment variable lets the command know to generate completions instead of running the command.
+By default, completions will be generated based on available flags and subcommands. Additional completions can be added by supplying a `CompletionHandlerFunc` on an Option or Command.
\ No newline at end of file
diff --git a/completion/all.go b/completion/all.go
new file mode 100644
index 0000000..b20c254
--- /dev/null
+++ b/completion/all.go
@@ -0,0 +1,94 @@
+package completion
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"os/user"
+	"path/filepath"
+	"strings"
+	"text/template"
+
+	"github.com/coder/serpent"
+)
+
+const (
+	BashShell  string = "bash"
+	FishShell  string = "fish"
+	ZShell     string = "zsh"
+	Powershell string = "powershell"
+)
+
+var shellCompletionByName = map[string]func(io.Writer, string) error{
+	BashShell:  generateCompletion(bashCompletionTemplate),
+	FishShell:  generateCompletion(fishCompletionTemplate),
+	ZShell:     generateCompletion(zshCompletionTemplate),
+	Powershell: generateCompletion(pshCompletionTemplate),
+}
+
+func ShellOptions(choice *string) *serpent.Enum {
+	return serpent.EnumOf(choice, BashShell, FishShell, ZShell, Powershell)
+}
+
+func WriteCompletion(writer io.Writer, shell string, cmdName string) error {
+	fn, ok := shellCompletionByName[shell]
+	if !ok {
+		return fmt.Errorf("unknown shell %q", shell)
+	}
+	fn(writer, cmdName)
+	return nil
+}
+
+func DetectUserShell() (string, error) {
+	// Attempt to get the SHELL environment variable first
+	if shell := os.Getenv("SHELL"); shell != "" {
+		return filepath.Base(shell), nil
+	}
+
+	// Fallback: Look up the current user and parse /etc/passwd
+	currentUser, err := user.Current()
+	if err != nil {
+		return "", err
+	}
+
+	// Open and parse /etc/passwd
+	passwdFile, err := os.ReadFile("/etc/passwd")
+	if err != nil {
+		return "", err
+	}
+
+	lines := strings.Split(string(passwdFile), "\n")
+	for _, line := range lines {
+		if strings.HasPrefix(line, currentUser.Username+":") {
+			parts := strings.Split(line, ":")
+			if len(parts) > 6 {
+				return filepath.Base(parts[6]), nil // The shell is typically the 7th field
+			}
+		}
+	}
+
+	return "", fmt.Errorf("default shell not found")
+}
+
+func generateCompletion(
+	scriptTemplate string,
+) func(io.Writer, string) error {
+	return func(w io.Writer, rootCmdName string) error {
+		tmpl, err := template.New("script").Parse(scriptTemplate)
+		if err != nil {
+			return fmt.Errorf("parse template: %w", err)
+		}
+
+		err = tmpl.Execute(
+			w,
+			map[string]string{
+				"Name": rootCmdName,
+			},
+		)
+		if err != nil {
+			return fmt.Errorf("execute template: %w", err)
+		}
+
+		return nil
+	}
+}
diff --git a/completion/bash.go b/completion/bash.go
new file mode 100644
index 0000000..fad4069
--- /dev/null
+++ b/completion/bash.go
@@ -0,0 +1,22 @@
+package completion
+
+const bashCompletionTemplate = `
+_generate_{{.Name}}_completions() {
+    # Capture the line excluding the command, and everything after the current word
+    local args=("${COMP_WORDS[@]:1:COMP_CWORD}")
+
+    # Set COMPLETION_MODE and call the command with the arguments, capturing the output
+    local completions=$(COMPLETION_MODE=1 "{{.Name}}" "${args[@]}")
+
+    # Use the command's output to generate completions for the current word
+    COMPREPLY=($(compgen -W "$completions" -- "${COMP_WORDS[COMP_CWORD]}"))
+
+    # Ensure no files are shown, even if there are no matches
+    if [ ${#COMPREPLY[@]} -eq 0 ]; then
+        COMPREPLY=()
+    fi
+}
+
+# Setup Bash to use the function for completions for '{{.Name}}'
+complete -F _generate_{{.Name}}_completions {{.Name}}
+`
diff --git a/completion/fish.go b/completion/fish.go
new file mode 100644
index 0000000..f9b2793
--- /dev/null
+++ b/completion/fish.go
@@ -0,0 +1,13 @@
+package completion
+
+const fishCompletionTemplate = `
+function _{{.Name}}_completions
+	# Capture the full command line as an array
+	set -l args (commandline -opc)
+	set -l current (commandline -ct)
+    COMPLETION_MODE=1 $args $current
+end
+
+# Setup Fish to use the function for completions for '{{.Name}}'
+complete -c {{.Name}} -f -a '(_{{.Name}}_completions)'
+`
diff --git a/completion/handlers.go b/completion/handlers.go
new file mode 100644
index 0000000..848bb06
--- /dev/null
+++ b/completion/handlers.go
@@ -0,0 +1,55 @@
+package completion
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/coder/serpent"
+)
+
+// FileHandler returns a handler that completes file names, using the
+// given filter func, which may be nil.
+func FileHandler(filter func(info os.FileInfo) bool) serpent.CompletionHandlerFunc {
+	return func(inv *serpent.Invocation) []string {
+		var out []string
+		_, word := inv.CurWords()
+
+		dir, _ := filepath.Split(word)
+		if dir == "" {
+			dir = "."
+		}
+		f, err := os.Open(dir)
+		if err != nil {
+			return out
+		}
+		defer f.Close()
+		if dir == "." {
+			dir = ""
+		}
+
+		infos, err := f.Readdir(0)
+		if err != nil {
+			return out
+		}
+
+		for _, info := range infos {
+			if filter != nil && !filter(info) {
+				continue
+			}
+
+			var cur string
+			if info.IsDir() {
+				cur = fmt.Sprintf("%s%s%c", dir, info.Name(), os.PathSeparator)
+			} else {
+				cur = fmt.Sprintf("%s%s", dir, info.Name())
+			}
+
+			if strings.HasPrefix(cur, word) {
+				out = append(out, cur)
+			}
+		}
+		return out
+	}
+}
diff --git a/completion/powershell.go b/completion/powershell.go
new file mode 100644
index 0000000..e30c61b
--- /dev/null
+++ b/completion/powershell.go
@@ -0,0 +1,42 @@
+package completion
+
+const pshCompletionTemplate = `
+
+# Escaping output sourced from:
+# https://github.com/spf13/cobra/blob/e94f6d0dd9a5e5738dca6bce03c4b1207ffbc0ec/powershell_completions.go#L47
+filter _{{.Name}}_escapeStringWithSpecialChars {
+` + "    $_ -replace '\\s|#|@|\\$|;|,|''|\\{|\\}|\\(|\\)|\"|`|\\||<|>|&','`$&'" + `
+}
+
+$_{{.Name}}_completions = {
+    param(
+        $wordToComplete,
+        $commandAst,
+        $cursorPosition
+    )
+    # Legacy space handling sourced from:
+	# https://github.com/spf13/cobra/blob/e94f6d0dd9a5e5738dca6bce03c4b1207ffbc0ec/powershell_completions.go#L107
+	if ($PSVersionTable.PsVersion -lt [version]'7.2.0' -or
+        ($PSVersionTable.PsVersion -lt [version]'7.3.0' -and -not [ExperimentalFeature]::IsEnabled("PSNativeCommandArgumentPassing")) -or
+        (($PSVersionTable.PsVersion -ge [version]'7.3.0' -or [ExperimentalFeature]::IsEnabled("PSNativeCommandArgumentPassing")) -and
+         $PSNativeCommandArgumentPassing -eq 'Legacy')) {
+        $Space =` + "' `\"`\"'" + `
+    } else {
+        $Space = ' ""'
+    }
+	$Command = $commandAst.ToString().Substring(0, $cursorPosition - 1)
+	if ($wordToComplete -ne "" ) {
+        $wordToComplete = $Command.Split(" ")[-1]
+    } else {
+        $Command = $Command + $Space
+    }
+    # Get completions by calling the command with the COMPLETION_MODE environment variable set to 1
+    $env:COMPLETION_MODE = 1
+    Invoke-Expression $Command | Where-Object { $_ -like "$wordToComplete*" } | ForEach-Object {
+        "$_" | _{{.Name}}_escapeStringWithSpecialChars
+    }
+    rm env:COMPLETION_MODE
+}
+
+Register-ArgumentCompleter -CommandName {{.Name}} -ScriptBlock $_{{.Name}}_completions
+`
diff --git a/completion/zsh.go b/completion/zsh.go
new file mode 100644
index 0000000..a8ee4a8
--- /dev/null
+++ b/completion/zsh.go
@@ -0,0 +1,12 @@
+package completion
+
+const zshCompletionTemplate = `
+_{{.Name}}_completions() {
+	local -a args completions
+	args=("${words[@]:1:$#words}")
+	completions=($(COMPLETION_MODE=1 "{{.Name}}" "${args[@]}"))
+	compadd -a completions
+}
+
+compdef _{{.Name}}_completions {{.Name}}
+`
diff --git a/completion_test.go b/completion_test.go
new file mode 100644
index 0000000..5ca160a
--- /dev/null
+++ b/completion_test.go
@@ -0,0 +1,203 @@
+package serpent_test
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"testing"
+
+	serpent "github.com/coder/serpent"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCompletion(t *testing.T) {
+	t.Parallel()
+
+	cmd := func() *serpent.Command { return sampleCommand(t) }
+
+	t.Run("SubcommandList", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "altfile\nfile\nrequired-flag\ntoupper\n--prefix\n--verbose\n", io.Stdout.String())
+	})
+
+	t.Run("SubcommandNoPartial", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("f")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "altfile\nfile\nrequired-flag\ntoupper\n--prefix\n--verbose\n", io.Stdout.String())
+	})
+
+	t.Run("SubcommandComplete", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("required-flag")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "required-flag\n", io.Stdout.String())
+	})
+
+	t.Run("ListFlags", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("required-flag", "")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "--req-array\n--req-bool\n--req-enum\n--req-string\n", io.Stdout.String())
+	})
+
+	t.Run("ListFlagsAfterArg", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("altfile", "")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "doesntexist.go\n--extra\n", io.Stdout.String())
+	})
+
+	t.Run("FlagExhaustive", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("required-flag", "--req-bool", "--req-string", "foo bar", "--req-array", "asdf", "--req-array", "qwerty")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "--req-array\n--req-enum\n", io.Stdout.String())
+	})
+
+	t.Run("FlagShorthand", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("required-flag", "-b", "-s", "foo bar", "-a", "asdf")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "--req-array\n--req-enum\n", io.Stdout.String())
+	})
+
+	t.Run("NoOptDefValueFlag", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("--verbose", "")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "altfile\nfile\nrequired-flag\ntoupper\n--prefix\n", io.Stdout.String())
+	})
+
+	t.Run("EnumOK", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("required-flag", "--req-enum", "")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "foo\nbar\nqux\n", io.Stdout.String())
+	})
+
+	t.Run("EnumEqualsOK", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("required-flag", "--req-enum", "--req-enum=")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "--req-enum=foo\n--req-enum=bar\n--req-enum=qux\n", io.Stdout.String())
+	})
+
+	t.Run("EnumEqualsBeginQuotesOK", func(t *testing.T) {
+		t.Parallel()
+		i := cmd().Invoke("required-flag", "--req-enum", "--req-enum=\"")
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "--req-enum=foo\n--req-enum=bar\n--req-enum=qux\n", io.Stdout.String())
+	})
+
+}
+
+func TestFileCompletion(t *testing.T) {
+	t.Parallel()
+
+	cmd := func() *serpent.Command { return sampleCommand(t) }
+
+	t.Run("DirOK", func(t *testing.T) {
+		t.Parallel()
+		tempDir := t.TempDir()
+		i := cmd().Invoke("file", tempDir)
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, fmt.Sprintf("%s%c\n", tempDir, os.PathSeparator), io.Stdout.String())
+	})
+
+	t.Run("EmptyDirOK", func(t *testing.T) {
+		t.Parallel()
+		tempDir := t.TempDir() + string(os.PathSeparator)
+		i := cmd().Invoke("file", tempDir)
+		i.Environ.Set(serpent.CompletionModeEnv, "1")
+		io := fakeIO(i)
+		err := i.Run()
+		require.NoError(t, err)
+		require.Equal(t, "", io.Stdout.String())
+	})
+
+	cases := []struct {
+		name     string
+		realPath string
+		paths    []string
+	}{
+		{
+			name:     "CurDirOK",
+			realPath: ".",
+			paths:    []string{"", "./", "././"},
+		},
+		{
+			name:     "PrevDirOK",
+			realPath: "..",
+			paths:    []string{"../", ".././"},
+		},
+		{
+			name:     "RootOK",
+			realPath: "/",
+			paths:    []string{"/", "/././"},
+		},
+	}
+	for _, tc := range cases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			for _, path := range tc.paths {
+				i := cmd().Invoke("file", path)
+				i.Environ.Set(serpent.CompletionModeEnv, "1")
+				io := fakeIO(i)
+				err := i.Run()
+				require.NoError(t, err)
+				output := strings.Split(io.Stdout.String(), "\n")
+				output = output[:len(output)-1]
+				for _, str := range output {
+					if strings.HasSuffix(str, string(os.PathSeparator)) {
+						require.DirExists(t, str)
+					} else {
+						require.FileExists(t, str)
+					}
+				}
+				files, err := os.ReadDir(tc.realPath)
+				require.NoError(t, err)
+				require.Equal(t, len(files), len(output))
+			}
+		})
+	}
+}
diff --git a/example/completetest/main.go b/example/completetest/main.go
new file mode 100644
index 0000000..920e705
--- /dev/null
+++ b/example/completetest/main.go
@@ -0,0 +1,126 @@
+package main
+
+import (
+	"os"
+	"strings"
+
+	"github.com/coder/serpent"
+	"github.com/coder/serpent/completion"
+)
+
+// installCommand returns a serpent command that helps
+// a user configure their shell to use serpent's completion.
+func installCommand() *serpent.Command {
+	defaultShell, err := completion.DetectUserShell()
+	if err != nil {
+		defaultShell = "bash"
+	}
+
+	var shell string
+	return &serpent.Command{
+		Use:   "completion",
+		Short: "Generate completion scripts for the given shell.",
+		Handler: func(inv *serpent.Invocation) error {
+			completion.WriteCompletion(inv.Stdout, shell, inv.Command.Parent.Name())
+			return nil
+		},
+		Options: serpent.OptionSet{
+			{
+				Flag:          "shell",
+				FlagShorthand: "s",
+				Default:       defaultShell,
+				Description:   "The shell to generate a completion script for.",
+				Value:         completion.ShellOptions(&shell),
+			},
+		},
+	}
+}
+
+func main() {
+	var (
+		print    bool
+		upper    bool
+		fileType string
+		fileArr  []string
+	)
+	cmd := serpent.Command{
+		Use:   "completetest <text>",
+		Short: "Prints the given text to the console.",
+		Options: serpent.OptionSet{
+			{
+				Name:        "different",
+				Value:       serpent.BoolOf(&upper),
+				Flag:        "different",
+				Description: "Do the command differently.",
+			},
+		},
+		Handler: func(inv *serpent.Invocation) error {
+			if len(inv.Args) == 0 {
+				inv.Stderr.Write([]byte("error: missing text\n"))
+				os.Exit(1)
+			}
+
+			text := inv.Args[0]
+			if upper {
+				text = strings.ToUpper(text)
+			}
+
+			inv.Stdout.Write([]byte(text))
+			return nil
+		},
+		Children: []*serpent.Command{
+			{
+				Use:   "sub",
+				Short: "A subcommand",
+				Handler: func(inv *serpent.Invocation) error {
+					inv.Stdout.Write([]byte("subcommand"))
+					return nil
+				},
+				Options: serpent.OptionSet{
+					{
+						Name:        "upper",
+						Value:       serpent.BoolOf(&upper),
+						Flag:        "upper",
+						Description: "Prints the text in upper case.",
+					},
+				},
+			},
+			{
+				Use: "file <file>",
+				Handler: func(inv *serpent.Invocation) error {
+					return nil
+				},
+				Options: serpent.OptionSet{
+					{
+						Name:        "print",
+						Value:       serpent.BoolOf(&print),
+						Flag:        "print",
+						Description: "Print the file.",
+					},
+					{
+						Name:        "type",
+						Value:       serpent.EnumOf(&fileType, "binary", "text"),
+						Flag:        "type",
+						Description: "The type of file.",
+					},
+					{
+						Name:        "extra",
+						Flag:        "extra",
+						Description: "Extra files.",
+						Value:       serpent.StringArrayOf(&fileArr),
+					},
+				},
+				CompletionHandler: completion.FileHandler(nil),
+				Middleware:        serpent.RequireNArgs(1),
+			},
+			installCommand(),
+		},
+	}
+
+	inv := cmd.Invoke().WithOS()
+
+	err := inv.Run()
+	if err != nil {
+		panic(err)
+	}
+}
diff --git a/option.go b/option.go
index 5545d07..fccc67e 100644
--- a/option.go
+++ b/option.go
@@ -65,6 +65,8 @@ type Option struct {
 	Hidden bool `json:"hidden,omitempty"`
 
 	ValueSource ValueSource `json:"value_source,omitempty"`
+
+	CompletionHandler CompletionHandlerFunc `json:"-"`
 }
 
 // optionNoMethods is just a wrapper around Option so we can defer to the
@@ -335,10 +337,22 @@ func (optSet *OptionSet) SetDefaults() error {
 
 // ByName returns the Option with the given name, or nil if no such option
 // exists.
-func (optSet *OptionSet) ByName(name string) *Option {
-	for i := range *optSet {
-		opt := &(*optSet)[i]
-		if opt.Name == name {
+func (optSet OptionSet) ByName(name string) *Option {
+	for i := range optSet {
+		if optSet[i].Name == name {
+			return &optSet[i]
+		}
+	}
+	return nil
+}
+
+func (optSet OptionSet) ByFlag(flag string) *Option {
+	if flag == "" {
+		return nil
+	}
+	for i := range optSet {
+		opt := &optSet[i]
+		if opt.Flag == flag || opt.FlagShorthand == flag {
 			return opt
 		}
 	}