diff --git a/cmd/md-to-html/main.go b/cmd/md-to-html/main.go
index ace3c45..c5ae54b 100644
--- a/cmd/md-to-html/main.go
+++ b/cmd/md-to-html/main.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -9,6 +10,7 @@ import (
 	"os/signal"
 	"syscall"
 
+	internalcli "github.com/fserg/md-to-html/internal/cli"
 	"github.com/fserg/md-to-html/internal/converter"
 	"github.com/fserg/md-to-html/internal/server"
 	"github.com/fserg/md-to-html/internal/version"
@@ -85,19 +87,15 @@ func runServe(args []string, stdout, stderr io.Writer) int {
 }
 
 func runCLI(args []string, stdout, stderr io.Writer) int {
-	fs := flag.NewFlagSet("cli", flag.ContinueOnError)
-	fs.SetOutput(io.Discard)
-	if err := fs.Parse(args); err != nil {
+	err := internalcli.Run(context.Background(), args, os.Stdin, stdout, stderr)
+	if err == nil {
+		return 0
+	}
+	if errors.Is(err, internalcli.ErrUsage) {
 		return 2
 	}
-
-	if fs.NArg() != 1 {
-		fmt.Fprintln(stderr, "usage: md-to-html cli <file.md>")
-		return 2
-	}
-
-	fmt.Fprintf(stdout, "cli not implemented yet: %s\n", fs.Arg(0))
-	return 0
+	fmt.Fprintln(stderr, err)
+	return 1
 }
 
 func runVersion(args []string, stdout, stderr io.Writer) int {
@@ -119,12 +117,12 @@ func runVersion(args []string, stdout, stderr io.Writer) int {
 func printUsage(w io.Writer) {
 	fmt.Fprint(w, `Usage:
   md-to-html serve
-  md-to-html cli <file.md>
+  md-to-html cli [--stdin|-|<file.md>] [--output path] [--title str]
   md-to-html version
 
 Commands:
   serve    Start the HTTP server
-  cli      Convert a Markdown file stub
+  cli      Convert Markdown from a file or stdin
   version  Print the build version
 `)
 }
diff --git a/internal/cli/cli.go b/internal/cli/cli.go
new file mode 100644
index 0000000..56236a4
--- /dev/null
+++ b/internal/cli/cli.go
@@ -0,0 +1,171 @@
+package cli
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/fserg/md-to-html/internal/converter"
+	webtemplate "github.com/fserg/md-to-html/web/template"
+)
+
+var ErrUsage = errors.New("cli usage error")
+
+func Run(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
+	if wantsHelp(args) {
+		printUsage(stdout)
+		return nil
+	}
+
+	normalized, err := normalizeArgs(args)
+	if err != nil {
+		printUsage(stderr)
+		return fmt.Errorf("%w: %v", ErrUsage, err)
+	}
+
+	fs := flag.NewFlagSet("cli", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+
+	var (
+		output   string
+		title    string
+		useStdin bool
+	)
+
+	fs.StringVar(&output, "output", "", "output file path")
+	fs.StringVar(&output, "o", "", "output file path")
+	fs.StringVar(&title, "title", "", "fallback title if markdown has no headings")
+	fs.BoolVar(&useStdin, "stdin", false, "read markdown from stdin")
+
+	if err := fs.Parse(normalized); err != nil {
+		printUsage(stderr)
+		return fmt.Errorf("%w: %v", ErrUsage, err)
+	}
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+
+	positional := fs.Args()
+	if len(positional) > 1 {
+		printUsage(stderr)
+		return fmt.Errorf("%w: expected a single input file or '-'", ErrUsage)
+	}
+
+	conv, err := converter.New(webtemplate.FS)
+	if err != nil {
+		return fmt.Errorf("init converter: %w", err)
+	}
+
+	var (
+		markdown      []byte
+		fallbackTitle = title
+		outputPath    = output
+		writeToStdout bool
+	)
+
+	switch {
+	case useStdin || (len(positional) == 1 && positional[0] == "-"):
+		markdown, err = io.ReadAll(stdin)
+		if err != nil {
+			return fmt.Errorf("read stdin: %w", err)
+		}
+		if fallbackTitle == "" {
+			fallbackTitle = "Document"
+		}
+		writeToStdout = outputPath == ""
+	case len(positional) == 1:
+		inputPath := positional[0]
+		markdown, err = os.ReadFile(inputPath)
+		if err != nil {
+			return fmt.Errorf("read %s: %w", inputPath, err)
+		}
+		if fallbackTitle == "" {
+			fallbackTitle = strings.TrimSuffix(filepath.Base(inputPath), filepath.Ext(inputPath))
+		}
+		if outputPath == "" {
+			outputPath = strings.TrimSuffix(inputPath, filepath.Ext(inputPath)) + ".html"
+		}
+	default:
+		printUsage(stderr)
+		return fmt.Errorf("%w: no input specified", ErrUsage)
+	}
+
+	result, err := conv.Convert(markdown, fallbackTitle)
+	if err != nil {
+		return fmt.Errorf("convert markdown: %w", err)
+	}
+
+	if writeToStdout {
+		_, err = stdout.Write(result.HTML)
+		if err != nil {
+			return fmt.Errorf("write stdout: %w", err)
+		}
+		return nil
+	}
+
+	if err := os.WriteFile(outputPath, result.HTML, 0o644); err != nil {
+		return fmt.Errorf("write %s: %w", outputPath, err)
+	}
+
+	return nil
+}
+
+func normalizeArgs(args []string) ([]string, error) {
+	flags := make([]string, 0, len(args))
+	positionals := make([]string, 0, 1)
+
+	for i := 0; i < len(args); i++ {
+		arg := args[i]
+		switch {
+		case arg == "--":
+			positionals = append(positionals, args[i+1:]...)
+			return append(flags, positionals...), nil
+		case arg == "-":
+			positionals = append(positionals, arg)
+		case !strings.HasPrefix(arg, "-"):
+			positionals = append(positionals, arg)
+		case strings.HasPrefix(arg, "--output="), strings.HasPrefix(arg, "--title="), strings.HasPrefix(arg, "-o="):
+			flags = append(flags, arg)
+		case arg == "--output" || arg == "-o" || arg == "--title":
+			if i+1 >= len(args) {
+				return nil, fmt.Errorf("flag needs an argument: %s", arg)
+			}
+			flags = append(flags, arg, args[i+1])
+			i++
+		default:
+			flags = append(flags, arg)
+		}
+	}
+
+	return append(flags, positionals...), nil
+}
+
+func wantsHelp(args []string) bool {
+	for _, arg := range args {
+		switch arg {
+		case "-h", "--help", "-help":
+			return true
+		}
+	}
+	return false
+}
+
+func printUsage(w io.Writer) {
+	fmt.Fprint(w, `Usage: md-to-html cli [--stdin|-|<file.md>] [--output path] [--title str]
+
+Options:
+  --stdin        Read markdown from stdin
+  -o, --output   Output file path (default: stdout for stdin, <input>.html for file)
+  --title        Fallback title if markdown has no headings
+  -h, --help     Show this help
+`)
+}
diff --git a/internal/cli/cli_test.go b/internal/cli/cli_test.go
new file mode 100644
index 0000000..4b01cf6
--- /dev/null
+++ b/internal/cli/cli_test.go
@@ -0,0 +1,124 @@
+package cli
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestCLIFileToFile(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	inputPath := filepath.Join(dir, "example.md")
+	if err := os.WriteFile(inputPath, []byte("# Hello\n\nBody"), 0o644); err != nil {
+		t.Fatalf("write input: %v", err)
+	}
+
+	var stdout, stderr bytes.Buffer
+	if err := Run(context.Background(), []string{inputPath}, strings.NewReader(""), &stdout, &stderr); err != nil {
+		t.Fatalf("run: %v", err)
+	}
+
+	outputPath := filepath.Join(dir, "example.html")
+	got, err := os.ReadFile(outputPath)
+	if err != nil {
+		t.Fatalf("read output: %v", err)
+	}
+	if !bytes.Contains(got, []byte("<!DOCTYPE html>")) {
+		t.Fatalf("output missing doctype: %s", got)
+	}
+	if stdout.Len() != 0 {
+		t.Fatalf("stdout = %q, want empty", stdout.String())
+	}
+	if stderr.Len() != 0 {
+		t.Fatalf("stderr = %q, want empty", stderr.String())
+	}
+}
+
+func TestCLIStdin(t *testing.T) {
+	t.Parallel()
+
+	stdin := strings.NewReader("# Привет\n\nТекст")
+	var stdout, stderr bytes.Buffer
+
+	if err := Run(context.Background(), []string{"--stdin"}, stdin, &stdout, &stderr); err != nil {
+		t.Fatalf("run: %v", err)
+	}
+
+	if !strings.Contains(stdout.String(), "<!DOCTYPE html>") {
+		t.Fatalf("stdout missing doctype: %s", stdout.String())
+	}
+	if stderr.Len() != 0 {
+		t.Fatalf("stderr = %q, want empty", stderr.String())
+	}
+}
+
+func TestCLIOutputFlag(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	inputPath := filepath.Join(dir, "example.md")
+	outputPath := filepath.Join(dir, "custom.html")
+	if err := os.WriteFile(inputPath, []byte("Plain text"), 0o644); err != nil {
+		t.Fatalf("write input: %v", err)
+	}
+
+	var stdout, stderr bytes.Buffer
+	if err := Run(context.Background(), []string{inputPath, "-o", outputPath}, strings.NewReader(""), &stdout, &stderr); err != nil {
+		t.Fatalf("run: %v", err)
+	}
+
+	if _, err := os.Stat(outputPath); err != nil {
+		t.Fatalf("stat output: %v", err)
+	}
+}
+
+func TestCLITitle(t *testing.T) {
+	t.Parallel()
+
+	var stdout, stderr bytes.Buffer
+	if err := Run(context.Background(), []string{"--stdin", "--title", "Custom"}, strings.NewReader(""), &stdout, &stderr); err != nil {
+		t.Fatalf("run: %v", err)
+	}
+
+	if !strings.Contains(stdout.String(), "<title>Custom</title>") {
+		t.Fatalf("stdout missing title: %s", stdout.String())
+	}
+}
+
+func TestCLINoInput(t *testing.T) {
+	t.Parallel()
+
+	var stdout, stderr bytes.Buffer
+	err := Run(context.Background(), nil, strings.NewReader(""), &stdout, &stderr)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !errors.Is(err, ErrUsage) {
+		t.Fatalf("error = %v, want ErrUsage", err)
+	}
+	if !strings.Contains(stderr.String(), "Usage: md-to-html cli") {
+		t.Fatalf("stderr missing usage: %s", stderr.String())
+	}
+}
+
+func TestCLIMissingFile(t *testing.T) {
+	t.Parallel()
+
+	var stdout, stderr bytes.Buffer
+	err := Run(context.Background(), []string{"missing.md"}, strings.NewReader(""), &stdout, &stderr)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if errors.Is(err, ErrUsage) {
+		t.Fatalf("error = %v, did not want ErrUsage", err)
+	}
+	if stdout.Len() != 0 {
+		t.Fatalf("stdout = %q, want empty", stdout.String())
+	}
+}