diff --git a/client.go b/client.go
index f4825ff..96f65c3 100644
--- a/client.go
+++ b/client.go
@@ -12,7 +12,7 @@ import (
 // Client is a mosh client. It connects to a mosh server over UDP,
 // handles the SSP transport, and provides send/recv for terminal I/O.
 type Client struct {
-	conn      *net.UDPConn
+	conn      Conn
 	transport *Transport
 	ocb       *OCB
 
@@ -32,8 +32,8 @@ type Client struct {
 	wg   sync.WaitGroup
 }
 
-// Dial connects to a mosh server. The key is the base64-encoded mosh key
-// (with or without padding).
+// Dial connects to a mosh server over UDP. The key is the base64-encoded
+// mosh key (with or without padding).
 func Dial(host string, port int, key string) (*Client, error) {
 	// Pad key for base64 if needed.
 	for len(key)%4 != 0 {
@@ -57,6 +57,12 @@ func Dial(host string, port int, key string) (*Client, error) {
 		return nil, err
 	}
 
+	return DialConn(conn, ocb)
+}
+
+// DialConn creates a mosh client over an existing datagram connection.
+// Use this with WebTransport or other non-UDP transports.
+func DialConn(conn Conn, ocb *OCB) (*Client, error) {
 	c := &Client{
 		conn:             conn,
 		transport:        NewTransport(ocb, false),
diff --git a/cmd/mosh-wasm/main.go b/cmd/mosh-wasm/main.go
new file mode 100644
index 0000000..0bb0bb7
--- /dev/null
+++ b/cmd/mosh-wasm/main.go
@@ -0,0 +1,143 @@
+//go:build js && wasm
+
+package main
+
+import (
+	"encoding/base64"
+	"fmt"
+	"sync"
+	"syscall/js"
+	"time"
+
+	mosh "github.com/unixshells/mosh-go"
+)
+
+func main() {
+	js.Global().Set("moshConnect", js.FuncOf(moshConnect))
+	select {} // keep alive
+}
+
+// moshConnect(url, key) → Promise<MoshSession>
+// url: WebTransport URL (e.g., "https://relay.example.com/mosh/user/device")
+// key: base64-encoded mosh key
+func moshConnect(this js.Value, args []js.Value) interface{} {
+	if len(args) < 2 {
+		return reject("moshConnect requires (url, key)")
+	}
+
+	url := args[0].String()
+	key := args[1].String()
+
+	handler := js.FuncOf(func(this js.Value, pargs []js.Value) interface{} {
+		resolve := pargs[0]
+		rejectFn := pargs[1]
+
+		go func() {
+			session, err := newSession(url, key)
+			if err != nil {
+				rejectFn.Invoke(err.Error())
+				return
+			}
+			resolve.Invoke(session.jsObject())
+		}()
+
+		return nil
+	})
+
+	return js.Global().Get("Promise").New(handler)
+}
+
+type session struct {
+	client *mosh.Client
+	conn   *wtConn
+	mu     sync.Mutex
+	closed bool
+}
+
+func newSession(url, key string) (*session, error) {
+	for len(key)%4 != 0 {
+		key += "="
+	}
+	rawKey, err := base64.StdEncoding.DecodeString(key)
+	if err != nil {
+		return nil, fmt.Errorf("bad key: %w", err)
+	}
+	ocb, err := mosh.NewOCB(rawKey)
+	if err != nil {
+		return nil, err
+	}
+
+	conn, err := dialWebTransport(url)
+	if err != nil {
+		return nil, err
+	}
+
+	client, err := mosh.DialConn(conn, ocb)
+	if err != nil {
+		conn.Close()
+		return nil, err
+	}
+
+	return &session{client: client, conn: conn}, nil
+}
+
+func (s *session) jsObject() js.Value {
+	obj := js.Global().Get("Object").New()
+
+	obj.Set("send", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		if len(args) < 1 {
+			return nil
+		}
+		data := args[0].String()
+		s.client.Send([]byte(data))
+		return nil
+	}))
+
+	obj.Set("resize", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		if len(args) < 2 {
+			return nil
+		}
+		cols := args[0].Int()
+		rows := args[1].Int()
+		s.client.Resize(uint16(cols), uint16(rows))
+		return nil
+	}))
+
+	// recv(callback) — calls callback(string) whenever output is available
+	obj.Set("recv", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		if len(args) < 1 {
+			return nil
+		}
+		cb := args[0]
+		go func() {
+			for {
+				out := s.client.Recv(60 * time.Second)
+				if out == nil {
+					s.mu.Lock()
+					closed := s.closed
+					s.mu.Unlock()
+					if closed {
+						return
+					}
+					continue
+				}
+				cb.Invoke(string(out))
+			}
+		}()
+		return nil
+	}))
+
+	obj.Set("close", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		s.mu.Lock()
+		s.closed = true
+		s.mu.Unlock()
+		s.client.Close()
+		return nil
+	}))
+
+	return obj
+}
+
+func reject(msg string) js.Value {
+	return js.Global().Get("Promise").Call("reject", msg)
+}
diff --git a/cmd/mosh-wasm/webtransport.go b/cmd/mosh-wasm/webtransport.go
new file mode 100644
index 0000000..f36ea1f
--- /dev/null
+++ b/cmd/mosh-wasm/webtransport.go
@@ -0,0 +1,159 @@
+//go:build js && wasm
+
+package main
+
+import (
+	"errors"
+	"sync"
+	"syscall/js"
+	"time"
+)
+
+// wtConn wraps a browser WebTransport connection as a mosh.Conn.
+// Uses WebTransport datagrams for unreliable, unordered delivery —
+// matching UDP semantics for the mosh protocol.
+type wtConn struct {
+	transport js.Value // WebTransport instance
+	reader    js.Value // datagrams.readable reader
+	writer    js.Value // datagrams.writable writer
+
+	incoming chan []byte
+	done     chan struct{}
+	once     sync.Once
+
+	mu       sync.Mutex
+	deadline time.Time
+}
+
+func dialWebTransport(url string) (*wtConn, error) {
+	wt := js.Global().Get("WebTransport").New(url)
+
+	// Wait for .ready promise
+	readyCh := make(chan error, 1)
+	wt.Get("ready").Call("then",
+		js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+			readyCh <- nil
+			return nil
+		}),
+		js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+			msg := "WebTransport connection failed"
+			if len(args) > 0 {
+				msg = args[0].Get("message").String()
+			}
+			readyCh <- errors.New(msg)
+			return nil
+		}),
+	)
+
+	select {
+	case err := <-readyCh:
+		if err != nil {
+			return nil, err
+		}
+	case <-time.After(10 * time.Second):
+		wt.Call("close")
+		return nil, errors.New("WebTransport connect timeout")
+	}
+
+	datagrams := wt.Get("datagrams")
+	reader := datagrams.Get("readable").Call("getReader")
+	writer := datagrams.Get("writable").Call("getWriter")
+
+	c := &wtConn{
+		transport: wt,
+		reader:    reader,
+		writer:    writer,
+		incoming:  make(chan []byte, 256),
+		done:      make(chan struct{}),
+	}
+
+	// Start reading datagrams in background
+	go c.readLoop()
+
+	return c, nil
+}
+
+func (c *wtConn) readLoop() {
+	for {
+		ch := make(chan js.Value, 1)
+		c.reader.Call("read").Call("then",
+			js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+				ch <- args[0]
+				return nil
+			}),
+			js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+				close(ch)
+				return nil
+			}),
+		)
+
+		select {
+		case <-c.done:
+			return
+		case result, ok := <-ch:
+			if !ok {
+				return
+			}
+			if result.Get("done").Bool() {
+				return
+			}
+			value := result.Get("value")
+			buf := make([]byte, value.Get("byteLength").Int())
+			js.CopyBytesToGo(buf, js.Global().Get("Uint8Array").New(value))
+			select {
+			case c.incoming <- buf:
+			default:
+				// drop if channel full
+			}
+		}
+	}
+}
+
+func (c *wtConn) Read(b []byte) (int, error) {
+	c.mu.Lock()
+	deadline := c.deadline
+	c.mu.Unlock()
+
+	var timer <-chan time.Time
+	if !deadline.IsZero() {
+		d := time.Until(deadline)
+		if d <= 0 {
+			return 0, errors.New("i/o timeout")
+		}
+		timer = time.After(d)
+	}
+
+	select {
+	case <-c.done:
+		return 0, errors.New("connection closed")
+	case data := <-c.incoming:
+		n := copy(b, data)
+		return n, nil
+	case <-timer:
+		return 0, errors.New("i/o timeout")
+	}
+}
+
+func (c *wtConn) Write(b []byte) (int, error) {
+	arr := js.Global().Get("Uint8Array").New(len(b))
+	js.CopyBytesToJS(arr, b)
+	c.writer.Call("write", arr)
+	return len(b), nil
+}
+
+func (c *wtConn) SetReadDeadline(t time.Time) error {
+	c.mu.Lock()
+	c.deadline = t
+	c.mu.Unlock()
+	return nil
+}
+
+func (c *wtConn) Close() error {
+	c.once.Do(func() {
+		close(c.done)
+		c.reader.Call("cancel")
+		c.writer.Call("close")
+		c.transport.Call("close")
+	})
+	return nil
+}
diff --git a/cmd/remote-test/main.go b/cmd/remote-test/main.go
new file mode 100644
index 0000000..c670208
--- /dev/null
+++ b/cmd/remote-test/main.go
@@ -0,0 +1,133 @@
+package main
+
+import (
+	"encoding/base64"
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+	"time"
+	"unicode/utf8"
+
+	mosh "github.com/unixshells/mosh-go"
+)
+
+func main() {
+	if len(os.Args) != 4 {
+		fmt.Fprintf(os.Stderr, "Usage: %s <host> <port> <base64key>\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	host := os.Args[1]
+	port, _ := strconv.Atoi(os.Args[2])
+	key, err := base64.StdEncoding.DecodeString(os.Args[3])
+	if err != nil {
+		// Try raw base64 without padding.
+		key, err = base64.RawStdEncoding.DecodeString(os.Args[2+1])
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "bad key: %v\n", err)
+			os.Exit(1)
+		}
+	}
+
+	ocb, err := mosh.NewOCB(key)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "ocb: %v\n", err)
+		os.Exit(1)
+	}
+
+	transport := mosh.NewTransport(ocb, false) // client mode
+
+	// Resolve host.
+	addrs, err := net.ResolveUDPAddr("udp", fmt.Sprintf("%s:%d", host, port))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "resolve: %v\n", err)
+		os.Exit(1)
+	}
+
+	conn, err := net.DialUDP("udp", nil, addrs)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "dial: %v\n", err)
+		os.Exit(1)
+	}
+	defer conn.Close()
+
+	flush := func() {
+		datagrams := transport.Tick()
+		for _, dg := range datagrams {
+			conn.Write(dg)
+		}
+	}
+
+	// Read incoming in background.
+	go func() {
+		buf := make([]byte, 65536)
+		for {
+			n, err := conn.Read(buf)
+			if err != nil {
+				return
+			}
+			diff := transport.Recv(buf[:n])
+			if diff == nil || len(diff) == 0 {
+				continue
+			}
+
+			instrs, err := mosh.UnmarshalHostMessage(diff)
+			if err != nil {
+				fmt.Printf("unmarshal error: %v\n", err)
+				continue
+			}
+			for _, hi := range instrs {
+				if len(hi.Hoststring) > 0 {
+					text := string(hi.Hoststring)
+					if !utf8.ValidString(text) {
+						text = fmt.Sprintf("(%d raw bytes)", len(hi.Hoststring))
+					}
+					fmt.Printf("=== hoststring (%d bytes) ===\n%s\n=== end ===\n", len(hi.Hoststring), text)
+				}
+				if hi.Width > 0 || hi.Height > 0 {
+					fmt.Printf("=== resize: %dx%d ===\n", hi.Width, hi.Height)
+				}
+			}
+		}
+	}()
+
+	// Tick timer.
+	ticker := time.NewTicker(20 * time.Millisecond)
+	go func() {
+		for range ticker.C {
+			flush()
+		}
+	}()
+
+	// Initial keepalive.
+	fmt.Println("Sending initial keepalive...")
+	transport.ForceNextSend()
+	flush()
+
+	// Resize.
+	fmt.Println("Sending resize 80x24...")
+	resize := mosh.MarshalUserMessage([]mosh.UserInstruction{
+		{Width: 80, Height: 24},
+	})
+	transport.SetPending(resize)
+	flush()
+
+	// Wait for initial output.
+	fmt.Println("Waiting 5 seconds for initial output...")
+	time.Sleep(5 * time.Second)
+
+	// Send ls + Enter.
+	fmt.Println("\nSending ls + Enter...")
+	keystroke := mosh.MarshalUserMessage([]mosh.UserInstruction{
+		{Keys: []byte("ls\r")},
+	})
+	transport.SetPending(keystroke)
+	flush()
+
+	// Wait for response.
+	time.Sleep(3 * time.Second)
+
+	fmt.Println("\nDone.")
+	ticker.Stop()
+}
diff --git a/conn.go b/conn.go
new file mode 100644
index 0000000..8612558
--- /dev/null
+++ b/conn.go
@@ -0,0 +1,36 @@
+package mosh
+
+import "time"
+
+// Conn is a datagram-oriented connection used by the mosh transport.
+// UDP, WebTransport, and other datagram transports implement this interface.
+type Conn interface {
+	// Read reads a datagram. Returns the number of bytes read.
+	// Must respect deadlines set by SetReadDeadline.
+	Read(b []byte) (int, error)
+
+	// Write writes a datagram.
+	Write(b []byte) (int, error)
+
+	// SetReadDeadline sets the deadline for Read calls.
+	SetReadDeadline(t time.Time) error
+
+	// Close closes the connection.
+	Close() error
+}
+
+const (
+	// Direction bits in the 64-bit nonce header.
+	dirToServer = uint64(0)          // client → server
+	dirToClient = uint64(1) << 63    // server → client
+	seqMask     = ^(uint64(1) << 63) // lower 63 bits
+
+	// Minimum wire datagram: 8 (nonce) + 16 (tag) = 24 bytes.
+	minDatagram = 24
+
+	// Maximum payload per UDP datagram.
+	maxPayload = 16384
+
+	// Tick interval matches mosh's SEND_MINDELAY (8ms) for keystroke batching.
+	tickInterval = 8 * time.Millisecond
+)
diff --git a/framebuffer.go b/framebuffer.go
index cb27341..e37688b 100644
--- a/framebuffer.go
+++ b/framebuffer.go
@@ -1,3 +1,5 @@
+//go:build !js
+
 package mosh
 
 import (
diff --git a/remote-test b/remote-test
new file mode 100755
index 0000000..8249965
Binary files /dev/null and b/remote-test differ
diff --git a/server.go b/server.go
index 5a09d54..1c9c8b6 100644
--- a/server.go
+++ b/server.go
@@ -1,3 +1,5 @@
+//go:build !js
+
 package mosh
 
 import (
@@ -18,20 +20,6 @@ import (
 )
 
 const (
-	// Direction bits in the 64-bit nonce header.
-	dirToServer = uint64(0)          // client → server
-	dirToClient = uint64(1) << 63    // server → client
-	seqMask     = ^(uint64(1) << 63) // lower 63 bits
-
-	// Minimum wire datagram: 8 (nonce) + 16 (tag) = 24 bytes.
-	minDatagram = 24
-
-	// Maximum payload per UDP datagram.
-	maxPayload = 16384
-
-	// Tick interval matches mosh's SEND_MINDELAY (8ms) for keystroke batching.
-	tickInterval = 8 * time.Millisecond
-
 	// Server gives up waiting for client after this.
 	associationTimeout = 60 * time.Second
 
diff --git a/server_test.go b/server_test.go
index 8667ce3..daf7119 100644
--- a/server_test.go
+++ b/server_test.go
@@ -1,3 +1,5 @@
+//go:build !js
+
 package mosh
 
 import (