diff --git a/cmd/load/main.go b/cmd/load/main.go
index 17a589d..78269d1 100644
--- a/cmd/load/main.go
+++ b/cmd/load/main.go
@@ -7,9 +7,9 @@ import (
 	"path/filepath"
 	"time"
 
+	"github.com/theater/picomap/lib/client"
 	"github.com/theater/picomap/lib/picoserial"
 	"github.com/theater/picomap/lib/picotool"
-	"github.com/theater/picomap/lib/wire"
 )
 
 func main() {
@@ -40,27 +40,18 @@ func run(buildDir string) error {
 		return err
 	}
 	if dev != "" {
-		fmt.Printf("Sending bootsel request to %s...\n", dev)
-		req, err := wire.EncodeMessage(&wire.RequestBOOTSEL{})
-		if err != nil {
-			return fmt.Errorf("encoding request: %w", err)
-		}
-		resp, err := picoserial.SendAndReceive(dev, req, 2*time.Second)
+		fmt.Printf("Sending BOOTSEL request to %s...\n", dev)
+		t, err := picoserial.Open(dev)
 		if err != nil {
 			return err
 		}
-		if len(resp) > 0 {
-			msg, err := wire.DecodeMessage(resp)
-			if err != nil {
-				fmt.Fprintf(os.Stderr, "warning: failed to decode response: %v\n", err)
-			} else {
-				switch msg.(type) {
-				case *wire.ResponseBOOTSEL:
-					fmt.Println("Device confirmed reboot into BOOTSEL mode.")
-				default:
-					fmt.Printf("Unexpected response type: %T\n", msg)
-				}
-			}
+		c := client.New(t, 2*time.Second)
+		err = c.BOOTSEL()
+		c.Close()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "warning: BOOTSEL request failed: %v\n", err)
+		} else {
+			fmt.Println("Device confirmed reboot into BOOTSEL mode.")
 		}
 		time.Sleep(2 * time.Second)
 	}
diff --git a/include/device.h b/include/device.h
new file mode 100644
index 0000000..5c31059
--- /dev/null
+++ b/include/device.h
@@ -0,0 +1,51 @@
+#pragma once
+#include <cstdint>
+#include <vector>
+#include "msgpackpp.h"
+#include "halfsiphash.h"
+#include "static_vector.h"
+#include "protocol.h"
+
+static constexpr uint8_t hash_key[8] = {};
+
+struct DecodedMessage {
+    uint32_t message_id;
+    int8_t type_id;
+};
+
+inline std::vector<uint8_t> pack_envelope(uint32_t message_id, const std::vector<uint8_t> &payload) {
+    uint32_t checksum = halfsiphash::hash32(payload.data(), payload.size(), hash_key);
+    msgpackpp::packer p;
+    p.pack(Envelope{message_id, checksum, payload});
+    return p.get_payload();
+}
+
+template <typename T>
+inline std::vector<uint8_t> encode_response(uint32_t message_id, const T &msg) {
+    msgpackpp::packer inner;
+    inner.pack(msg);
+    return pack_envelope(message_id, inner.get_payload());
+}
+
+inline msgpackpp::result<DecodedMessage> try_decode(const uint8_t *data, size_t len) {
+    msgpackpp::parser p(data, static_cast<int>(len));
+
+    Envelope env;
+    auto r = msgpackpp::unpack(p, env);
+    if (!r) return std::unexpected(r.error());
+
+    uint32_t expected = halfsiphash::hash32(env.payload.data(), env.payload.size(), hash_key);
+    if (env.checksum != expected) return std::unexpected(msgpackpp::error_code::invalid);
+
+    msgpackpp::parser inner(env.payload.data(), static_cast<int>(env.payload.size()));
+    if (!inner.is_ext()) return std::unexpected(msgpackpp::error_code::type_error);
+    auto ext = inner.get_ext();
+    if (!ext) return std::unexpected(ext.error());
+
+    return DecodedMessage{env.message_id, std::get<0>(*ext)};
+}
+
+template <size_t N>
+inline msgpackpp::result<DecodedMessage> try_decode(const static_vector<uint8_t, N> &buf) {
+    return try_decode(buf.data(), buf.size());
+}
diff --git a/include/msgpackpp.h b/include/msgpackpp.h
index 6c960f0..78fa97f 100644
--- a/include/msgpackpp.h
+++ b/include/msgpackpp.h
@@ -718,6 +718,13 @@ inline result<parser> unpack(const parser &p, std::string_view &out) {
   return p.next();
 }
 
+inline result<parser> unpack(const parser &p, std::string &out) {
+  auto v = p.get_string();
+  if (!v) return std::unexpected(v.error());
+  out = std::string(v->data(), v->size());
+  return p.next();
+}
+
 inline result<parser> unpack(const parser &p, std::vector<uint8_t> &out) {
   auto v = p.get_binary_view();
   if (!v) return std::unexpected(v.error());
diff --git a/include/protocol.h b/include/protocol.h
new file mode 100644
index 0000000..5cf4d19
--- /dev/null
+++ b/include/protocol.h
@@ -0,0 +1,34 @@
+#pragma once
+#include <cstdint>
+#include <string>
+#include <tuple>
+#include <vector>
+
+struct ResponseBOOTSEL {
+    static constexpr int8_t ext_id = 1;
+    auto as_tuple() const { return std::tie(); }
+    auto as_tuple() { return std::tie(); }
+};
+
+struct RequestBOOTSEL {
+    static constexpr int8_t ext_id = 2;
+    auto as_tuple() const { return std::tie(); }
+    auto as_tuple() { return std::tie(); }
+};
+
+struct DeviceError {
+    static constexpr int8_t ext_id = 3;
+    uint32_t code;
+    std::string message;
+    auto as_tuple() const { return std::tie(code, message); }
+    auto as_tuple() { return std::tie(code, message); }
+};
+
+struct Envelope {
+    static constexpr int8_t ext_id = 0;
+    uint32_t message_id;
+    uint32_t checksum;
+    std::vector<uint8_t> payload;
+    auto as_tuple() const { return std::tie(message_id, checksum, payload); }
+    auto as_tuple() { return std::tie(message_id, checksum, payload); }
+};
diff --git a/lib/client/client.go b/lib/client/client.go
new file mode 100644
index 0000000..9b7cb6b
--- /dev/null
+++ b/lib/client/client.go
@@ -0,0 +1,90 @@
+package client
+
+import (
+	"fmt"
+	"sync/atomic"
+	"time"
+
+	"github.com/theater/picomap/lib/halfsiphash"
+	"github.com/theater/picomap/lib/msgpack"
+	"github.com/theater/picomap/lib/transport"
+)
+
+var HashKey = [8]byte{}
+
+type Client struct {
+	transport transport.Transport
+	timeout   time.Duration
+	nextID    atomic.Uint32
+}
+
+func New(t transport.Transport, timeout time.Duration) *Client {
+	return &Client{transport: t, timeout: timeout}
+}
+
+func (c *Client) Close() error {
+	return c.transport.Close()
+}
+
+func (c *Client) send(msg any) (uint32, error) {
+	id := c.nextID.Add(1)
+	payload, err := msgpack.Marshal(msg)
+	if err != nil {
+		return 0, fmt.Errorf("encode inner: %w", err)
+	}
+	checksum := halfsiphash.Sum32(payload, HashKey)
+	env := Envelope{MessageID: id, Checksum: checksum, Payload: payload}
+	data, err := msgpack.Marshal(&env)
+	if err != nil {
+		return 0, fmt.Errorf("encode envelope: %w", err)
+	}
+	return id, c.transport.Send(data)
+}
+
+func (c *Client) receive(expectedID uint32) (any, error) {
+	data, err := c.transport.Receive(c.timeout)
+	if err != nil {
+		return nil, fmt.Errorf("receive: %w", err)
+	}
+	if len(data) == 0 {
+		return nil, fmt.Errorf("no response")
+	}
+	var env Envelope
+	if err := msgpack.Unmarshal(data, &env); err != nil {
+		return nil, fmt.Errorf("decode envelope: %w", err)
+	}
+	if env.MessageID != expectedID {
+		return nil, fmt.Errorf("message id mismatch: got %d, want %d", env.MessageID, expectedID)
+	}
+	expected := halfsiphash.Sum32(env.Payload, HashKey)
+	if env.Checksum != expected {
+		return nil, fmt.Errorf("checksum mismatch: got %08x, want %08x", env.Checksum, expected)
+	}
+	var inner any
+	if err := msgpack.Unmarshal(env.Payload, &inner); err != nil {
+		return nil, fmt.Errorf("decode inner: %w", err)
+	}
+	if devErr, ok := inner.(*DeviceError); ok {
+		return nil, devErr
+	}
+	return inner, nil
+}
+
+func (c *Client) roundTrip(req any) (any, error) {
+	id, err := c.send(req)
+	if err != nil {
+		return nil, err
+	}
+	return c.receive(id)
+}
+
+func (c *Client) BOOTSEL() error {
+	resp, err := c.roundTrip(&RequestBOOTSEL{})
+	if err != nil {
+		return err
+	}
+	if _, ok := resp.(*ResponseBOOTSEL); !ok {
+		return fmt.Errorf("unexpected response: %T", resp)
+	}
+	return nil
+}
diff --git a/lib/client/types.go b/lib/client/types.go
new file mode 100644
index 0000000..5a7b347
--- /dev/null
+++ b/lib/client/types.go
@@ -0,0 +1,28 @@
+package client
+
+import "github.com/theater/picomap/lib/msgpack"
+
+type ResponseBOOTSEL struct{}
+type RequestBOOTSEL struct{}
+
+type DeviceError struct {
+	Code    uint32
+	Message string
+}
+
+func (e *DeviceError) Error() string {
+	return e.Message
+}
+
+type Envelope struct {
+	MessageID uint32
+	Checksum  uint32
+	Payload   []byte
+}
+
+func init() {
+	msgpack.RegisterExt(0, (*Envelope)(nil))
+	msgpack.RegisterExt(1, (*ResponseBOOTSEL)(nil))
+	msgpack.RegisterExt(2, (*RequestBOOTSEL)(nil))
+	msgpack.RegisterExt(3, (*DeviceError)(nil))
+}
diff --git a/lib/picoserial/picoserial.go b/lib/picoserial/picoserial.go
index de8ec27..e1b401e 100644
--- a/lib/picoserial/picoserial.go
+++ b/lib/picoserial/picoserial.go
@@ -21,29 +21,29 @@ func FindDevice() (string, error) {
 	return "", nil
 }
 
-func Open(portName string) (serial.Port, error) {
-	return serial.Open(portName, &serial.Mode{BaudRate: 115200})
+type SerialTransport struct {
+	port serial.Port
 }
 
-// SendAndReceive sends data and reads the response with a timeout.
-func SendAndReceive(portName string, data []byte, timeout time.Duration) ([]byte, error) {
-	port, err := Open(portName)
+func Open(portName string) (*SerialTransport, error) {
+	port, err := serial.Open(portName, &serial.Mode{BaudRate: 115200})
 	if err != nil {
 		return nil, fmt.Errorf("opening %s: %w", portName, err)
 	}
-	defer port.Close()
+	return &SerialTransport{port: port}, nil
+}
 
-	port.SetReadTimeout(timeout)
-
-	_, err = port.Write(data)
-	if err != nil {
-		return nil, fmt.Errorf("writing to %s: %w", portName, err)
-	}
+func (t *SerialTransport) Send(data []byte) error {
+	_, err := t.port.Write(data)
+	return err
+}
 
+func (t *SerialTransport) Receive(timeout time.Duration) ([]byte, error) {
+	t.port.SetReadTimeout(timeout)
 	var resp []byte
 	buf := make([]byte, 256)
 	for {
-		n, err := port.Read(buf)
+		n, err := t.port.Read(buf)
 		if n > 0 {
 			resp = append(resp, buf[:n]...)
 		}
@@ -53,3 +53,7 @@ func SendAndReceive(portName string, data []byte, timeout time.Duration) ([]byte
 	}
 	return resp, nil
 }
+
+func (t *SerialTransport) Close() error {
+	return t.port.Close()
+}
diff --git a/lib/transport/transport.go b/lib/transport/transport.go
new file mode 100644
index 0000000..664fc23
--- /dev/null
+++ b/lib/transport/transport.go
@@ -0,0 +1,9 @@
+package transport
+
+import "time"
+
+type Transport interface {
+	Send(data []byte) error
+	Receive(timeout time.Duration) ([]byte, error)
+	Close() error
+}
diff --git a/lib/wire/wire.go b/lib/wire/wire.go
deleted file mode 100644
index 6c36f0f..0000000
--- a/lib/wire/wire.go
+++ /dev/null
@@ -1,57 +0,0 @@
-package wire
-
-import (
-	"fmt"
-
-	"github.com/theater/picomap/lib/halfsiphash"
-	"github.com/theater/picomap/lib/msgpack"
-)
-
-var HashKey = [8]byte{}
-
-type ResponseBOOTSEL struct{}
-type RequestBOOTSEL struct{}
-
-type Envelope struct {
-	Checksum uint32
-	Payload  []byte
-}
-
-func init() {
-	msgpack.RegisterExt(0, (*Envelope)(nil))
-	msgpack.RegisterExt(1, (*ResponseBOOTSEL)(nil))
-	msgpack.RegisterExt(2, (*RequestBOOTSEL)(nil))
-}
-
-func EncodeMessage(msg any) ([]byte, error) {
-	payload, err := msgpack.Marshal(msg)
-	if err != nil {
-		return nil, fmt.Errorf("encode inner: %w", err)
-	}
-
-	checksum := halfsiphash.Sum32(payload, HashKey)
-	env := Envelope{Checksum: checksum, Payload: payload}
-	data, err := msgpack.Marshal(&env)
-	if err != nil {
-		return nil, fmt.Errorf("encode envelope: %w", err)
-	}
-	return data, nil
-}
-
-func DecodeMessage(data []byte) (any, error) {
-	var env Envelope
-	if err := msgpack.Unmarshal(data, &env); err != nil {
-		return nil, fmt.Errorf("decode envelope: %w", err)
-	}
-
-	expected := halfsiphash.Sum32(env.Payload, HashKey)
-	if env.Checksum != expected {
-		return nil, fmt.Errorf("checksum mismatch: got %08x, want %08x", env.Checksum, expected)
-	}
-
-	var inner any
-	if err := msgpack.Unmarshal(env.Payload, &inner); err != nil {
-		return nil, fmt.Errorf("decode inner: %w", err)
-	}
-	return inner, nil
-}
diff --git a/picomap.cpp b/picomap.cpp
index fe99ee8..8204897 100644
--- a/picomap.cpp
+++ b/picomap.cpp
@@ -1,40 +1,7 @@
 #include <stdio.h>
-#include <string.h>
-#include <vector>
 #include "pico/stdlib.h"
 #include "pico/bootrom.h"
-#include "msgpackpp.h"
-#include "halfsiphash.h"
-#include "static_vector.h"
-
-static constexpr uint8_t hash_key[8] = {};
-
-struct ResponseBOOTSEL {
-    static constexpr int8_t ext_id = 1;
-    auto as_tuple() const { return std::tie(); }
-    auto as_tuple() { return std::tie(); }
-};
-
-struct RequestBOOTSEL {
-    static constexpr int8_t ext_id = 2;
-    auto as_tuple() const { return std::tie(); }
-    auto as_tuple() { return std::tie(); }
-};
-
-struct Envelope {
-    static constexpr int8_t ext_id = 0;
-    uint32_t checksum;
-    std::vector<uint8_t> payload;
-    auto as_tuple() const { return std::tie(checksum, payload); }
-    auto as_tuple() { return std::tie(checksum, payload); }
-};
-
-static std::vector<uint8_t> pack_envelope(const std::vector<uint8_t> &payload) {
-    uint32_t checksum = halfsiphash::hash32(payload.data(), payload.size(), hash_key);
-    msgpackpp::packer p;
-    p.pack(Envelope{checksum, payload});
-    return p.get_payload();
-}
+#include "device.h"
 
 static void send_bytes(const std::vector<uint8_t> &data) {
     for (auto b : data) {
@@ -43,31 +10,6 @@ static void send_bytes(const std::vector<uint8_t> &data) {
     stdio_flush();
 }
 
-template <typename T>
-static void send_message(const T &msg) {
-    msgpackpp::packer inner;
-    inner.pack(msg);
-    auto envelope = pack_envelope(inner.get_payload());
-    send_bytes(envelope);
-}
-
-static int8_t try_decode(const static_vector<uint8_t, 256> &buf) {
-    msgpackpp::parser p(buf.data(), static_cast<int>(buf.size()));
-
-    Envelope env;
-    if (!msgpackpp::unpack(p, env)) return -1;
-
-    uint32_t expected = halfsiphash::hash32(env.payload.data(), env.payload.size(), hash_key);
-    if (env.checksum != expected) return -1;
-
-    msgpackpp::parser inner(env.payload.data(), static_cast<int>(env.payload.size()));
-    if (!inner.is_ext()) return -1;
-    auto ext = inner.get_ext();
-    if (!ext) return -1;
-
-    return std::get<0>(*ext);
-}
-
 int main() {
     stdio_init_all();
 
@@ -79,17 +21,17 @@ int main() {
 
         rx_buf.push_back(static_cast<uint8_t>(c));
 
-        int8_t msg_type = try_decode(rx_buf);
-        if (msg_type < 0) {
+        auto msg = try_decode(rx_buf);
+        if (!msg) {
             if (rx_buf.full()) rx_buf.clear();
             continue;
         }
 
         rx_buf.clear();
 
-        switch (msg_type) {
+        switch (msg->type_id) {
         case RequestBOOTSEL::ext_id:
-            send_message(ResponseBOOTSEL{});
+            send_bytes(encode_response(msg->message_id, ResponseBOOTSEL{}));
             sleep_ms(100);
             reset_usb_boot(0, 0);
             break;