diff --git a/muxterm/protocol.go b/muxterm/protocol.go
new file mode 100644
index 0000000..b5167fe
--- /dev/null
+++ b/muxterm/protocol.go
@@ -0,0 +1,94 @@
+package main
+
+import "encoding/json"
+
+// --- Client-to-server message types ---
+
+// AttachMsg requests attaching to a named tmux session.
+type AttachMsg struct {
+	Attach string `json:"attach"`
+}
+
+// ResizeMsg requests a PTY resize.
+type ResizeMsg struct {
+	Resize struct {
+		Cols int `json:"cols"`
+		Rows int `json:"rows"`
+	} `json:"resize"`
+}
+
+// DetachMsg requests detaching from the current session.
+type DetachMsg struct {
+	Detach bool `json:"detach"`
+}
+
+// --- Server-to-client message types ---
+
+// AttachedMsg confirms a successful attach.
+type AttachedMsg struct {
+	Attached string `json:"attached"`
+}
+
+// ErrorMsg reports an error to the client.
+type ErrorMsg struct {
+	Error string `json:"error"`
+}
+
+// ExitedMsg reports that a session's process has exited.
+type ExitedMsg struct {
+	Exited string `json:"exited"`
+}
+
+// ParseControlMessage unmarshals a JSON control message and returns the
+// message kind and a pointer to the typed value. Returns ("invalid", nil)
+// for unparseable JSON and ("unknown", nil) for unrecognised keys.
+func ParseControlMessage(data []byte) (string, interface{}) {
+	var raw map[string]json.RawMessage
+	if err := json.Unmarshal(data, &raw); err != nil {
+		return "invalid", nil
+	}
+
+	if _, ok := raw["attach"]; ok {
+		var msg AttachMsg
+		if err := json.Unmarshal(data, &msg); err != nil {
+			return "invalid", nil
+		}
+		return "attach", &msg
+	}
+
+	if _, ok := raw["resize"]; ok {
+		var msg ResizeMsg
+		if err := json.Unmarshal(data, &msg); err != nil {
+			return "invalid", nil
+		}
+		return "resize", &msg
+	}
+
+	if _, ok := raw["detach"]; ok {
+		var msg DetachMsg
+		if err := json.Unmarshal(data, &msg); err != nil {
+			return "invalid", nil
+		}
+		return "detach", &msg
+	}
+
+	return "unknown", nil
+}
+
+// MarshalAttached returns JSON for an AttachedMsg.
+func MarshalAttached(session string) []byte {
+	b, _ := json.Marshal(AttachedMsg{Attached: session})
+	return b
+}
+
+// MarshalError returns JSON for an ErrorMsg.
+func MarshalError(msg string) []byte {
+	b, _ := json.Marshal(ErrorMsg{Error: msg})
+	return b
+}
+
+// MarshalExited returns JSON for an ExitedMsg.
+func MarshalExited(session string) []byte {
+	b, _ := json.Marshal(ExitedMsg{Exited: session})
+	return b
+}
\ No newline at end of file
diff --git a/muxterm/protocol_test.go b/muxterm/protocol_test.go
new file mode 100644
index 0000000..596e393
--- /dev/null
+++ b/muxterm/protocol_test.go
@@ -0,0 +1,202 @@
+package main
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+// --- Client-to-server message type tests ---
+
+func TestAttachMsgJSON(t *testing.T) {
+	msg := AttachMsg{Attach: "my-session"}
+	data, err := json.Marshal(msg)
+	if err != nil {
+		t.Fatalf("marshal AttachMsg: %v", err)
+	}
+	var m map[string]interface{}
+	json.Unmarshal(data, &m)
+	if m["attach"] != "my-session" {
+		t.Errorf("expected attach=my-session, got %v", m["attach"])
+	}
+}
+
+func TestResizeMsgJSON(t *testing.T) {
+	msg := ResizeMsg{Resize: struct {
+		Cols int `json:"cols"`
+		Rows int `json:"rows"`
+	}{Cols: 80, Rows: 24}}
+	data, err := json.Marshal(msg)
+	if err != nil {
+		t.Fatalf("marshal ResizeMsg: %v", err)
+	}
+	var m map[string]interface{}
+	json.Unmarshal(data, &m)
+	resize, ok := m["resize"].(map[string]interface{})
+	if !ok {
+		t.Fatalf("expected resize to be a map, got %T", m["resize"])
+	}
+	if resize["cols"].(float64) != 80 {
+		t.Errorf("expected cols=80, got %v", resize["cols"])
+	}
+	if resize["rows"].(float64) != 24 {
+		t.Errorf("expected rows=24, got %v", resize["rows"])
+	}
+}
+
+func TestDetachMsgJSON(t *testing.T) {
+	msg := DetachMsg{Detach: true}
+	data, err := json.Marshal(msg)
+	if err != nil {
+		t.Fatalf("marshal DetachMsg: %v", err)
+	}
+	var m map[string]interface{}
+	json.Unmarshal(data, &m)
+	if m["detach"] != true {
+		t.Errorf("expected detach=true, got %v", m["detach"])
+	}
+}
+
+// --- Server-to-client message type tests ---
+
+func TestAttachedMsgJSON(t *testing.T) {
+	msg := AttachedMsg{Attached: "sess1"}
+	data, err := json.Marshal(msg)
+	if err != nil {
+		t.Fatalf("marshal AttachedMsg: %v", err)
+	}
+	var m map[string]interface{}
+	json.Unmarshal(data, &m)
+	if m["attached"] != "sess1" {
+		t.Errorf("expected attached=sess1, got %v", m["attached"])
+	}
+}
+
+func TestErrorMsgJSON(t *testing.T) {
+	msg := ErrorMsg{Error: "something broke"}
+	data, err := json.Marshal(msg)
+	if err != nil {
+		t.Fatalf("marshal ErrorMsg: %v", err)
+	}
+	var m map[string]interface{}
+	json.Unmarshal(data, &m)
+	if m["error"] != "something broke" {
+		t.Errorf("expected error='something broke', got %v", m["error"])
+	}
+}
+
+func TestExitedMsgJSON(t *testing.T) {
+	msg := ExitedMsg{Exited: "sess1"}
+	data, err := json.Marshal(msg)
+	if err != nil {
+		t.Fatalf("marshal ExitedMsg: %v", err)
+	}
+	var m map[string]interface{}
+	json.Unmarshal(data, &m)
+	if m["exited"] != "sess1" {
+		t.Errorf("expected exited=sess1, got %v", m["exited"])
+	}
+}
+
+// --- ParseControlMessage tests ---
+
+func TestParseControlMessage_Attach(t *testing.T) {
+	data := []byte(`{"attach":"dev"}`)
+	kind, val := ParseControlMessage(data)
+	if kind != "attach" {
+		t.Fatalf("expected kind=attach, got %q", kind)
+	}
+	msg, ok := val.(*AttachMsg)
+	if !ok {
+		t.Fatalf("expected *AttachMsg, got %T", val)
+	}
+	if msg.Attach != "dev" {
+		t.Errorf("expected Attach=dev, got %q", msg.Attach)
+	}
+}
+
+func TestParseControlMessage_Resize(t *testing.T) {
+	data := []byte(`{"resize":{"cols":120,"rows":40}}`)
+	kind, val := ParseControlMessage(data)
+	if kind != "resize" {
+		t.Fatalf("expected kind=resize, got %q", kind)
+	}
+	msg, ok := val.(*ResizeMsg)
+	if !ok {
+		t.Fatalf("expected *ResizeMsg, got %T", val)
+	}
+	if msg.Resize.Cols != 120 || msg.Resize.Rows != 40 {
+		t.Errorf("expected 120x40, got %dx%d", msg.Resize.Cols, msg.Resize.Rows)
+	}
+}
+
+func TestParseControlMessage_Detach(t *testing.T) {
+	data := []byte(`{"detach":true}`)
+	kind, val := ParseControlMessage(data)
+	if kind != "detach" {
+		t.Fatalf("expected kind=detach, got %q", kind)
+	}
+	msg, ok := val.(*DetachMsg)
+	if !ok {
+		t.Fatalf("expected *DetachMsg, got %T", val)
+	}
+	if !msg.Detach {
+		t.Errorf("expected Detach=true")
+	}
+}
+
+func TestParseControlMessage_Unknown(t *testing.T) {
+	data := []byte(`{"foo":"bar"}`)
+	kind, val := ParseControlMessage(data)
+	if kind != "unknown" {
+		t.Errorf("expected kind=unknown, got %q", kind)
+	}
+	if val != nil {
+		t.Errorf("expected nil val for unknown, got %v", val)
+	}
+}
+
+func TestParseControlMessage_Invalid(t *testing.T) {
+	data := []byte(`not json at all`)
+	kind, val := ParseControlMessage(data)
+	if kind != "invalid" {
+		t.Errorf("expected kind=invalid, got %q", kind)
+	}
+	if val != nil {
+		t.Errorf("expected nil val for invalid, got %v", val)
+	}
+}
+
+// --- Marshal function tests ---
+
+func TestMarshalAttached(t *testing.T) {
+	data := MarshalAttached("sess1")
+	var m map[string]interface{}
+	if err := json.Unmarshal(data, &m); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if m["attached"] != "sess1" {
+		t.Errorf("expected attached=sess1, got %v", m["attached"])
+	}
+}
+
+func TestMarshalError(t *testing.T) {
+	data := MarshalError("bad thing")
+	var m map[string]interface{}
+	if err := json.Unmarshal(data, &m); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if m["error"] != "bad thing" {
+		t.Errorf("expected error='bad thing', got %v", m["error"])
+	}
+}
+
+func TestMarshalExited(t *testing.T) {
+	data := MarshalExited("sess1")
+	var m map[string]interface{}
+	if err := json.Unmarshal(data, &m); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if m["exited"] != "sess1" {
+		t.Errorf("expected exited=sess1, got %v", m["exited"])
+	}
+}
\ No newline at end of file