diff --git a/internal/protocol/frame.go b/internal/protocol/frame.go
new file mode 100644
index 0000000..26e5964
--- /dev/null
+++ b/internal/protocol/frame.go
@@ -0,0 +1,164 @@
+// Package protocol defines the daemon/client control frames shared by
+// transports. It intentionally contains data shapes only; app behavior stays
+// in internal/app until the headless daemon split is complete.
+package protocol
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+// FrameType identifies one protocol message kind.
+type FrameType string
+
+const (
+	FrameHello          FrameType = "hello"
+	FrameAuthChallenge  FrameType = "auth_challenge"
+	FrameAuthOK         FrameType = "auth_ok"
+	FrameAttach         FrameType = "attach"
+	FrameDetach         FrameType = "detach"
+	FrameProjectList    FrameType = "project_list"
+	FrameChrome         FrameType = "chrome"
+	FramePaneSnapshot   FrameType = "pane_snapshot"
+	FramePaneChunk      FrameType = "pane_chunk"
+	FrameLifecycle      FrameType = "lifecycle"
+	FrameAttention      FrameType = "attention"
+	FrameTrustPrompt    FrameType = "trust_prompt"
+	FrameInput          FrameType = "input"
+	FrameFocus          FrameType = "focus"
+	FrameSwitchProject  FrameType = "switch_project"
+	FrameOpenProject    FrameType = "open_project"
+	FramePaletteCommand FrameType = "palette_command"
+	FrameTrustResponse  FrameType = "trust_response"
+	FrameResize         FrameType = "resize"
+)
+
+// Frame is the transport envelope. Payload is deliberately raw JSON so
+// network transports can frame without knowing every message type; loopback
+// transports may pass the same bytes without JSON re-encoding.
+type Frame struct {
+	Type      FrameType       `json:"type"`
+	RequestID string          `json:"request_id,omitempty"`
+	Payload   json.RawMessage `json:"payload,omitempty"`
+}
+
+// NewFrame marshals payload into a protocol frame.
+func NewFrame[T any](typ FrameType, payload T) (Frame, error) {
+	b, err := json.Marshal(payload)
+	if err != nil {
+		return Frame{}, fmt.Errorf("protocol: marshal %s: %w", typ, err)
+	}
+	return Frame{Type: typ, Payload: b}, nil
+}
+
+// Decode unmarshals f.Payload into v.
+func Decode[T any](f Frame) (T, error) {
+	var v T
+	if len(f.Payload) == 0 {
+		return v, nil
+	}
+	if err := json.Unmarshal(f.Payload, &v); err != nil {
+		return v, fmt.Errorf("protocol: decode %s: %w", f.Type, err)
+	}
+	return v, nil
+}
+
+type Hello struct {
+	Version    int    `json:"version"`
+	DaemonID   string `json:"daemon_id,omitempty"`
+	ClientID   string `json:"client_id,omitempty"`
+	ProjectKey string `json:"project_key,omitempty"`
+}
+
+type Attach struct {
+	Token      string `json:"token,omitempty"`
+	ProjectKey string `json:"project_key,omitempty"`
+	TermSize   Size   `json:"term_size"`
+}
+
+type Detach struct {
+	ClientID string `json:"client_id,omitempty"`
+}
+
+type Size struct {
+	Cols uint16 `json:"cols"`
+	Rows uint16 `json:"rows"`
+}
+
+type Project struct {
+	Key        string    `json:"key"`
+	Path       string    `json:"path"`
+	Name       string    `json:"name"`
+	LastActive time.Time `json:"last_active,omitempty"`
+	TabCount   int       `json:"tab_count"`
+}
+
+type ProjectList struct {
+	Projects []Project `json:"projects"`
+}
+
+type Chrome struct {
+	ProjectKey string          `json:"project_key"`
+	Model      json.RawMessage `json:"model"`
+}
+
+type PaneSnapshot struct {
+	PaneID string `json:"pane_id"`
+	Bytes  []byte `json:"bytes"`
+}
+
+type PaneChunk struct {
+	PaneID string `json:"pane_id"`
+	Bytes  []byte `json:"bytes"`
+}
+
+type LifecycleKind string
+
+const (
+	LifecycleSpawned      LifecycleKind = "spawned"
+	LifecycleExited       LifecycleKind = "exited"
+	LifecycleClosed       LifecycleKind = "closed"
+	LifecycleStateChanged LifecycleKind = "state_changed"
+)
+
+type Lifecycle struct {
+	Kind       LifecycleKind   `json:"kind"`
+	ProjectKey string          `json:"project_key,omitempty"`
+	ChildID    string          `json:"child_id,omitempty"`
+	Child      json.RawMessage `json:"child,omitempty"`
+	State      string          `json:"state,omitempty"`
+}
+
+type Input struct {
+	PaneID string `json:"pane_id"`
+	Bytes  []byte `json:"bytes"`
+}
+
+type Focus struct {
+	PaneID string `json:"pane_id,omitempty"`
+	Pad    string `json:"pad,omitempty"`
+}
+
+type SwitchProject struct {
+	Key string `json:"key"`
+}
+
+type OpenProject struct {
+	Path string `json:"path"`
+}
+
+type PaletteCommand struct {
+	Kind string          `json:"kind"`
+	Data json.RawMessage `json:"data,omitempty"`
+}
+
+type TrustResponse struct {
+	ProcessID string `json:"process_id"`
+	Preset    string `json:"preset"`
+	Allow     bool   `json:"allow"`
+}
+
+type Resize struct {
+	Size Size `json:"size"`
+}
diff --git a/internal/protocol/loopback.go b/internal/protocol/loopback.go
new file mode 100644
index 0000000..d8f61ae
--- /dev/null
+++ b/internal/protocol/loopback.go
@@ -0,0 +1,67 @@
+package protocol
+
+import (
+	"sync"
+)
+
+const defaultLoopbackBuffer = 64
+
+// NewLoopbackPair returns connected in-process transports. Frames cross the
+// same Send/Recv boundary as network transports, but payload bytes are passed
+// directly without JSON re-encoding.
+func NewLoopbackPair() (client Transport, daemon Transport) {
+	c2d := make(chan Frame, defaultLoopbackBuffer)
+	d2c := make(chan Frame, defaultLoopbackBuffer)
+	return &loopbackTransport{send: c2d, recv: d2c}, &loopbackTransport{send: d2c, recv: c2d}
+}
+
+type loopbackTransport struct {
+	send chan<- Frame
+	recv <-chan Frame
+	once sync.Once
+	done chan struct{}
+}
+
+func (t *loopbackTransport) init() {
+	if t.done == nil {
+		t.done = make(chan struct{})
+	}
+}
+
+func (t *loopbackTransport) Send(f Frame) error {
+	t.init()
+	select {
+	case <-t.done:
+		return ErrTransportClosed
+	case t.send <- cloneFrame(f):
+		return nil
+	}
+}
+
+func (t *loopbackTransport) Recv() (Frame, error) {
+	t.init()
+	select {
+	case <-t.done:
+		return Frame{}, ErrTransportClosed
+	case f, ok := <-t.recv:
+		if !ok {
+			return Frame{}, ErrTransportClosed
+		}
+		return f, nil
+	}
+}
+
+func (t *loopbackTransport) Close() error {
+	t.init()
+	t.once.Do(func() {
+		close(t.done)
+	})
+	return nil
+}
+
+func cloneFrame(f Frame) Frame {
+	if len(f.Payload) > 0 {
+		f.Payload = append([]byte(nil), f.Payload...)
+	}
+	return f
+}
diff --git a/internal/protocol/loopback_test.go b/internal/protocol/loopback_test.go
new file mode 100644
index 0000000..ab001d9
--- /dev/null
+++ b/internal/protocol/loopback_test.go
@@ -0,0 +1,51 @@
+package protocol
+
+import "testing"
+
+func TestLoopbackUsesFramePayload(t *testing.T) {
+	client, daemon := NewLoopbackPair()
+	defer client.Close()
+	defer daemon.Close()
+
+	sent, err := NewFrame(FrameInput, Input{PaneID: "p_123456", Bytes: []byte("hello")})
+	if err != nil {
+		t.Fatalf("NewFrame: %v", err)
+	}
+	if err := client.Send(sent); err != nil {
+		t.Fatalf("Send: %v", err)
+	}
+	got, err := daemon.Recv()
+	if err != nil {
+		t.Fatalf("Recv: %v", err)
+	}
+	if got.Type != FrameInput {
+		t.Fatalf("type = %q, want %q", got.Type, FrameInput)
+	}
+	payload, err := Decode[Input](got)
+	if err != nil {
+		t.Fatalf("Decode: %v", err)
+	}
+	if payload.PaneID != "p_123456" || string(payload.Bytes) != "hello" {
+		t.Fatalf("payload = %#v", payload)
+	}
+}
+
+func TestLoopbackCopiesPayloadOnSend(t *testing.T) {
+	client, daemon := NewLoopbackPair()
+	defer client.Close()
+	defer daemon.Close()
+
+	f := Frame{Type: FramePaneChunk, Payload: []byte(`{"pane_id":"p","bytes":"aGVsbG8="}`)}
+	if err := client.Send(f); err != nil {
+		t.Fatalf("Send: %v", err)
+	}
+	f.Payload[0] = 'x'
+
+	got, err := daemon.Recv()
+	if err != nil {
+		t.Fatalf("Recv: %v", err)
+	}
+	if got.Payload[0] != '{' {
+		t.Fatalf("payload was retained instead of copied: %q", string(got.Payload))
+	}
+}
diff --git a/internal/protocol/transport.go b/internal/protocol/transport.go
new file mode 100644
index 0000000..8d431b5
--- /dev/null
+++ b/internal/protocol/transport.go
@@ -0,0 +1,73 @@
+package protocol
+
+import (
+	"bufio"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+)
+
+var ErrTransportClosed = errors.New("protocol: transport closed")
+
+// Transport carries framed daemon/client protocol messages.
+type Transport interface {
+	Send(Frame) error
+	Recv() (Frame, error)
+	Close() error
+}
+
+// ConnTransport is a JSON-lines implementation over a stream connection.
+type ConnTransport struct {
+	conn net.Conn
+	r    *bufio.Reader
+	w    *bufio.Writer
+}
+
+func NewConnTransport(conn net.Conn) *ConnTransport {
+	return &ConnTransport{
+		conn: conn,
+		r:    bufio.NewReader(conn),
+		w:    bufio.NewWriter(conn),
+	}
+}
+
+func (t *ConnTransport) Send(f Frame) error {
+	if t == nil || t.conn == nil {
+		return ErrTransportClosed
+	}
+	b, err := json.Marshal(f)
+	if err != nil {
+		return fmt.Errorf("protocol: encode frame: %w", err)
+	}
+	if _, err := t.w.Write(append(b, '\n')); err != nil {
+		return err
+	}
+	return t.w.Flush()
+}
+
+func (t *ConnTransport) Recv() (Frame, error) {
+	if t == nil || t.conn == nil {
+		return Frame{}, ErrTransportClosed
+	}
+	line, err := t.r.ReadBytes('\n')
+	if err != nil {
+		if errors.Is(err, io.EOF) {
+			return Frame{}, ErrTransportClosed
+		}
+		return Frame{}, err
+	}
+	var f Frame
+	if err := json.Unmarshal(line, &f); err != nil {
+		return Frame{}, fmt.Errorf("protocol: decode frame: %w", err)
+	}
+	return f, nil
+}
+
+func (t *ConnTransport) Close() error {
+	if t == nil || t.conn == nil {
+		return nil
+	}
+	return t.conn.Close()
+}