diff --git a/agent/src/protocol.rs b/agent/src/protocol.rs
new file mode 100644
index 0000000..0164440
--- /dev/null
+++ b/agent/src/protocol.rs
@@ -0,0 +1,162 @@
+//! Message types for communication between the agent and the Butterfly server.
+//!
+//! These mirror the server's `WsMessage` enum (serde-tagged with `msg_type`).
+//! The agent sends **Agent → Server** messages and receives **Server → Agent** messages.
+
+use serde::{Deserialize, Serialize};
+
+// ── Agent → Server ────────────────────────────────────────────────────────────
+
+/// A single display frame, base64-encoded JPEG.
+#[derive(Debug, Clone, Serialize)]
+pub struct DisplayFrame {
+    pub session_id: String,
+    pub data: String,
+    pub timestamp: Option<f64>,
+}
+
+/// An audio chunk (future: PCM/Opus encoded).
+#[derive(Debug, Clone, Serialize)]
+pub struct AudioFrame {
+    pub session_id: String,
+    pub data: String,
+    pub timestamp: Option<f64>,
+}
+
+/// Agent announces its capabilities on connect.
+#[derive(Debug, Clone, Serialize)]
+pub struct AgentInfo {
+    pub session_id: String,
+    pub agent_id: String,
+    pub resolution: Option<String>,
+    pub hostname: String,
+}
+
+/// Keep-alive ping.
+#[derive(Debug, Clone, Serialize)]
+pub struct Heartbeat;
+
+// ── Server → Agent ────────────────────────────────────────────────────────────
+
+/// A HUD command forwarded from a viewer (mouse, keyboard, etc.).
+#[derive(Debug, Clone, Deserialize)]
+pub struct ForwardHudCommand {
+    pub command: String,
+    pub params: serde_json::Value,
+}
+
+/// A resize request forwarded from a viewer.
+#[derive(Debug, Clone, Deserialize)]
+pub struct ForwardResize {
+    pub width: u32,
+    pub height: u32,
+}
+
+/// Server tells the agent to start/stop streaming.
+#[derive(Debug, Clone, Deserialize)]
+pub struct StreamControl {
+    pub action: String,
+}
+
+/// Generic acknowledgment from server.
+#[derive(Debug, Clone, Deserialize)]
+pub struct Ack {
+    pub message: String,
+}
+
+/// Error from server.
+#[derive(Debug, Clone, Deserialize)]
+pub struct ErrorMsg {
+    pub message: String,
+}
+
+// ── Unified message envelope ──────────────────────────────────────────────────
+
+/// All messages use a `msg_type` discriminator for routing.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "msg_type", rename_all = "snake_case")]
+pub enum AgentWsMessage {
+    // Outgoing (Agent → Server)
+    #[serde(rename = "display_frame")]
+    DisplayFrame {
+        session_id: String,
+        data: String,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        timestamp: Option<f64>,
+    },
+    #[serde(rename = "audio_frame")]
+    AudioFrame {
+        session_id: String,
+        data: String,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        timestamp: Option<f64>,
+    },
+    #[serde(rename = "agent_info")]
+    AgentInfo {
+        session_id: String,
+        agent_id: String,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        resolution: Option<String>,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        hostname: Option<String>,
+    },
+    #[serde(rename = "heartbeat")]
+    Heartbeat,
+
+    // Incoming (Server → Agent) — included for deserialization
+    #[serde(rename = "forward_hud_command")]
+    ForwardHudCommand {
+        command: String,
+        params: serde_json::Value,
+    },
+    #[serde(rename = "forward_resize")]
+    ForwardResize {
+        width: u32,
+        height: u32,
+    },
+    #[serde(rename = "stream_control")]
+    StreamControl { action: String },
+    #[serde(rename = "ack")]
+    Ack { message: String },
+    #[serde(rename = "error")]
+    Error { message: String },
+}
+
+// ── Helpers ───────────────────────────────────────────────────────────────────
+
+/// Build a display frame message.
+pub fn display_frame_msg(session_id: &str, data: String) -> String {
+    let msg = AgentWsMessage::DisplayFrame {
+        session_id: session_id.to_string(),
+        data,
+        timestamp: None,
+    };
+    serde_json::to_string(&msg).unwrap_or_default()
+}
+
+/// Build an audio frame message.
+pub fn audio_frame_msg(session_id: &str, data: String) -> String {
+    let msg = AgentWsMessage::AudioFrame {
+        session_id: session_id.to_string(),
+        data,
+        timestamp: None,
+    };
+    serde_json::to_string(&msg).unwrap_or_default()
+}
+
+/// Build an agent info message.
+pub fn agent_info_msg(session_id: &str, agent_id: &str, resolution: Option<&str>, hostname: Option<&str>) -> String {
+    let msg = AgentWsMessage::AgentInfo {
+        session_id: session_id.to_string(),
+        agent_id: agent_id.to_string(),
+        resolution: resolution.map(String::from),
+        hostname: hostname.map(String::from),
+    };
+    serde_json::to_string(&msg).unwrap_or_default()
+}
+
+/// Build a heartbeat message.
+pub fn heartbeat_msg() -> String {
+    let msg = AgentWsMessage::Heartbeat;
+    serde_json::to_string(&msg).unwrap_or_default()
+}