diff --git a/frontend/src/api/gateway.ts b/frontend/src/api/gateway.ts
index 408cf233..ce30cf5e 100644
--- a/frontend/src/api/gateway.ts
+++ b/frontend/src/api/gateway.ts
@@ -8,6 +8,8 @@ export interface JoinedAgent {
 	label: string;
 	address: string;
 	registered_at: number;
+	/// Unix timestamp of the last heartbeat from this agent.
+	last_seen: number;
 	/// Project this agent is assigned to, if any.
 	assigned_project?: string;
 }
@@ -102,4 +104,11 @@ export const gatewayApi = {
 			{ method: "DELETE" },
 		);
 	},
+
+	/// Send a heartbeat for an agent to update its last-seen timestamp.
+	heartbeat(id: string): Promise<void> {
+		return gatewayRequest<void>(`/gateway/agents/${id}/heartbeat`, {
+			method: "POST",
+		});
+	},
 };
diff --git a/frontend/src/components/GatewayPanel.tsx b/frontend/src/components/GatewayPanel.tsx
index 96e77f1e..d2430160 100644
--- a/frontend/src/components/GatewayPanel.tsx
+++ b/frontend/src/components/GatewayPanel.tsx
@@ -3,12 +3,42 @@
 /// Provides:
 /// - An "Add Agent" button that generates a one-time join token.
 /// - Instructions for running a build agent with the token.
-/// - A list of connected agents with per-agent project assignment and "Remove" buttons.
+/// - A list of connected agents with per-agent status, project assignment, and "Remove" buttons.
+/// - Auto-refresh every 5 seconds so new agents and disconnections appear without a page reload.
 
 import * as React from "react";
 import { gatewayApi, type JoinedAgent, type GatewayProject } from "../api/gateway";
 
-const { useCallback, useEffect, useState } = React;
+const { useCallback, useEffect, useRef, useState } = React;
+
+/// Seconds of silence before an agent is considered disconnected.
+const DISCONNECT_THRESHOLD_SECS = 60;
+
+/// Poll the agent list this often (milliseconds).
+const POLL_INTERVAL_MS = 5_000;
+
+type AgentStatus = "idle" | "working" | "disconnected";
+
+/// Derive an agent's display status from its last-seen timestamp and project assignment.
+function agentStatus(agent: JoinedAgent): AgentStatus {
+	const nowSecs = Date.now() / 1000;
+	if (nowSecs - agent.last_seen > DISCONNECT_THRESHOLD_SECS) {
+		return "disconnected";
+	}
+	return agent.assigned_project ? "working" : "idle";
+}
+
+const STATUS_COLORS: Record<AgentStatus, string> = {
+	idle: "#6e7681",
+	working: "#3fb950",
+	disconnected: "#f85149",
+};
+
+const STATUS_LABELS: Record<AgentStatus, string> = {
+	idle: "Idle",
+	working: "Working",
+	disconnected: "Disconnected",
+};
 
 function TokenDisplay({ token }: { token: string }) {
 	const [copied, setCopied] = useState(false);
@@ -100,7 +130,9 @@ function AgentRow({
 	onAssign: (id: string, project: string | null) => void;
 }) {
 	const registeredAt = new Date(agent.registered_at * 1000).toLocaleString();
-	const isAssigned = Boolean(agent.assigned_project);
+	const status = agentStatus(agent);
+	const statusColor = STATUS_COLORS[status];
+	const statusLabel = STATUS_LABELS[status];
 
 	return (
 		<div
@@ -121,18 +153,38 @@ function AgentRow({
 					width: "8px",
 					height: "8px",
 					borderRadius: "50%",
-					background: isAssigned ? "#3fb950" : "#6e7681",
+					background: statusColor,
 					flexShrink: 0,
 				}}
-				title={isAssigned ? "Assigned" : "Idle (unassigned)"}
+				title={statusLabel}
 			/>
 			<div style={{ flex: 1 }}>
-				<div style={{ fontWeight: 600, color: "#e6edf3" }}>{agent.label}</div>
+				<div style={{ display: "flex", alignItems: "center", gap: "8px" }}>
+					<span style={{ fontWeight: 600, color: "#e6edf3" }}>{agent.label}</span>
+					<span
+						data-testid={`agent-status-${agent.id}`}
+						style={{
+							fontSize: "0.75em",
+							padding: "1px 6px",
+							borderRadius: "10px",
+							background: `${statusColor}22`,
+							color: statusColor,
+							border: `1px solid ${statusColor}44`,
+						}}
+					>
+						{statusLabel}
+					</span>
+				</div>
 				<div style={{ fontSize: "0.8em", color: "#8b949e" }}>
 					{agent.address}
 				</div>
 				<div style={{ fontSize: "0.75em", color: "#6e7681" }}>
 					Registered {registeredAt}
+					{agent.assigned_project && (
+						<span style={{ marginLeft: "8px", color: "#8b949e" }}>
+							· Project: {agent.assigned_project}
+						</span>
+					)}
 				</div>
 			</div>
 			<select
@@ -191,7 +243,13 @@ export function GatewayPanel() {
 	const [newProjectUrl, setNewProjectUrl] = useState("");
 	const [addingProject, setAddingProject] = useState(false);
 
+	// Keep a stable ref to setAgents so the polling interval doesn't need to
+	// be recreated when the agents list changes.
+	const setAgentsRef = useRef(setAgents);
+	setAgentsRef.current = setAgents;
+
 	useEffect(() => {
+		// Initial load.
 		gatewayApi
 			.listAgents()
 			.then(setAgents)
@@ -200,6 +258,19 @@ export function GatewayPanel() {
 			.getGatewayInfo()
 			.then((info) => setProjects(info.projects))
 			.catch(() => setProjects([]));
+
+		// Poll the agent list so the dashboard auto-updates when agents connect
+		// or disconnect.
+		const timer = setInterval(() => {
+			gatewayApi
+				.listAgents()
+				.then((updated) => setAgentsRef.current(updated))
+				.catch(() => {
+					// Swallow poll errors to avoid spamming the error banner.
+				});
+		}, POLL_INTERVAL_MS);
+
+		return () => clearInterval(timer);
 	}, []);
 
 	const handleAddAgent = useCallback(async () => {
diff --git a/server/src/gateway.rs b/server/src/gateway.rs
index 61834715..2c07b4be 100644
--- a/server/src/gateway.rs
+++ b/server/src/gateway.rs
@@ -65,6 +65,10 @@ pub struct JoinedAgent {
     pub address: String,
     /// Unix timestamp when the agent registered.
     pub registered_at: f64,
+    /// Unix timestamp of the last heartbeat from this agent. Defaults to `registered_at`
+    /// for agents loaded from persisted state that predate the heartbeat feature.
+    #[serde(default)]
+    pub last_seen: f64,
     /// Project this agent is assigned to, if any.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub assigned_project: Option<String>,
@@ -691,11 +695,13 @@ pub async fn gateway_register_agent_handler(
     tokens.remove(&req.token);
     drop(tokens);
 
+    let now = chrono::Utc::now().timestamp() as f64;
     let agent = JoinedAgent {
         id: Uuid::new_v4().to_string(),
         label: req.label,
         address: req.address,
-        registered_at: chrono::Utc::now().timestamp() as f64,
+        registered_at: now,
+        last_seen: now,
         assigned_project: None,
     };
 
@@ -815,6 +821,38 @@ pub async fn gateway_assign_agent_handler(
     }
 }
 
+/// `POST /gateway/agents/:id/heartbeat` — update an agent's last-seen timestamp.
+///
+/// Build agents should call this periodically (e.g. every 30 s) so the gateway
+/// can distinguish live agents from disconnected ones.  Returns 204 No Content on
+/// success or 404 if the agent ID is not found.
+#[handler]
+pub async fn gateway_heartbeat_handler(
+    PoemPath(id): PoemPath<String>,
+    state: Data<&Arc<GatewayState>>,
+) -> Response {
+    let found = {
+        let mut agents = state.joined_agents.write().await;
+        match agents.iter_mut().find(|a| a.id == id) {
+            None => false,
+            Some(a) => {
+                a.last_seen = chrono::Utc::now().timestamp() as f64;
+                true
+            }
+        }
+    };
+
+    if found {
+        Response::builder()
+            .status(StatusCode::NO_CONTENT)
+            .body(Body::empty())
+    } else {
+        Response::builder()
+            .status(StatusCode::NOT_FOUND)
+            .body(Body::from("agent not found"))
+    }
+}
+
 // ── Health aggregation endpoint ──────────────────────────────────────
 
 /// HTTP GET `/health` handler for the gateway — aggregates health from all projects.
@@ -1620,6 +1658,10 @@ pub async fn run(config_path: &Path, port: u16) -> Result<(), std::io::Error> {
             "/gateway/agents/:id/assign",
             poem::post(gateway_assign_agent_handler),
         )
+        .at(
+            "/gateway/agents/:id/heartbeat",
+            poem::post(gateway_heartbeat_handler),
+        )
         // Serve the embedded React frontend so the gateway has a UI.
         .at(
             "/assets/*path",
@@ -2064,6 +2106,7 @@ enabled = false
             label: "agent-1".into(),
             address: "ws://a:3001/crdt-sync".into(),
             registered_at: 0.0,
+            last_seen: 0.0,
             assigned_project: None,
         });
         let app = poem::Route::new()
@@ -2085,6 +2128,7 @@ enabled = false
             label: "to-delete".into(),
             address: "ws://x:3001/crdt-sync".into(),
             registered_at: 0.0,
+            last_seen: 0.0,
             assigned_project: None,
         });
         let app = poem::Route::new()
@@ -2112,4 +2156,45 @@ enabled = false
         let resp = cli.delete("/gateway/agents/no-such-id").send().await;
         assert_eq!(resp.0.status(), StatusCode::NOT_FOUND);
     }
+
+    #[tokio::test]
+    async fn heartbeat_updates_last_seen() {
+        let state = make_test_state();
+        state.joined_agents.write().await.push(JoinedAgent {
+            id: "hb-id".into(),
+            label: "hb-agent".into(),
+            address: "ws://hb:3001/crdt-sync".into(),
+            registered_at: 0.0,
+            last_seen: 0.0,
+            assigned_project: None,
+        });
+        let app = poem::Route::new()
+            .at(
+                "/gateway/agents/:id/heartbeat",
+                poem::post(gateway_heartbeat_handler),
+            )
+            .data(state.clone());
+        let cli = poem::test::TestClient::new(app);
+        let resp = cli.post("/gateway/agents/hb-id/heartbeat").send().await;
+        assert_eq!(resp.0.status(), StatusCode::NO_CONTENT);
+        let agents = state.joined_agents.read().await;
+        assert!(agents[0].last_seen > 0.0);
+    }
+
+    #[tokio::test]
+    async fn heartbeat_unknown_id_returns_not_found() {
+        let state = make_test_state();
+        let app = poem::Route::new()
+            .at(
+                "/gateway/agents/:id/heartbeat",
+                poem::post(gateway_heartbeat_handler),
+            )
+            .data(state.clone());
+        let cli = poem::test::TestClient::new(app);
+        let resp = cli
+            .post("/gateway/agents/no-such-id/heartbeat")
+            .send()
+            .await;
+        assert_eq!(resp.0.status(), StatusCode::NOT_FOUND);
+    }
 }