diff --git a/packages/coding-agent/src/collab/relay-client.ts b/packages/coding-agent/src/collab/relay-client.ts
index b64e525bc..8f2ea5c88 100644
--- a/packages/coding-agent/src/collab/relay-client.ts
+++ b/packages/coding-agent/src/collab/relay-client.ts
@@ -177,8 +177,10 @@ export class CollabSocket {
 		ws.onopen = () => {
 			if (this.#ws !== ws) return;
 			this.#attempt = 0;
-			for (const envelope of this.#pendingSends) ws.send(envelope);
-			this.#pendingSends.length = 0;
+			if (this.#pendingSends.length > 0) {
+				this.#drainPendingSends(ws);
+				if (this.#pendingSends.length > 0) this.#scheduleBackpressureDrain(ws);
+			}
 			this.onOpen?.();
 		};
 		ws.onmessage = (event: MessageEvent) => {
diff --git a/packages/coding-agent/test/collab/relay-client-backpressure.test.ts b/packages/coding-agent/test/collab/relay-client-backpressure.test.ts
new file mode 100644
index 000000000..673070b90
--- /dev/null
+++ b/packages/coding-agent/test/collab/relay-client-backpressure.test.ts
@@ -0,0 +1,124 @@
+import { afterEach, describe, expect, it, vi } from "bun:test";
+import { CollabSocket } from "../../src/collab/relay-client";
+
+const ORIGINAL_WEBSOCKET = globalThis.WebSocket;
+const HIGH_WATER_MARK = 64 * 1024;
+const DRAIN_RETRY_MS = 25;
+
+class BackpressuredWebSocket {
+	static readonly CONNECTING = 0;
+	static readonly OPEN = 1;
+	static readonly CLOSING = 2;
+	static readonly CLOSED = 3;
+	static initialBufferedAmount = 0;
+	static instances: BackpressuredWebSocket[] = [];
+
+	readonly url: string;
+	binaryType = "arraybuffer";
+	bufferedAmount: number;
+	onclose: ((event: CloseEvent) => void) | null = null;
+	onerror: ((event: Event) => void) | null = null;
+	onmessage: ((event: MessageEvent) => void) | null = null;
+	onopen: ((event: Event) => void) | null = null;
+	readyState = BackpressuredWebSocket.CONNECTING;
+	sent: Uint8Array[] = [];
+
+	constructor(url: string) {
+		this.url = url;
+		this.bufferedAmount = BackpressuredWebSocket.initialBufferedAmount;
+		BackpressuredWebSocket.instances.push(this);
+	}
+
+	send(data: Uint8Array): void {
+		this.sent.push(data);
+		this.bufferedAmount += data.byteLength;
+	}
+
+	open(): void {
+		this.readyState = BackpressuredWebSocket.OPEN;
+		this.onopen?.(new Event("open"));
+	}
+
+	close(): void {
+		if (this.readyState === BackpressuredWebSocket.CLOSED) return;
+		this.readyState = BackpressuredWebSocket.CLOSED;
+		this.onclose?.({ code: 1000, reason: "closed" } as CloseEvent);
+	}
+}
+
+describe("CollabSocket send backpressure", () => {
+	afterEach(() => {
+		globalThis.WebSocket = ORIGINAL_WEBSOCKET;
+		vi.restoreAllMocks();
+		vi.useRealTimers();
+	});
+
+	it("queues open-socket sends while bufferedAmount is above the high-water mark", async () => {
+		vi.useFakeTimers();
+		vi.spyOn(crypto.subtle, "encrypt").mockResolvedValue(new Uint8Array([1, 2, 3, 4]).buffer);
+		BackpressuredWebSocket.instances = [];
+		BackpressuredWebSocket.initialBufferedAmount = HIGH_WATER_MARK;
+		globalThis.WebSocket = BackpressuredWebSocket as unknown as typeof WebSocket;
+		const socket = new CollabSocket({
+			wsUrl: "ws://localhost:8788/r/backpressure",
+			role: "host",
+			key: {} as CryptoKey,
+		});
+
+		try {
+			socket.connect();
+			const ws = BackpressuredWebSocket.instances[0];
+			if (!ws) throw new Error("CollabSocket did not construct a WebSocket");
+			ws.open();
+			socket.send({ t: "bye", reason: "slow relay" });
+			for (let flush = 0; flush < 5; flush++) await Promise.resolve();
+			expect(ws.sent).toHaveLength(0);
+
+			vi.advanceTimersByTime(DRAIN_RETRY_MS);
+			for (let flush = 0; flush < 5; flush++) await Promise.resolve();
+			expect(ws.sent).toHaveLength(0);
+
+			ws.bufferedAmount = 0;
+			vi.advanceTimersByTime(DRAIN_RETRY_MS);
+			for (let flush = 0; flush < 5; flush++) await Promise.resolve();
+			expect(ws.sent).toHaveLength(1);
+		} finally {
+			socket.close();
+		}
+	});
+
+	it("drains reconnect backlog through the same backpressure gate", async () => {
+		vi.useFakeTimers();
+		vi.spyOn(crypto.subtle, "encrypt").mockResolvedValue(new Uint8Array([1, 2, 3, 4]).buffer);
+		BackpressuredWebSocket.instances = [];
+		BackpressuredWebSocket.initialBufferedAmount = HIGH_WATER_MARK;
+		globalThis.WebSocket = BackpressuredWebSocket as unknown as typeof WebSocket;
+		const socket = new CollabSocket({
+			wsUrl: "ws://localhost:8788/r/backpressure",
+			role: "host",
+			key: {} as CryptoKey,
+		});
+
+		try {
+			socket.connect();
+			const ws = BackpressuredWebSocket.instances[0];
+			if (!ws) throw new Error("CollabSocket did not construct a WebSocket");
+			socket.send({ t: "bye", reason: "queued while disconnected" });
+			for (let flush = 0; flush < 5; flush++) await Promise.resolve();
+			expect(ws.sent).toHaveLength(0);
+
+			ws.open();
+			expect(ws.sent).toHaveLength(0);
+			vi.advanceTimersByTime(DRAIN_RETRY_MS);
+			for (let flush = 0; flush < 5; flush++) await Promise.resolve();
+			expect(ws.sent).toHaveLength(0);
+
+			ws.bufferedAmount = 0;
+			vi.advanceTimersByTime(DRAIN_RETRY_MS);
+			for (let flush = 0; flush < 5; flush++) await Promise.resolve();
+			expect(ws.sent).toHaveLength(1);
+		} finally {
+			socket.close();
+		}
+	});
+});