diff --git a/packages/runtime-browser/src/index.ts b/packages/runtime-browser/src/index.ts
index 8f84a20..6a3efb9 100644
--- a/packages/runtime-browser/src/index.ts
+++ b/packages/runtime-browser/src/index.ts
@@ -32,7 +32,7 @@ export interface AutterBrowserOptions {
 	release?: string;
 	/** Send a session_start ping on init (default true). */
 	sessionTracking?: boolean;
-	/** Last-chance hook: mutate or drop (return null) an event before send. */
+	/** Last-chance hook: mutate or drop (return null) an event before send. Throwing drops the event. */
 	beforeSend?: (event: BrowserEvent) => BrowserEvent | null;
 	/** Observe failed fetch and XHR requests and 5xx responses (default true). */
 	captureNetworkFailures?: boolean;
@@ -311,9 +311,14 @@ function enqueue(event: BrowserEvent, urgent?: boolean): void {
 	// Scrub before beforeSend so the last-chance hook sees the final form.
 	if (event.context) event.context = redactContext(event.context);
 	if (opts.beforeSend) {
-		const mapped = opts.beforeSend(event);
-		if (!mapped) return;
-		event = mapped;
+		try {
+			const mapped = opts.beforeSend(event);
+			if (!mapped) return;
+			event = mapped;
+		} catch {
+			// A telemetry hook must not change application request outcomes.
+			return;
+		}
 	}
 	queue.push(event);
 	if (queue.length >= MAX_QUEUE) {
diff --git a/packages/runtime-browser/test/before-send.test.mjs b/packages/runtime-browser/test/before-send.test.mjs
new file mode 100644
index 0000000..2db1f10
--- /dev/null
+++ b/packages/runtime-browser/test/before-send.test.mjs
@@ -0,0 +1,59 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { captureMessage, flush, initAutterBrowser } from "../dist/index.js";
+
+test("beforeSend failures preserve application requests and later delivery", async () => {
+	const response = new Response("unavailable", { status: 503 });
+	const networkError = new Error("original network failure");
+	const hookError = new Error("beforeSend failure");
+	const sent = [];
+	let rejectRequest = false;
+	let hookMode = "throw";
+	globalThis.window = {
+		fetch: async () => {
+			if (rejectRequest) throw networkError;
+			return response;
+		},
+		addEventListener() {},
+	};
+	globalThis.document = { addEventListener() {}, visibilityState: "visible" };
+	globalThis.location = { href: "https://app.example.test/", pathname: "/" };
+	Object.defineProperty(globalThis, "navigator", {
+		configurable: true,
+		value: {
+			sendBeacon(_url, body) { sent.push(body); return true; },
+		},
+	});
+	initAutterBrowser({
+		endpoint: "/api/autter-runtime",
+		service: "web",
+		sessionTracking: false,
+		captureActions: false,
+		captureTimings: false,
+		beforeSend(event) {
+			if (hookMode === "throw") throw hookError;
+			if (hookMode === "drop") return null;
+			return { ...event, message: "mapped message" };
+		},
+	});
+
+	assert.equal(await window.fetch("/checkout"), response);
+	rejectRequest = true;
+	await assert.rejects(window.fetch("/offline"), (error) => error === networkError);
+	assert.doesNotThrow(() => captureMessage("failed hook"));
+	flush();
+	assert.equal(sent.length, 0);
+
+	hookMode = "drop";
+	captureMessage("dropped message");
+	flush();
+	assert.equal(sent.length, 0);
+
+	hookMode = "map";
+	captureMessage("healthy message");
+	flush();
+	assert.equal(sent.length, 1);
+	const payload = JSON.parse(await sent[0].text());
+	assert.equal(payload.events.length, 1);
+	assert.equal(payload.events[0].message, "mapped message");
+});