diff --git a/apps/backend/AGENTS.md b/apps/backend/AGENTS.md
index 6185635d..4315bcf8 100644
--- a/apps/backend/AGENTS.md
+++ b/apps/backend/AGENTS.md
@@ -260,6 +260,30 @@ failure is logged before the next step runs; `exit(0)` is still attempted after
 three steps. Later signals remain ignored. The direct regression coverage lives in
 `src/main.test.ts` under `termination shutdown lifecycle`.
 
+## SIGUSR2 UDEV HOTPLUG HOOK [EXISTS]
+
+`main.ts`'s `process.on("SIGUSR2", udevDeviceUpdate)` re-scans Cam Link USB2 +
+audio devices on an Elgato/USB-audio hot(un)plug. The signal is delivered by two
+udev rules in `deployment/` (`98-ceralive-audio.rules`,
+`99-ceralive-check-usb-devices.rules`) that MUST target the unit's main pid:
+
+```
+RUN+="/usr/bin/systemctl kill --kill-whom=main --signal=SIGUSR2 ceralive.service"
+```
+
+NOT `pkill -f ceralive` (the retired form). `pkill -f` substring-matched
+avahi-daemon's process title `avahi-daemon: registering [ceralive.local]` (the
+device hostname is `ceralive.local`) and killed mDNS on every hotplug.
+`--kill-whom=main` scopes the signal to the tracked MAIN pid via the unit cgroup —
+mirroring the old `pkill -o` single-process intent — so the whole-cgroup default
+(`all`) can never collaterally SIGUSR2-terminate `srtla_send`/`bcrpt` (which share
+`ceralive.service`'s cgroup while streaming and do NOT handle SIGUSR2). `systemctl
+kill` from a udev `RUN+=` is safe: it creates no systemd job (returns after the
+PID-1 D-Bus request), so the `--no-block` / deadlock caveats that apply to
+`start`/`stop`/`restart` do not apply. Regression lock:
+`src/tests/udev-rules-sigusr2-scope.test.ts` (static assertion on the shipped rule
+files). Do NOT reintroduce a broad `pkill`, and do NOT drop `--kill-whom=main`.
+
 ## BROADCAST EVENTS
 
 The backend pushes typed events to all connected clients via `rpc/events.ts`. Each event type carries a monotonic `seq` counter (`Map<string, number>`) that resets to 0 on server restart.
diff --git a/apps/backend/src/tests/udev-rules-sigusr2-scope.test.ts b/apps/backend/src/tests/udev-rules-sigusr2-scope.test.ts
new file mode 100644
index 00000000..a3293d3b
--- /dev/null
+++ b/apps/backend/src/tests/udev-rules-sigusr2-scope.test.ts
@@ -0,0 +1,119 @@
+/*
+ * Regression guard for the SIGUSR2 udev-hotplug delivery mechanism.
+ *
+ * BUG (confirmed live on real hardware): the two hotplug udev rules signalled the
+ * backend with `pkill -o -SIGUSR2 -f ceralive`. `pkill -f` substring-matches the
+ * FULL command line of EVERY process. Avahi renames its own process title to
+ * `avahi-daemon: registering [<hostname>.local]` once it registers a hostname;
+ * this device's hostname is `ceralive.local`, so avahi-daemon's title contains
+ * the substring "ceralive" and was caught by the same broad pkill. avahi-daemon
+ * (no `Restart=`) died on every USB/audio hotplug and mDNS stayed dead until
+ * reboot.
+ *
+ * FIX: deliver the signal through `systemctl kill --kill-whom=main
+ * --signal=SIGUSR2 ceralive.service`, which targets exactly the unit's tracked
+ * MAIN pid via its cgroup — never another process. `--kill-whom=main` also
+ * preserves the old `pkill -o` single-process intent, so the whole-cgroup default
+ * (`all`) can never collaterally SIGUSR2-terminate srtla_send/bcrpt (which share
+ * the cgroup while streaming and do NOT handle SIGUSR2).
+ *
+ * This suite is a static assertion on the shipped rule FILES (the source of truth
+ * copied verbatim into the .deb via scripts/build/shared-build-functions.sh), so
+ * a future edit that reintroduces the broad `pkill -f` — or drops the main-pid
+ * scoping — goes red here.
+ */
+import { describe, expect, it } from "bun:test";
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+// src/tests -> src -> backend -> apps -> CeraUI repo root, then deployment/.
+const DEPLOYMENT_DIR = join(
+	import.meta.dir,
+	"..",
+	"..",
+	"..",
+	"..",
+	"deployment",
+);
+
+const AUDIO_RULES = join(DEPLOYMENT_DIR, "98-ceralive-audio.rules");
+const USB_RULES = join(DEPLOYMENT_DIR, "99-ceralive-check-usb-devices.rules");
+
+// The exact scoped delivery idiom every RUN+= hotplug signal must now use.
+const SCOPED_SIGNAL =
+	"/usr/bin/systemctl kill --kill-whom=main --signal=SIGUSR2 ceralive.service";
+
+/** Non-comment, non-blank rule lines (udev comments start with `#`). */
+function ruleLines(contents: string): string[] {
+	return contents
+		.split("\n")
+		.map((l) => l.trim())
+		.filter((l) => l.length > 0 && !l.startsWith("#"));
+}
+
+/** Every RUN+= assignment target in the file, unquoted. */
+function runTargets(contents: string): string[] {
+	const targets: string[] = [];
+	const re = /RUN\+="([^"]*)"/g;
+	for (const line of ruleLines(contents)) {
+		let m: RegExpExecArray | null = re.exec(line);
+		while (m !== null) {
+			// biome-ignore lint/style/noNonNullAssertion: capture group 1 always present on match
+			targets.push(m[1]!);
+			m = re.exec(line);
+		}
+	}
+	return targets;
+}
+
+describe("udev SIGUSR2 hotplug rules — scoped, avahi-safe delivery", () => {
+	const audio = readFileSync(AUDIO_RULES, "utf8");
+	const usb = readFileSync(USB_RULES, "utf8");
+
+	it("neither rule uses the broad `pkill -f ceralive` that killed avahi-daemon", () => {
+		// Assert against ACTIVE rule lines only — a `#`-comment may legitimately
+		// name the retired `pkill -f ceralive` idiom to explain the fix.
+		const activeLines = [...ruleLines(audio), ...ruleLines(usb)];
+		expect(activeLines.length).toBeGreaterThan(0);
+		for (const line of activeLines) {
+			// The literal defect: a full-command-line substring match on "ceralive".
+			expect(line).not.toMatch(/pkill[^\n]*-f[^\n]*ceralive/);
+		}
+		// No `pkill` at all remains in either RUN+= target.
+		for (const target of [...runTargets(audio), ...runTargets(usb)]) {
+			expect(target).not.toContain("pkill");
+		}
+	});
+
+	it("both rules deliver SIGUSR2 via the unit-scoped systemctl kill idiom", () => {
+		expect(runTargets(audio)).toContain(SCOPED_SIGNAL);
+		expect(runTargets(usb)).toContain(SCOPED_SIGNAL);
+	});
+
+	it("every RUN+= target scopes the signal to the ceralive.service MAIN pid", () => {
+		const targets = [...runTargets(audio), ...runTargets(usb)];
+		expect(targets.length).toBeGreaterThan(0);
+		for (const target of targets) {
+			// Must name the unit explicitly (cgroup scoping) …
+			expect(target).toContain("ceralive.service");
+			// … and target only the main pid, so a whole-cgroup SIGUSR2 can never
+			// terminate srtla_send/bcrpt during a live stream.
+			expect(target).toContain("--kill-whom=main");
+			expect(target).toContain("--signal=SIGUSR2");
+			// udev RUN+= is exec'd without a shell → absolute path is mandatory.
+			expect(target.startsWith("/usr/bin/systemctl ")).toBe(true);
+		}
+	});
+
+	it("audio rule: every signal path converges on the single scoped RUN+= line", () => {
+		// All GOTO branches jump to LABEL="signal_ceralive"; the label must be
+		// backed by exactly one RUN+= line so no path can reach an unscoped kill.
+		const gotoCount = (audio.match(/GOTO="signal_ceralive"/g) ?? []).length;
+		expect(gotoCount).toBeGreaterThan(0);
+		expect(audio).toContain('LABEL="signal_ceralive"');
+
+		const audioTargets = runTargets(audio);
+		expect(audioTargets).toHaveLength(1);
+		expect(audioTargets[0]).toBe(SCOPED_SIGNAL);
+	});
+});
diff --git a/deployment/98-ceralive-audio.rules b/deployment/98-ceralive-audio.rules
index 30de6d78..9c8e1fae 100644
--- a/deployment/98-ceralive-audio.rules
+++ b/deployment/98-ceralive-audio.rules
@@ -22,6 +22,11 @@ ATTR{id}=="OsmoAction5*", ATTR{id}="OsmoAction5", GOTO="signal_ceralive"
 ATTR{id}="usbaudio"
 
 LABEL="signal_ceralive"
-RUN+="/usr/bin/pkill -o -SIGUSR2 -f ceralive"
+# Target the ceralive.service unit's MAIN pid via its cgroup — NOT
+# `pkill -f ceralive`, which substring-matched avahi-daemon's process title
+# (`avahi-daemon: registering [ceralive.local]`) and killed mDNS. --kill-whom=main
+# preserves the old `pkill -o` single-process intent without signalling
+# srtla_send/bcrpt in the same cgroup (SIGUSR2 would terminate them).
+RUN+="/usr/bin/systemctl kill --kill-whom=main --signal=SIGUSR2 ceralive.service"
 
 LABEL="end"
diff --git a/deployment/99-ceralive-check-usb-devices.rules b/deployment/99-ceralive-check-usb-devices.rules
index 11545167..d835dd50 100644
--- a/deployment/99-ceralive-check-usb-devices.rules
+++ b/deployment/99-ceralive-check-usb-devices.rules
@@ -1 +1,7 @@
-ENV{DEVTYPE}=="usb_device", ENV{ID_VENDOR_ID}=="0fd9", RUN+="/usr/bin/pkill -o -SIGUSR2 -f ceralive"
+# Elgato/Cam Link (vendor 0fd9) hot(un)plug: poke the CeraUI backend to re-scan
+# USB2 capture + audio devices. Target the ceralive.service unit's MAIN pid via
+# its cgroup — NOT `pkill -f ceralive`, which substring-matched avahi-daemon's
+# process title (`avahi-daemon: registering [ceralive.local]`) and killed mDNS.
+# --kill-whom=main preserves the old `pkill -o` single-process intent without
+# signalling srtla_send/bcrpt in the same cgroup (SIGUSR2 would terminate them).
+ENV{DEVTYPE}=="usb_device", ENV{ID_VENDOR_ID}=="0fd9", RUN+="/usr/bin/systemctl kill --kill-whom=main --signal=SIGUSR2 ceralive.service"