diff --git a/app/src/lib/channels/use-channel-events.ts b/app/src/lib/channels/use-channel-events.ts
index 51958ebb8..ed714cc50 100644
--- a/app/src/lib/channels/use-channel-events.ts
+++ b/app/src/lib/channels/use-channel-events.ts
@@ -20,10 +20,50 @@ export type ChannelResyncEvent = { resync: true };
 /** What arrives on the socket. `resync` is the discriminant; an activity event never carries it. */
 export type ChannelSocketMessage = ChannelActivityEvent | ChannelResyncEvent;
 
-export function isResync(
-  message: ChannelSocketMessage,
-): message is ChannelResyncEvent {
-  return (message as ChannelResyncEvent).resync === true;
+export function isResync(message: unknown): message is ChannelResyncEvent {
+  return (
+    typeof message === "object" &&
+    message !== null &&
+    (message as ChannelResyncEvent).resync === true
+  );
+}
+
+/**
+ * Whether a parsed socket payload has the shape of anything this roster handles.
+ *
+ * The `try` around `JSON.parse` is not enough: `JSON.parse("null")` succeeds with
+ * `null`, and `JSON.parse("5")` succeeds with `5`, and both used to reach `isResync`
+ * — `null.resync` throwing a `TypeError` inside `onmessage` for the first, and a
+ * spurious roster-wide refetch for the second when the channel id came back
+ * `undefined`. Binary frames arrive as `Blob` rather than text and never parse.
+ */
+export function isChannelSocketMessage(
+  value: unknown,
+): value is ChannelSocketMessage {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}
+
+/**
+ * Parse one socket frame into something the roster can act on, or `null` to drop it.
+ *
+ * Pure and exported so the drop rules are provable without a socket: unparseable
+ * text, non-object JSON (`null`, numbers, strings, arrays) and activity events with
+ * no string channel id are all ignored rather than crashing or refetching the roster.
+ */
+export function parseChannelSocketMessage(
+  data: unknown,
+): ChannelSocketMessage | null {
+  if (typeof data !== "string") return null;
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(data);
+  } catch {
+    return null;
+  }
+  if (!isChannelSocketMessage(parsed)) return null;
+  if (isResync(parsed)) return parsed;
+  if (typeof parsed.channelId !== "string") return null;
+  return parsed;
 }
 
 export type ChannelActivityEvent = {
@@ -183,12 +223,8 @@ export function useChannelEvents() {
       };
 
       socket.onmessage = (message) => {
-        let parsed: ChannelSocketMessage;
-        try {
-          parsed = JSON.parse(message.data as string);
-        } catch {
-          return;
-        }
+        const parsed = parseChannelSocketMessage(message.data);
+        if (!parsed) return;
 
         // Refetch rather than patch: there is no delta to apply. Checked before anything reads
         // `channelId`, because this message has none.
diff --git a/app/tests/channel-socket-message.test.ts b/app/tests/channel-socket-message.test.ts
new file mode 100644
index 000000000..7a9330b25
--- /dev/null
+++ b/app/tests/channel-socket-message.test.ts
@@ -0,0 +1,80 @@
+import { describe, expect, test } from "bun:test";
+import {
+  isChannelSocketMessage,
+  isResync,
+  parseChannelSocketMessage,
+} from "../src/lib/channels/use-channel-events";
+
+const activity = {
+  channelId: "a",
+  lastMessage: "Said something.",
+  lastMessageAt: "2024-04-01T00:00:00.000Z",
+  lastMessageAgentId: null,
+};
+
+describe("isResync", () => {
+  test("matches a resync event", () => {
+    expect(isResync({ resync: true })).toBe(true);
+  });
+
+  test("rejects an activity event", () => {
+    expect(isResync(activity)).toBe(false);
+  });
+
+  for (const value of [null, undefined, 5, "x", [], {}, { resync: false }]) {
+    test(`does not throw for ${String(JSON.stringify(value))}`, () => {
+      expect(isResync(value)).toBe(false);
+    });
+  }
+});
+
+describe("isChannelSocketMessage", () => {
+  test.each([{ resync: true }, activity, { channelId: "a" }])(
+    "accepts %p",
+    (value) => {
+      expect(isChannelSocketMessage(value)).toBe(true);
+    },
+  );
+
+  for (const value of [null, undefined, 5, "x", [], [activity]]) {
+    test(`rejects ${String(JSON.stringify(value))}`, () => {
+      expect(isChannelSocketMessage(value)).toBe(false);
+    });
+  }
+});
+
+describe("parseChannelSocketMessage", () => {
+  test("parses a resync event", () => {
+    expect(parseChannelSocketMessage('{"resync":true}')).toEqual({
+      resync: true,
+    });
+  });
+
+  test("parses an activity event", () => {
+    expect(parseChannelSocketMessage(JSON.stringify(activity))).toEqual(
+      activity,
+    );
+  });
+
+  for (const raw of ["null", "5", '"x"', "[1]", ""]) {
+    test(`drops JSON payload ${raw}`, () => {
+      expect(parseChannelSocketMessage(raw)).toBeNull();
+    });
+  }
+
+  test("drops unparseable text", () => {
+    expect(parseChannelSocketMessage("not json {")).toBeNull();
+  });
+
+  test("drops an activity event with no channel id", () => {
+    expect(
+      parseChannelSocketMessage(JSON.stringify({ lastMessage: "hi" })),
+    ).toBeNull();
+  });
+
+  for (const data of [null, undefined, 5, {}, []]) {
+    test(`drops non-string frame ${String(JSON.stringify(data))}`, () => {
+      expect(parseChannelSocketMessage(data)).toBeNull();
+    });
+  }
+});