diff --git a/packages/web/src/utils/notificationFilters.test.ts b/packages/web/src/utils/notificationFilters.test.ts
new file mode 100644
index 00000000..593f9bfe
--- /dev/null
+++ b/packages/web/src/utils/notificationFilters.test.ts
@@ -0,0 +1,135 @@
+import { describe, it, expect } from 'vitest';
+import { shouldPlayMessageSound } from './notificationFilters';
+
+describe('shouldPlayMessageSound', () => {
+  const myIds = new Set(['local-snowflake', 'home-uid-42']);
+
+  it('suppresses messages authored by self (local id)', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'local-snowflake',
+        myIds,
+        isDmChannel: true,
+        content: 'hi',
+        allChannels: false,
+      }),
+    ).toBe(false);
+  });
+
+  it('suppresses messages authored by self (home id)', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'home-uid-42',
+        myIds,
+        isDmChannel: true,
+        content: 'hi',
+        allChannels: false,
+      }),
+    ).toBe(false);
+  });
+
+  it('plays for DM messages from others', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'someone-else',
+        myIds,
+        isDmChannel: true,
+        content: 'yo',
+        allChannels: false,
+      }),
+    ).toBe(true);
+  });
+
+  it('suppresses non-DM, non-mention messages from others', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'someone-else',
+        myIds,
+        isDmChannel: false,
+        content: 'general chatter',
+        allChannels: false,
+      }),
+    ).toBe(false);
+  });
+
+  it('plays when content mentions me by local id', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'someone-else',
+        myIds,
+        isDmChannel: false,
+        content: 'hey <@local-snowflake> look',
+        allChannels: false,
+      }),
+    ).toBe(true);
+  });
+
+  it('plays when content mentions me by home id (federated)', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'someone-else',
+        myIds,
+        isDmChannel: false,
+        content: 'cc <@home-uid-42>',
+        allChannels: false,
+      }),
+    ).toBe(true);
+  });
+
+  it('does not play for mentions of someone else', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'someone-else',
+        myIds,
+        isDmChannel: false,
+        content: 'pinging <@third-party>',
+        allChannels: false,
+      }),
+    ).toBe(false);
+  });
+
+  it('plays for any non-self message when allChannels=true', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'someone-else',
+        myIds,
+        isDmChannel: false,
+        content: 'general chatter',
+        allChannels: true,
+      }),
+    ).toBe(true);
+  });
+
+  it('still suppresses self-authored even when allChannels=true', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'local-snowflake',
+        myIds,
+        isDmChannel: false,
+        content: 'my own message',
+        allChannels: true,
+      }),
+    ).toBe(false);
+  });
+
+  it('handles null content (attachment-only) gracefully', () => {
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'someone-else',
+        myIds,
+        isDmChannel: true,
+        content: null,
+        allChannels: false,
+      }),
+    ).toBe(true);
+    expect(
+      shouldPlayMessageSound({
+        authorUserId: 'someone-else',
+        myIds,
+        isDmChannel: false,
+        content: null,
+        allChannels: false,
+      }),
+    ).toBe(false);
+  });
+});
diff --git a/packages/web/src/utils/notificationFilters.ts b/packages/web/src/utils/notificationFilters.ts
new file mode 100644
index 00000000..073d61f8
--- /dev/null
+++ b/packages/web/src/utils/notificationFilters.ts
@@ -0,0 +1,29 @@
+/**
+ * Decides whether a freshly-arrived chat message should fire the in-app
+ * `message.mp3` cue. Pure, federation-aware (matches against any of the
+ * caller's known self-ids).
+ *
+ * Rule (Discord-default):
+ *   - Suppress messages authored by self (any id in myIds).
+ *   - When allChannels=true, fire for every non-self message.
+ *   - Otherwise, fire only if the channel is a DM, or if the content contains
+ *     a `<@${id}>` mention for any id in myIds.
+ */
+export interface ShouldPlayMessageSoundInput {
+  authorUserId: string;
+  myIds: Set<string>;
+  isDmChannel: boolean;
+  content: string | null;
+  allChannels: boolean;
+}
+
+export function shouldPlayMessageSound(input: ShouldPlayMessageSoundInput): boolean {
+  if (input.myIds.has(input.authorUserId)) return false;
+  if (input.allChannels) return true;
+  if (input.isDmChannel) return true;
+  if (!input.content) return false;
+  for (const id of input.myIds) {
+    if (input.content.includes(`<@${id}>`)) return true;
+  }
+  return false;
+}