diff --git a/packages/app/src/attachments/utils.ts b/packages/app/src/attachments/utils.ts
index f6a78f6a0..397a67f65 100644
--- a/packages/app/src/attachments/utils.ts
+++ b/packages/app/src/attachments/utils.ts
@@ -98,7 +98,7 @@ export function createPreviewAttachmentId(input: {
 export async function blobToBase64(blob: Blob): Promise<string> {
   return await new Promise<string>((resolve, reject) => {
     const reader = new FileReader();
-    reader.onload = () => {
+    reader.addEventListener("load", () => {
       if (typeof reader.result !== "string") {
         reject(new Error("Unexpected FileReader result while encoding attachment."));
         return;
@@ -109,10 +109,10 @@ export async function blobToBase64(blob: Blob): Promise<string> {
         return;
       }
       resolve(payload);
-    };
-    reader.onerror = () => {
+    });
+    reader.addEventListener("error", () => {
       reject(reader.error ?? new Error("Failed to read attachment blob."));
-    };
+    });
     reader.readAsDataURL(blob);
   });
 }
diff --git a/packages/app/src/attachments/web/indexeddb-attachment-store.ts b/packages/app/src/attachments/web/indexeddb-attachment-store.ts
index e2352749b..7d5ee4483 100644
--- a/packages/app/src/attachments/web/indexeddb-attachment-store.ts
+++ b/packages/app/src/attachments/web/indexeddb-attachment-store.ts
@@ -40,13 +40,13 @@ function openAttachmentDb(): Promise<IDBDatabase> {
       }
     };
 
-    request.onsuccess = () => {
+    request.addEventListener("success", () => {
       resolve(request.result);
-    };
+    });
 
-    request.onerror = () => {
+    request.addEventListener("error", () => {
       reject(request.error ?? new Error("Failed to open attachment IndexedDB."));
-    };
+    });
   });
 }
 
@@ -60,17 +60,17 @@ function runTx<T>(
     const store = transaction.objectStore(STORE_NAME);
     const request = run(store);
 
-    request.onsuccess = () => {
+    request.addEventListener("success", () => {
       resolve(request.result as T);
-    };
+    });
 
-    request.onerror = () => {
+    request.addEventListener("error", () => {
       reject(request.error ?? new Error("IndexedDB transaction request failed."));
-    };
+    });
 
-    transaction.onerror = () => {
+    transaction.addEventListener("error", () => {
       reject(transaction.error ?? new Error("IndexedDB transaction failed."));
-    };
+    });
   });
 }
 
@@ -203,13 +203,13 @@ export function createIndexedDbAttachmentStore(): AttachmentStore {
           const store = tx.objectStore(STORE_NAME);
           const cursorRequest = store.openCursor();
 
-          cursorRequest.onerror = () => {
+          cursorRequest.addEventListener("error", () => {
             reject(
               cursorRequest.error ?? new Error("Failed to iterate IndexedDB attachment store."),
             );
-          };
+          });
 
-          cursorRequest.onsuccess = () => {
+          cursorRequest.addEventListener("success", () => {
             const cursor = cursorRequest.result;
             if (!cursor) {
               resolve();
@@ -221,11 +221,11 @@ export function createIndexedDbAttachmentStore(): AttachmentStore {
               cursor.delete();
             }
             cursor.continue();
-          };
+          });
 
-          tx.onerror = () => {
+          tx.addEventListener("error", () => {
             reject(tx.error ?? new Error("Failed to garbage collect IndexedDB attachments."));
-          };
+          });
         });
       } finally {
         db.close();
diff --git a/packages/app/src/hooks/use-audio-recorder.web.ts b/packages/app/src/hooks/use-audio-recorder.web.ts
index 2e783bac3..65794d450 100644
--- a/packages/app/src/hooks/use-audio-recorder.web.ts
+++ b/packages/app/src/hooks/use-audio-recorder.web.ts
@@ -242,10 +242,10 @@ export function useAudioRecorder(config?: AudioCaptureConfig) {
       }
     };
 
-    recorder.onerror = (event) => {
+    recorder.addEventListener("error", (event) => {
       const error = (event as { error?: Error }).error;
       console.error("[AudioRecorder][Web] Recorder error", error ?? event);
-    };
+    });
 
     const timeslice = options?.enableContinuousRecording ? 1000 : undefined;
 
diff --git a/packages/app/src/hooks/use-dictation-audio-source.web.ts b/packages/app/src/hooks/use-dictation-audio-source.web.ts
index 5e20c6a34..344f8a05a 100644
--- a/packages/app/src/hooks/use-dictation-audio-source.web.ts
+++ b/packages/app/src/hooks/use-dictation-audio-source.web.ts
@@ -283,9 +283,9 @@ export function useDictationAudioSource(config: DictationAudioSourceConfig): Dic
         recorderRefs.audioChunks.push(data);
       }
     };
-    recorder.onerror = (event: any) => {
+    recorder.addEventListener("error", (event: Event) => {
       recorderRefs.stoppedReject?.(event);
-    };
+    });
     recorder.addEventListener("stop", () => {
       try {
         const blob =
diff --git a/packages/app/src/utils/os-notifications.ts b/packages/app/src/utils/os-notifications.ts
index 16bff8c30..8762c5d88 100644
--- a/packages/app/src/utils/os-notifications.ts
+++ b/packages/app/src/utils/os-notifications.ts
@@ -14,7 +14,7 @@ export interface WebNotificationClickDetail {
 }
 
 interface WebNotificationInstance {
-  onclick?: ((event: Event) => void) | null;
+  addEventListener: (type: "click", listener: (event: Event) => void) => void;
 }
 
 export const WEB_NOTIFICATION_CLICK_EVENT = "paseo:web-notification-click";
@@ -144,12 +144,12 @@ function attachWebClickHandler(
   notification: WebNotificationInstance,
   data: Record<string, unknown> | undefined,
 ): void {
-  notification.onclick = () => {
+  notification.addEventListener("click", () => {
     const handledByApp = dispatchWebNotificationClick({ data });
     if (!handledByApp) {
       fallbackNavigateToNotificationTarget(data);
     }
-  };
+  });
 }
 
 export async function sendOsNotification(payload: OsNotificationPayload): Promise<boolean> {
diff --git a/packages/app/src/voice/audio-engine.web.ts b/packages/app/src/voice/audio-engine.web.ts
index e377c4f36..d6b17c609 100644
--- a/packages/app/src/voice/audio-engine.web.ts
+++ b/packages/app/src/voice/audio-engine.web.ts
@@ -192,9 +192,9 @@ export function createAudioEngine(
         fn();
       };
 
-      source.onended = () => {
+      source.addEventListener("ended", () => {
         settle(() => resolve(durationSec));
-      };
+      });
 
       try {
         source.start();
@@ -384,7 +384,6 @@ export function createAudioEngine(
         const active = refs.activePlayback;
         refs.activePlayback = null;
         try {
-          active.source.onended = null;
           active.source.stop();
         } catch {
           // Ignore best-effort stop errors.