Error: Unable to Write File to applicationDirectory in Capacitor Filesystem

Problem Summary

Capacitor’s applicationDirectory points to the app bundle – the folder that contains the compiled resources. On both iOS and Android this folder is read‑only after installation, so any attempt to write a file there will be rejected by the underlying platform.

Typical Error Messages

• iOS: Error: Failed to write file: Permission denied (code 0x80000000)
• Android: Error: Failed to write file: Permission denied (Permission denied) (code 0x80000000)

Choosing the Correct Mutable Directory

Capacitor exposes several writable directories. Pick the one that matches your data’s persistence and backup requirements:

• documentsDirectory – persistent, backed up to iCloud (iOS) or device backup (Android).
• libraryDirectory – persistent on iOS; use Library/NoBackup subfolder to exclude from iCloud backup.
• cacheDirectory – temporary, not backed up; suitable for transient or large data.

Excluding Files from iCloud Backups

On iOS you can prevent a file from being backed up by setting the NSURLIsExcludedFromBackupKey attribute after creation, or by creating it directly in Library/NoBackup (which iOS treats as excluded by default). Example (Swift):

let url = FileManager.default.urls(for: .libraryDirectory, in: .userDomainMask).first!.appendingPathComponent("NoBackup").appendingPathComponent("secret.txt")
try? FileManager.default.createDirectory(at: url.deletingLastPathComponent(), withIntermediateDirectories: true, attributes: nil)
var values = URLResourceValues()
values.isExcludedFromBackup = true
try? url.setResourceValues(values)

Quick Fix Steps

1. Remove any writeFile calls that target applicationDirectory.
2. Choose documentsDirectory (or libraryDirectory for iOS) for data that must persist.
3. If the data must not be backed up, write it to Library/NoBackup on iOS or cacheDirectory on Android.
4. After creating the file, set the backup‑exclusion flag on iOS if needed.
5. Re‑run the app and verify that the file appears in the chosen directory and that no error is thrown.

Verification Checklist

• On iOS: Inspect the app bundle in Xcode’s Devices window – it should be read‑only.
• On iOS: Use FileManager to confirm isExcludedFromBackup is true for the file.
• On Android: Use Android Studio’s Device File Explorer to ensure the file lives in cache or files and not in the assets folder.

When to Ask for More Details

If you are using a Capacitor version earlier than 4, the directory constants and error codes differ slightly. In that case, let us know your Capacitor version so we can adjust the guidance.