[android - fs] Writing to a file provided via dialog (content:// URI) results in a 0 byte file in the Documents picker

[NyaomiDEV]

Describe the bug

If you get a URI via the dialog plugin dialog.save() and then write to it via fs.writeFile(), the resulting document will be written, but it will be seen as being empty by other applications and Android's documents picker.

Reproduction

• const uri = await dialog.save()
• Choose the Downloads folder for example
• await fs.saveFile(uri, "test")
• Open the Documents UI and see that the file is listed as having 0 bytes

Expected behavior

No response

Full tauri info output

[✔] Environment
    - OS: Artix Linux Rolling Release x86_64 (X64) (Unknown DE on wayland)
    ✔ webkit2gtk-4.1: 2.50.6
    ✔ rsvg2: 2.61.4
    ✔ rustc: 1.86.0 (05f9846f8 2025-03-31)
    ✔ cargo: 1.86.0 (adf9b6ad1 2025-02-28)
    ✔ rustup: 1.28.2 (2026-02-27)
    ✔ Rust toolchain: stable-x86_64-unknown-linux-gnu (overridden by environment variable RUSTUP_TOOLCHAIN)
    - node: 25.8.1
    - yarn: 1.22.22
    - npm: 11.11.1
    - deno: deno 2.7.7

[-] Packages
    - tauri 🦀: 2.10.2
    - tauri-build 🦀: 2.5.5
    - wry 🦀: 0.54.1
    - tao 🦀: 0.34.5
    - tauri-cli 🦀: 2.7.0
    - @tauri-apps/api : 2.10.1
    - @tauri-apps/cli : 2.6.3

[-] Plugins
    - tauri-plugin-biometric 🦀: 2.3.2
    - @tauri-apps/plugin-biometric : 2.3.2
    - tauri-plugin-opener 🦀: 2.5.3
    - @tauri-apps/plugin-opener : 2.5.3
    - tauri-plugin-process 🦀: 2.3.1
    - @tauri-apps/plugin-process : 2.3.1
    - tauri-plugin-dialog 🦀: 2.6.0
    - @tauri-apps/plugin-dialog : 2.6.0
    - tauri-plugin-os 🦀: 2.3.2
    - @tauri-apps/plugin-os : 2.3.2
    - tauri-plugin-store 🦀: 2.4.2
    - @tauri-apps/plugin-store : 2.4.2
    - tauri-plugin-fs 🦀: 2.4.5
    - @tauri-apps/plugin-fs : 2.4.5
    - tauri-plugin-http 🦀: 2.5.7
    - @tauri-apps/plugin-http : 2.5.7

[-] App
    - build-type: bundle
    - CSP: unset
    - frontendDist: ../dist
    - devUrl: http://localhost:5173/
    - framework: Vue.js
    - bundler: Vite

Stack trace

Additional context

No response

[onehumandev]

We ran into this same issue, and created a local fix. We'll be posting a possible fix MR for tauri-plugin-fs as well, but this can act as a band-aid for the time being.

Fix explanation:

When tauri-plugin-fs opens a content:// URI for writing, its Kotlin implementation (FsPlugin.getFileDescriptor) calls ParcelFileDescriptor.detachFd() to hand a raw fd to Rust.
Detaching immediately invalidates the Java-side ParcelFileDescriptor, so the content provider (e.g. the Downloads provider) receives its close signal before any bytes are written.
The provider finalizes the file metadata at that point — recording 0 bytes — and subsequent writes through the orphaned fd never update the provider's bookkeeping.

The result is a 0-byte file on disk.

By performing the entire copy on the native Android side we keep the OutputStream (and its underlying ParcelFileDescriptor) alive for the full duration of the write, then close it normally so the content provider sees the correct final size.

Android implementation for fix, to be invoked as a Tauri command:

@InvokeArg
class CopyFileToUriArgs {
    lateinit var sourcePath: String
    lateinit var destUri: String
}

@TauriPlugin
class ContentResolverPlugin(private val activity: Activity): Plugin(activity) {

    /**
     * Copies a file from a local filesystem path to a content:// URI.
     *
     * @param invoke Tauri invoke containing [CopyFileToUriArgs]:
     *   - sourcePath: absolute filesystem path inside the app sandbox
     *   - destUri:    content:// URI obtained from a save-file dialog
     */
    @Command
    fun copyFileToUri(invoke: Invoke) {
        try {
            val args = invoke.parseArgs(CopyFileToUriArgs::class.java)
            val sourceFile = File(args.sourcePath)

            if (!sourceFile.exists()) {
                throw Error("Source file does not exist: ${args.sourcePath}")
            }

            val destUriParsed = Uri.parse(args.destUri)
            val outputStream = activity.contentResolver.openOutputStream(destUriParsed)
                ?: throw Error("Cannot open output stream for URI: ${args.destUri}")

            outputStream.use { output ->
                FileInputStream(sourceFile).use { input ->
                    input.copyTo(output)
                }
            }

            invoke.resolve(JSObject())
        } catch (t: Throwable) {
            invoke.reject(t.message ?: "Failed to copy file to content URI")
        }
    }
}

c. @velocitysystems