diff --git a/opencloudApp/src/main/java/eu/opencloud/android/workers/ContentUriUploadCacheValidator.kt b/opencloudApp/src/main/java/eu/opencloud/android/workers/ContentUriUploadCacheValidator.kt
new file mode 100644
index 0000000000..3ffbf1cadc
--- /dev/null
+++ b/opencloudApp/src/main/java/eu/opencloud/android/workers/ContentUriUploadCacheValidator.kt
@@ -0,0 +1,27 @@
+/**
+ * openCloud Android client application
+ *
+ * Copyright (C) 2026 OpenCloud GmbH.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package eu.opencloud.android.workers
+
+internal object ContentUriUploadCacheValidator {
+    fun isValidCacheSize(
+        actualSize: Long,
+        expectedSize: Long,
+    ): Boolean =
+        actualSize > 0 && (expectedSize <= 0 || actualSize == expectedSize)
+}
diff --git a/opencloudApp/src/main/java/eu/opencloud/android/workers/UploadFileFromContentUriWorker.kt b/opencloudApp/src/main/java/eu/opencloud/android/workers/UploadFileFromContentUriWorker.kt
index 08eedd26e2..71782420c2 100644
--- a/opencloudApp/src/main/java/eu/opencloud/android/workers/UploadFileFromContentUriWorker.kt
+++ b/opencloudApp/src/main/java/eu/opencloud/android/workers/UploadFileFromContentUriWorker.kt
@@ -147,18 +147,37 @@ class UploadFileFromContentUriWorker(
         cachePath = localStorageProvider.getTemporalPath(account.name, ocTransfer.spaceId) +
                 File.separator + flatCacheName
 
-        // Re-copy if the cache file is missing or empty. A previous run may have copied it
-        // and then had it removed (e.g. by removeCacheFile() at the end of a successful run
-        // that the OS killed before bookkeeping). Only the contentUri from worker params is
-        // authoritative.
         val cacheFile = File(cachePath)
-        if (!cacheFile.exists() || cacheFile.length() == 0L) {
+        if (!isCacheFileReadyForUpload(cacheFile)) {
             checkDocumentFileExists()
             checkPermissionsToReadDocumentAreGranted()
             copyFileToLocalStorage()
         }
     }
 
+    private fun isCacheFileReadyForUpload(cacheFile: File): Boolean {
+        if (!cacheFile.exists()) return false
+
+        val cacheSize = cacheFile.length()
+        val isValidCacheSize = ContentUriUploadCacheValidator.isValidCacheSize(
+            actualSize = cacheSize,
+            expectedSize = ocTransfer.fileSize,
+        )
+        if (isValidCacheSize) return true
+
+        Timber.w(
+            "Cached upload file for %s has invalid size. expected=%d actual=%d. Deleting and recopying.",
+            contentUri,
+            ocTransfer.fileSize,
+            cacheSize,
+        )
+        if (!cacheFile.delete()) {
+            Timber.w("Could not delete invalid cached upload file: %s", cacheFile.absolutePath)
+        }
+        clearTusState()
+        return false
+    }
+
     private fun areParametersValid(): Boolean {
         val paramAccountName = workerParameters.inputData.getString(KEY_PARAM_ACCOUNT_NAME)
         val paramUploadPath = workerParameters.inputData.getString(KEY_PARAM_UPLOAD_PATH)
@@ -208,11 +227,14 @@ class UploadFileFromContentUriWorker(
     private fun copyFileToLocalStorage() {
         val documentFile = DocumentFile.fromSingleUri(appContext, contentUri)
         val cacheFile = File(cachePath)
+        val partFile = File("$cachePath.part")
         val cacheDir = cacheFile.parentFile
         if (cacheDir != null && !cacheDir.exists()) {
             cacheDir.mkdirs()
         }
-        cacheFile.createNewFile()
+        if (partFile.exists() && !partFile.delete()) {
+            Timber.w("Could not delete stale partial cache file: %s", partFile.absolutePath)
+        }
 
         // openInputStream can return null if the content provider is unavailable or permissions were revoked.
         // Failing here avoids silently uploading a 0-byte file.
@@ -221,18 +243,57 @@ class UploadFileFromContentUriWorker(
             Timber.e("Failed to open input stream for %s — content provider unavailable or permissions revoked", contentUri)
             throw LocalFileNotFoundException()
         }
-        val outputStream = FileOutputStream(cachePath)
-        inputStream.use { input ->
-            outputStream.use { output ->
-                input.copyTo(output)
+        try {
+            val outputStream = FileOutputStream(partFile)
+            inputStream.use { input ->
+                outputStream.use { output ->
+                    input.copyTo(output)
+                }
             }
+        } catch (throwable: Throwable) {
+            partFile.delete()
+            throw throwable
         }
 
-        // Guard against a truncated or empty copy (e.g. file deleted mid-read).
-        if (cacheFile.length() == 0L) {
-            Timber.e("Cache file is 0 bytes after copy from %s — source may have been deleted mid-read", contentUri)
+        val copiedSize = partFile.length()
+        if (!ContentUriUploadCacheValidator.isValidCacheSize(copiedSize, ocTransfer.fileSize)) {
+            Timber.e(
+                "Partial cache copy from %s. expected=%d actual=%d",
+                contentUri,
+                ocTransfer.fileSize,
+                copiedSize,
+            )
+            partFile.delete()
+            clearTusState()
+            throw IOException(
+                "Cache copy size mismatch for $contentUri: " +
+                    "expected ${ocTransfer.fileSize} bytes, copied $copiedSize bytes"
+            )
+        }
+
+        if (cacheFile.exists() && !cacheFile.delete()) {
+            partFile.delete()
+            throw IOException("Could not replace cached upload file: ${cacheFile.absolutePath}")
+        }
+        if (!partFile.renameTo(cacheFile)) {
+            partFile.delete()
+            throw IOException("Could not finalize cached upload file: ${cacheFile.absolutePath}")
+        }
+
+        val finalSize = cacheFile.length()
+        if (!ContentUriUploadCacheValidator.isValidCacheSize(finalSize, ocTransfer.fileSize)) {
+            Timber.e(
+                "Invalid finalized cache copy from %s. expected=%d actual=%d",
+                contentUri,
+                ocTransfer.fileSize,
+                finalSize,
+            )
             cacheFile.delete()
-            throw LocalFileNotFoundException()
+            clearTusState()
+            throw IOException(
+                "Final cache copy size mismatch for $contentUri: " +
+                    "expected ${ocTransfer.fileSize} bytes, copied $finalSize bytes"
+            )
         }
 
         transferRepository.updateTransferSourcePath(uploadIdInStorageManager, contentUri.toString())
diff --git a/opencloudApp/src/test/java/eu/opencloud/android/workers/ContentUriUploadCacheValidatorTest.kt b/opencloudApp/src/test/java/eu/opencloud/android/workers/ContentUriUploadCacheValidatorTest.kt
new file mode 100644
index 0000000000..0b2d88a9a1
--- /dev/null
+++ b/opencloudApp/src/test/java/eu/opencloud/android/workers/ContentUriUploadCacheValidatorTest.kt
@@ -0,0 +1,88 @@
+/**
+ * openCloud Android client application
+ *
+ * Copyright (C) 2026 OpenCloud GmbH.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package eu.opencloud.android.workers
+
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class ContentUriUploadCacheValidatorTest {
+
+    @Test
+    fun `exact large cache size is valid`() {
+        val fileSize = 5_832_800_958L
+
+        val isValid = ContentUriUploadCacheValidator.isValidCacheSize(
+            actualSize = fileSize,
+            expectedSize = fileSize,
+        )
+
+        assertTrue(isValid)
+    }
+
+    @Test
+    fun `partial non-zero cache size is invalid`() {
+        val isValid = ContentUriUploadCacheValidator.isValidCacheSize(
+            actualSize = 1_180_123_136L,
+            expectedSize = 5_832_800_958L,
+        )
+
+        assertFalse(isValid)
+    }
+
+    @Test
+    fun `larger than expected cache size is invalid`() {
+        val isValid = ContentUriUploadCacheValidator.isValidCacheSize(
+            actualSize = 5_832_800_959L,
+            expectedSize = 5_832_800_958L,
+        )
+
+        assertFalse(isValid)
+    }
+
+    @Test
+    fun `zero byte cache size is invalid`() {
+        val isValid = ContentUriUploadCacheValidator.isValidCacheSize(
+            actualSize = 0L,
+            expectedSize = 5_832_800_958L,
+        )
+
+        assertFalse(isValid)
+    }
+
+    @Test
+    fun `unknown expected size keeps existing non-zero behavior`() {
+        val isValid = ContentUriUploadCacheValidator.isValidCacheSize(
+            actualSize = 42L,
+            expectedSize = -1L,
+        )
+
+        assertTrue(isValid)
+    }
+
+    @Test
+    fun `non-positive expected size still rejects zero byte cache`() {
+        val isValid = ContentUriUploadCacheValidator.isValidCacheSize(
+            actualSize = 0L,
+            expectedSize = -1L,
+        )
+
+        assertFalse(isValid)
+    }
+}