diff --git a/Sources/NextcloudKit/NKSession.swift b/Sources/NextcloudKit/NKSession.swift
index fe598bfd..e8b69b2b 100644
--- a/Sources/NextcloudKit/NKSession.swift
+++ b/Sources/NextcloudKit/NKSession.swift
@@ -65,7 +65,8 @@ public struct NKSession: Sendable {
          groupIdentifier: String,
          httpMaximumConnectionsPerHost: Int,
          httpMaximumConnectionsPerHostInDownload: Int,
-         httpMaximumConnectionsPerHostInUpload: Int) {
+         httpMaximumConnectionsPerHostInUpload: Int,
+         sessionDataOverride: Alamofire.Session? = nil) {
         self.urlBase = urlBase
         self.user = user
         self.userId = userId
@@ -91,12 +92,12 @@ public struct NKSession: Sendable {
         #endif
 
         configurationSessionData.httpCookieStorage = HTTPCookieStorage.sharedCookieStorage(forGroupContainerIdentifier: sharedCookieStorage)
-        sessionData = Alamofire.Session(configuration: configurationSessionData,
-                                        delegate: NextcloudKitSessionDelegate(nkCommonInstance: nkCommonInstance),
-                                        rootQueue: nkCommonInstance.rootQueue,
-                                        requestQueue: nkCommonInstance.requestQueue,
-                                        serializationQueue: nkCommonInstance.serializationQueue,
-                                        eventMonitors: [NKMonitor(nkCommonInstance: nkCommonInstance)])
+        sessionData = sessionDataOverride ?? Alamofire.Session(configuration: configurationSessionData,
+                                                               delegate: NextcloudKitSessionDelegate(nkCommonInstance: nkCommonInstance),
+                                                               rootQueue: nkCommonInstance.rootQueue,
+                                                               requestQueue: nkCommonInstance.requestQueue,
+                                                               serializationQueue: nkCommonInstance.serializationQueue,
+                                                               eventMonitors: [NKMonitor(nkCommonInstance: nkCommonInstance)])
 
         // SessionDataNoCache Alamofire
         let configurationSessionDataNoCache = URLSessionConfiguration.af.default
diff --git a/Sources/NextcloudKit/NextcloudKit+Upload.swift b/Sources/NextcloudKit/NextcloudKit+Upload.swift
index 0f112319..1779ac73 100644
--- a/Sources/NextcloudKit/NextcloudKit+Upload.swift
+++ b/Sources/NextcloudKit/NextcloudKit+Upload.swift
@@ -438,7 +438,7 @@ public extension NextcloudKit {
         options.customHeader?["If-Match"] = nil
 
         guard moveRes.error == .success else {
-            return (account, nil)
+            throw moveRes.error
         }
 
         try Task.checkCancellation()
diff --git a/Tests/NextcloudKitUnitTests/ChunkedUploadAssemblyTests.swift b/Tests/NextcloudKitUnitTests/ChunkedUploadAssemblyTests.swift
index f409c915..3eb0c62e 100644
--- a/Tests/NextcloudKitUnitTests/ChunkedUploadAssemblyTests.swift
+++ b/Tests/NextcloudKitUnitTests/ChunkedUploadAssemblyTests.swift
@@ -2,11 +2,10 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 import XCTest
+import Alamofire
 @testable import NextcloudKit
 
-/// Unit tests for deriving the assembled file's `NKFile` from a chunk-assembly MOVE response's
-/// headers — the primary path that lets `uploadChunkAsync` skip the fragile PROPFIND read-back
-/// (whose failure otherwise surfaced as `errorChunkMoveFile`, code -9997).
+/// Unit tests for chunk-assembly MOVE failures and deriving the assembled file from response headers.
 final class ChunkedUploadAssemblyTests: XCTestCase {
     private func makeKit() -> NextcloudKit {
         #if swift(<6.0)
@@ -16,6 +15,64 @@ final class ChunkedUploadAssemblyTests: XCTestCase {
         #endif
     }
 
+    private func uploadThroughMockedAssembly(moveStatus: Int) async throws -> (error: NKError?, file: NKFile?) {
+        let kit = makeKit()
+        let account = UUID().uuidString
+        let urlBase = "https://assembly-\(moveStatus).test"
+        let configuration = URLSessionConfiguration.af.default
+        configuration.protocolClasses = [ChunkedUploadURLProtocol.self]
+        let session = NKSession(nkCommonInstance: kit.nkCommonInstance,
+                                urlBase: urlBase,
+                                user: "user",
+                                userId: "user",
+                                password: "password",
+                                account: account,
+                                userAgent: "NextcloudKitTests",
+                                groupIdentifier: "",
+                                httpMaximumConnectionsPerHost: 1,
+                                httpMaximumConnectionsPerHostInDownload: 1,
+                                httpMaximumConnectionsPerHostInUpload: 1,
+                                sessionDataOverride: Alamofire.Session(configuration: configuration))
+        kit.nkCommonInstance.nksessions.append(session)
+        defer { kit.nkCommonInstance.nksessions.remove(account: account) }
+
+        let directory = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+        try FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: directory) }
+        try Data("chunk".utf8).write(to: directory.appendingPathComponent("source.txt"))
+        try Data("chunk".utf8).write(to: directory.appendingPathComponent("00001"))
+
+        do {
+            let result = try await kit.uploadChunkAsync(directory: directory.path,
+                                                        fileName: "source.txt",
+                                                        date: nil, creationDate: nil,
+                                                        serverUrl: urlBase + "/remote.php/dav/files/user",
+                                                        chunkFolder: "chunks",
+                                                        filesChunk: [(fileName: "00001", size: 5)],
+                                                        chunkSize: 5,
+                                                        account: account)
+            return (nil, result.file)
+        } catch let error as NKError {
+            return (error, nil)
+        }
+    }
+
+    func test_failedAssemblyMove_throwsServerError() async throws {
+        let result = try await uploadThroughMockedAssembly(moveStatus: 507)
+        XCTAssertEqual(result.error?.errorCode, 507)
+    }
+
+    func test_failedAssemblyMove_throwsPreconditionError() async throws {
+        let result = try await uploadThroughMockedAssembly(moveStatus: 412)
+        XCTAssertEqual(result.error?.errorCode, 412)
+    }
+
+    func test_successfulAssemblyMove_returnsFile() async throws {
+        let result = try await uploadThroughMockedAssembly(moveStatus: 201)
+        XCTAssertNil(result.error)
+        XCTAssertEqual(result.file?.ocId, "assembled-file-id")
+    }
+
     func test_assembledFile_withOCFileId_derivesNKFileFromHeaders() {
         let kit = makeKit()
         let headers: [AnyHashable: Any] = [
diff --git a/Tests/NextcloudKitUnitTests/ChunkedUploadURLProtocol.swift b/Tests/NextcloudKitUnitTests/ChunkedUploadURLProtocol.swift
new file mode 100644
index 00000000..8807121a
--- /dev/null
+++ b/Tests/NextcloudKitUnitTests/ChunkedUploadURLProtocol.swift
@@ -0,0 +1,45 @@
+// SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+import Foundation
+
+final class ChunkedUploadURLProtocol: URLProtocol {
+    override class func canInit(with request: URLRequest) -> Bool {
+        guard let host = request.url?.host else { return false }
+        return host.hasPrefix("assembly-") && host.hasSuffix(".test")
+    }
+
+    override class func canonicalRequest(for request: URLRequest) -> URLRequest {
+        request
+    }
+
+    override func startLoading() {
+        guard let url = request.url else {
+            client?.urlProtocol(self, didFailWithError: URLError(.badURL))
+            return
+        }
+
+        let statusCode: Int
+        var headers: [String: String] = [:]
+        switch request.httpMethod {
+        case "PROPFIND":
+            statusCode = 404
+        case "MKCOL", "PUT":
+            statusCode = 201
+        case "MOVE":
+            statusCode = Int(url.host?.split(separator: "-").last?.split(separator: ".").first ?? "") ?? 500
+            if statusCode == 201 {
+                headers = ["OC-FileID": "assembled-file-id", "OC-ETag": "assembled-etag"]
+            }
+        default:
+            statusCode = 500
+        }
+
+        let response = HTTPURLResponse(url: url, statusCode: statusCode, httpVersion: "HTTP/1.1", headerFields: headers)!
+        client?.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)
+        client?.urlProtocol(self, didLoad: Data())
+        client?.urlProtocolDidFinishLoading(self)
+    }
+
+    override func stopLoading() {}
+}