diff --git a/lib/Controller/FileController.php b/lib/Controller/FileController.php
index ad3f0fec9a..2cfc25f5f0 100644
--- a/lib/Controller/FileController.php
+++ b/lib/Controller/FileController.php
@@ -27,6 +27,7 @@
 use OCA\Libresign\Service\Policy\ValidationEffectivePolicyService;
 use OCA\Libresign\Service\RequestSignatureService;
 use OCA\Libresign\Service\SessionService;
+use OCA\Libresign\Service\Validation\FileInputValidator;
 use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\AppFramework\Http;
 use OCP\AppFramework\Http\Attribute\ApiRoute;
@@ -84,6 +85,7 @@ public function __construct(
 		private ValidateHelper $validateHelper,
 		private SettingsLoader $settingsLoader,
 		private IURLGenerator $urlGenerator,
+		private FileInputValidator $fileInputValidator,
 	) {
 		parent::__construct(Application::APP_ID, $request);
 	}
@@ -742,12 +744,18 @@ private function prepareFilesForSaving(array $file, array $files, array $setting
 		}
 
 		if (!empty($files)) {
-			/** @var list<array{fileNode?: Node, name?: string}> $files */
-			return $files;
+			/** @var list<array{fileNode?: Node, name?: string}> $normalizedFiles */
+			$normalizedFiles = array_map(
+				fn (mixed $each): mixed => is_array($each) ? $this->fileInputValidator->normalizeNodeId($each) : $each,
+				$files,
+			);
+			return $normalizedFiles;
 		}
 
 		if (!empty($file)) {
-			return [$file];
+			/** @var array{fileNode?: Node, name?: string} $normalizedFile */
+			$normalizedFile = $this->fileInputValidator->normalizeNodeId($file);
+			return [$normalizedFile];
 		}
 
 		// TRANSLATORS Error shown when creating or updating a signature request without a file.
diff --git a/lib/Controller/RequestSignatureController.php b/lib/Controller/RequestSignatureController.php
index c46e0aa692..952f0c3e39 100644
--- a/lib/Controller/RequestSignatureController.php
+++ b/lib/Controller/RequestSignatureController.php
@@ -62,7 +62,7 @@ public function __construct(
 	 * @param LibresignNewSigner[] $signers Collection of signers who must sign the document. Use identifyMethods as the canonical format. Other supported fields: displayName, description, notify, signingOrder, status, geolocationRequired
 	 * @param string $name The name of file to sign
 	 * @param LibresignFolderSettings $settings Settings to define how and where the file should be stored
-	 * @param LibresignNewFile $file File object. Supports nodeId, url, base64 or path.
+	 * @param LibresignNewFile $file File object. Supports nodeId (a non-negative integer or its canonical decimal string, as Nextcloud node ids can exceed a JavaScript number), url, base64 or path.
 	 * @param list<LibresignNewFile> $files Multiple files to create an envelope (optional, use either file or files). Each file supports nodeId, url, base64 or path.
 	 * @param string|null $callback URL that will receive a POST after the document is signed
 	 * @param integer|null $status Numeric code of status * 0 - no signers * 1 - signed * 2 - pending
@@ -134,7 +134,7 @@ public function requestSignature(
 	 * @param LibresignNewSigner[]|null $signers Collection of signers who must sign the document. Use identifyMethods as the canonical format.
 	 * @param string|null $uuid UUID of sign request. The signer UUID is what the person receives via email when asked to sign. This is not the file UUID.
 	 * @param LibresignVisibleElement[]|null $visibleElements Visible elements on document
-	 * @param LibresignNewFile|null $file File object. Supports nodeId, url, base64 or path when creating a new request.
+	 * @param LibresignNewFile|null $file File object. Supports nodeId (a non-negative integer or its canonical decimal string), url, base64 or path when creating a new request.
 	 * @param integer|null $status Numeric code of status * 0 - no signers * 1 - signed * 2 - pending
 	 * @param array<string, mixed>|null $policy Structured policy payload with request-level overrides and active context.
 	 * @param string|null $name The name of file to sign
diff --git a/lib/ResponseDefinitions.php b/lib/ResponseDefinitions.php
index 7bda4b0a0e..a021e58731 100644
--- a/lib/ResponseDefinitions.php
+++ b/lib/ResponseDefinitions.php
@@ -72,7 +72,7 @@
  * }
  * @psalm-type LibresignNewFile = array{
  *     base64?: string,
- *     nodeId?: non-negative-int,
+ *     nodeId?: non-negative-int|numeric-string,
  *     path?: string,
  *     url?: string,
  *     name?: string,
diff --git a/lib/Service/IdDocsService.php b/lib/Service/IdDocsService.php
index 95e0354839..88c2de7d43 100644
--- a/lib/Service/IdDocsService.php
+++ b/lib/Service/IdDocsService.php
@@ -82,6 +82,28 @@ private function validateIdDoc(int $fileIndex, array $file, IUser $user): void {
 		}
 	}
 
+	/**
+	 * `file` of each entry is the HTTP `LibresignNewFile` payload; its node
+	 * id is normalized once here, the same boundary the signature request has.
+	 */
+	private function normalizeNodeIds(array $files): array {
+		foreach ($files as $fileIndex => $fileData) {
+			if (!is_array($fileData) || !is_array($fileData['file'] ?? null)) {
+				continue;
+			}
+			try {
+				$files[$fileIndex]['file'] = $this->fileInputValidator->normalizeNodeId($fileData['file'], FileInputValidator::TYPE_ACCOUNT_DOCUMENT);
+			} catch (LibresignException $e) {
+				throw new LibresignException(json_encode([
+					'type' => 'danger',
+					'file' => $fileIndex,
+					'message' => $e->getMessage(),
+				]));
+			}
+		}
+		return $files;
+	}
+
 	public function validateIdDocs(array $files, IUser $user): void {
 		foreach ($files as $fileIndex => $file) {
 			$this->validateTypeOfFile($fileIndex, $file);
@@ -90,6 +112,7 @@ public function validateIdDocs(array $files, IUser $user): void {
 	}
 
 	public function addIdDocs(array $files, IUser $user): void {
+		$files = $this->normalizeNodeIds($files);
 		$this->validateIdDocs($files, $user);
 		foreach ($files as $fileData) {
 			$dataToSave = $fileData;
@@ -119,6 +142,7 @@ public function addFilesToDocumentFolder(
 		array $files,
 		SignRequest $signRequest,
 	): void {
+		$files = $this->normalizeNodeIds($files);
 		foreach ($files as $fileIndex => $file) {
 			$this->validateTypeOfFile($fileIndex, $file);
 		}
diff --git a/lib/Service/RequestSignatureService.php b/lib/Service/RequestSignatureService.php
index 88b465e600..2ad694cc09 100644
--- a/lib/Service/RequestSignatureService.php
+++ b/lib/Service/RequestSignatureService.php
@@ -361,15 +361,15 @@ public function saveFile(array $data): FileEntity {
 			}
 			return $this->fileStatusService->updateFileStatusIfUpgrade($file, $data['status'] ?? 0);
 		}
-		$fileId = null;
+		$nodeId = null;
 		if (isset($data['file']['fileNode']) && $data['file']['fileNode'] instanceof Node) {
-			$fileId = $data['file']['fileNode']->getId();
+			$nodeId = $data['file']['fileNode']->getId();
 		} elseif (!empty($data['file']['nodeId'])) {
-			$fileId = $data['file']['nodeId'];
+			$nodeId = $data['file']['nodeId'];
 		}
-		if (!is_null($fileId)) {
+		if (!is_null($nodeId)) {
 			try {
-				$file = $this->fileMapper->getByNodeId($fileId);
+				$file = $this->fileMapper->getByNodeId($nodeId);
 				$this->filePolicyApplier->syncAllPolicies($file, $data);
 				return $this->fileStatusService->updateFileStatusIfUpgrade($file, $data['status'] ?? 0);
 			} catch (\Throwable) {
diff --git a/lib/Service/RequestSignatureWorkflowService.php b/lib/Service/RequestSignatureWorkflowService.php
index 6c5096ae76..cf65e87e82 100644
--- a/lib/Service/RequestSignatureWorkflowService.php
+++ b/lib/Service/RequestSignatureWorkflowService.php
@@ -26,6 +26,7 @@ public function __construct(
 		private SignerValidator $signerValidator,
 		private VisibleElementValidator $visibleElementValidator,
 		private FileMapper $fileMapper,
+		private FileInputValidator $fileInputValidator,
 	) {
 	}
 
@@ -67,6 +68,8 @@ public function createRequest(
 			throw new LibresignException($this->l10n->t('File or files parameter is required'));
 		}
 
+		$file = $this->fileInputValidator->normalizeNodeId($file);
+		$files = $this->normalizeNodeIds($files);
 		$resolvedPolicy = $this->resolvePolicyPayload($policy);
 		$data = [
 			'file' => $file,
@@ -128,6 +131,7 @@ public function updateExistingRequest(
 		?string $name = null,
 		array $settings = [],
 	): array {
+		$file = $this->fileInputValidator->normalizeNodeId($file);
 		$resolvedPolicy = $this->resolvePolicyPayload($policy);
 		$data = [
 			'uuid' => $uuid,
@@ -159,6 +163,17 @@ public function updateExistingRequest(
 		];
 	}
 
+	/**
+	 * @param list<array<string, mixed>> $files
+	 * @return list<array<string, mixed>>
+	 */
+	private function normalizeNodeIds(array $files): array {
+		return array_map(
+			fn (mixed $file): mixed => is_array($file) ? $this->fileInputValidator->normalizeNodeId($file) : $file,
+			$files,
+		);
+	}
+
 	/** @return list<FileEntity> */
 	private function loadChildFilesIfEnvelope(FileEntity $fileEntity): array {
 		return $fileEntity->getParentFileId() === null || $fileEntity->isEnvelope()
diff --git a/lib/Service/Validation/FileInputValidator.php b/lib/Service/Validation/FileInputValidator.php
index 02c76ae253..691eba7012 100644
--- a/lib/Service/Validation/FileInputValidator.php
+++ b/lib/Service/Validation/FileInputValidator.php
@@ -38,6 +38,38 @@ public function __construct(
 	) {
 	}
 
+	/**
+	 * Normalize the node id of a file payload received over HTTP.
+	 *
+	 * The API accepts `nodeId` as a non-negative int or as its canonical
+	 * decimal string (digits only, no sign, no leading zeros, within the int
+	 * range): the Files app exposes node ids as strings (`Node.id` of
+	 * `@nextcloud/files`) and, since Nextcloud 33, they can exceed what a
+	 * JavaScript number holds. Call it once at the boundary: past it,
+	 * `nodeId` is either absent or the non-negative `int` the Nextcloud Files
+	 * API works with, and anything else is rejected here instead of reaching
+	 * a later cast.
+	 *
+	 * @param array<string, mixed> $file
+	 * @return array<string, mixed>
+	 * @throws LibresignException when `nodeId` is present and is neither a non-negative int nor its canonical decimal string
+	 */
+	public function normalizeNodeId(array $file, int $type = self::TYPE_TO_SIGN): array {
+		$nodeId = $file['nodeId'] ?? null;
+		if ($nodeId === null) {
+			return $file;
+		}
+		if (is_string($nodeId) && ctype_digit($nodeId)) {
+			// FILTER_VALIDATE_INT also rejects leading zeros and overflow.
+			$nodeId = filter_var($nodeId, FILTER_VALIDATE_INT);
+		}
+		if (is_int($nodeId) && $nodeId >= 0) {
+			$file['nodeId'] = $nodeId;
+			return $file;
+		}
+		throw new LibresignException($this->l10n->t('File type: %s. Invalid fileID.', [$this->getTypeOfFile($type)]));
+	}
+
 	public function validateNewFile(array $data, int $type = self::TYPE_TO_SIGN, ?IUser $user = null): void {
 		$this->validateFile($data, $type, $user);
 		if (!empty($data['file']['nodeId'])) {
diff --git a/openapi-full.json b/openapi-full.json
index b63f319c65..c15632786a 100644
--- a/openapi-full.json
+++ b/openapi-full.json
@@ -2119,9 +2119,16 @@
                         "type": "string"
                     },
                     "nodeId": {
-                        "type": "integer",
-                        "format": "int64",
-                        "minimum": 0
+                        "oneOf": [
+                            {
+                                "type": "integer",
+                                "format": "int64",
+                                "minimum": 0
+                            },
+                            {
+                                "type": "string"
+                            }
+                        ]
                     },
                     "path": {
                         "type": "string"
@@ -10146,7 +10153,7 @@
                                     "file": {
                                         "$ref": "#/components/schemas/NewFile",
                                         "default": [],
-                                        "description": "File object. Supports nodeId, url, base64 or path."
+                                        "description": "File object. Supports nodeId (a non-negative integer or its canonical decimal string, as Nextcloud node ids can exceed a JavaScript number), url, base64 or path."
                                     },
                                     "files": {
                                         "type": "array",
@@ -10322,7 +10329,7 @@
                                     "file": {
                                         "$ref": "#/components/schemas/NewFile",
                                         "nullable": true,
-                                        "description": "File object. Supports nodeId, url, base64 or path when creating a new request."
+                                        "description": "File object. Supports nodeId (a non-negative integer or its canonical decimal string), url, base64 or path when creating a new request."
                                     },
                                     "status": {
                                         "type": "integer",
diff --git a/openapi.json b/openapi.json
index ee97f68c77..687a58df74 100644
--- a/openapi.json
+++ b/openapi.json
@@ -1583,9 +1583,16 @@
                         "type": "string"
                     },
                     "nodeId": {
-                        "type": "integer",
-                        "format": "int64",
-                        "minimum": 0
+                        "oneOf": [
+                            {
+                                "type": "integer",
+                                "format": "int64",
+                                "minimum": 0
+                            },
+                            {
+                                "type": "string"
+                            }
+                        ]
                     },
                     "path": {
                         "type": "string"
@@ -9444,7 +9451,7 @@
                                     "file": {
                                         "$ref": "#/components/schemas/NewFile",
                                         "default": [],
-                                        "description": "File object. Supports nodeId, url, base64 or path."
+                                        "description": "File object. Supports nodeId (a non-negative integer or its canonical decimal string, as Nextcloud node ids can exceed a JavaScript number), url, base64 or path."
                                     },
                                     "files": {
                                         "type": "array",
@@ -9620,7 +9627,7 @@
                                     "file": {
                                         "$ref": "#/components/schemas/NewFile",
                                         "nullable": true,
-                                        "description": "File object. Supports nodeId, url, base64 or path when creating a new request."
+                                        "description": "File object. Supports nodeId (a non-negative integer or its canonical decimal string), url, base64 or path when creating a new request."
                                     },
                                     "status": {
                                         "type": "integer",
diff --git a/src/components/RightSidebar/AppFilesTab.vue b/src/components/RightSidebar/AppFilesTab.vue
index 0e716b7c42..f0072f0e66 100644
--- a/src/components/RightSidebar/AppFilesTab.vue
+++ b/src/components/RightSidebar/AppFilesTab.vue
@@ -47,7 +47,9 @@ type PendingEnvelope = {
 }
 
 type FileInfo = {
-	id: number
+	// Nextcloud node id as tab.ts sends it: the numeric `fileid` when the node
+	// has one, otherwise the string `Node.id` of `@nextcloud/files`.
+	id: number | string
 	type?: string
 	name?: string
 	path?: string
diff --git a/src/store/files.js b/src/store/files.js
index 1784b9e1d8..8736bf8bd0 100644
--- a/src/store/files.js
+++ b/src/store/files.js
@@ -961,6 +961,24 @@ const _filesStore = defineStore('files', () => {
 			.filter((signer) => signer && signer.identifyMethods?.length)
 	}
 
+	/**
+	 * Whether a value identifies a Nextcloud node. Besides the historical
+	 * positive number, `@nextcloud/files` exposes `Node.id` as a string and
+	 * that is what the Files sidebar hands to AppFilesTab (#8363). The string
+	 * is kept as is: node ids are 64-bit and converting them with Number()
+	 * could change the value above Number.MAX_SAFE_INTEGER. The API accepts
+	 * both representations.
+	 *
+	 * @param {unknown} value
+	 * @return {value is number | string}
+	 */
+	function isNodeId(value) {
+		if (typeof value === 'number') {
+			return Number.isInteger(value) && value > 0
+		}
+		return typeof value === 'string' && /^[1-9][0-9]*$/.test(value)
+	}
+
 	/** @param {EditableFileReferenceDraft | ApiFileRecord | EditableFileDraft | string | null | undefined} file */
 	function serializeRequestFile(file, { preferNodeId = false } = {}) {
 		if (typeof file === 'string') {
@@ -972,7 +990,7 @@ const _filesStore = defineStore('files', () => {
 		if (typeof file.path === 'string' && file.path.length > 0) {
 			return { path: file.path }
 		}
-		if (preferNodeId && typeof file.nodeId === 'number' && file.nodeId > 0) {
+		if (preferNodeId && isNodeId(file.nodeId)) {
 			return { nodeId: file.nodeId }
 		}
 		if (typeof file.fileId === 'number' && file.fileId > 0) {
@@ -983,7 +1001,7 @@ const _filesStore = defineStore('files', () => {
 				return { fileId: file.id }
 			}
 		}
-		if (typeof file.nodeId === 'number' && file.nodeId > 0) {
+		if (isNodeId(file.nodeId)) {
 			return { nodeId: file.nodeId }
 		}
 		if (typeof file.url === 'string' && file.url.length > 0) {
diff --git a/src/tests/components/RightSidebar/AppFilesTab.spec.ts b/src/tests/components/RightSidebar/AppFilesTab.spec.ts
index 4c2c7402b2..7f154d5828 100644
--- a/src/tests/components/RightSidebar/AppFilesTab.spec.ts
+++ b/src/tests/components/RightSidebar/AppFilesTab.spec.ts
@@ -72,7 +72,7 @@ type TitleObserver = {
 }
 
 type FileInfo = {
-	id: number
+	id: number | string
 	type?: string
 	name?: string
 	path?: string
@@ -286,6 +286,28 @@ describe('AppFilesTab', () => {
 			expect(sidebarStore.activeRequestSignatureTab).toHaveBeenCalled()
 		})
 
+		it('passes a string node id through unchanged when adding the file (#8363)', async () => {
+			filesStore.selectFileByNodeId = vi.fn().mockResolvedValue(null)
+			filesStore.addFile = vi.fn()
+			filesStore.selectFile = vi.fn()
+			sidebarStore.activeRequestSignatureTab = vi.fn()
+			wrapper = createWrapper()
+
+			// tab.ts sends `Node.id` (a string) when the node has no numeric fileid
+			await wrapper.vm.update({
+				id: '9007199254740993',
+				name: 'copy of contract.pdf',
+				path: '/Documents',
+			})
+
+			expect(filesStore.selectFileByNodeId).toHaveBeenCalledWith('9007199254740993')
+			expect(filesStore.addFile).toHaveBeenCalledWith(expect.objectContaining({
+				nodeId: '9007199254740993',
+				name: 'copy of contract.pdf',
+			}))
+			expect(sidebarStore.activeRequestSignatureTab).toHaveBeenCalled()
+		})
+
 		it('returns early when pending envelope processed', async () => {
 			window.OCA = {
 				Libresign: {
diff --git a/src/tests/store/files.spec.ts b/src/tests/store/files.spec.ts
index e5eb4d12fe..d110cdd1c4 100644
--- a/src/tests/store/files.spec.ts
+++ b/src/tests/store/files.spec.ts
@@ -1522,6 +1522,79 @@ describe('files store - critical business rules', () => {
 				expect(config.data.file).toEqual({ nodeId })
 			})
 
+			/**
+			 * Regression #8363: `@nextcloud/files` exposes `Node.id` as a string and
+			 * the Files sidebar hands it to AppFilesTab as is (a file copied in the
+			 * Files app, or any node id above Number.MAX_SAFE_INTEGER). The store
+			 * keeps it as nodeId; the request must carry it unchanged instead of
+			 * dropping the whole "file" (422 "File or files parameter is required").
+			 */
+			it('includes file.nodeId as the string the Files sidebar provided', async () => {
+				const store = useFilesStore()
+				const nodeId = '9007199254740993'
+				const tempId = -Number(nodeId)
+				store.files[tempId] = {
+					id: tempId,
+					nodeId,
+					name: 'copy of contract.pdf',
+					signers: [{ email: 'signer@example.com', identifyMethods: [{ method: 'email', value: 'signer@example.com', requirement: 'optional' }] }],
+					signatureFlow: 'parallel',
+				}
+				store.selectedFileId = tempId
+
+				axiosMock.mockResolvedValue({
+					data: { ocs: { data: { id: 77, nodeId: 77, signatureFlow: 'parallel', signers: [] } } },
+				})
+
+				await store.saveOrUpdateSignatureRequest({})
+
+				const config = axiosMock.mock.calls[0][0]
+				expect(config.data.file).toEqual({ nodeId: '9007199254740993' })
+			})
+
+			it('serializes envelope files with string and number node ids as they are', async () => {
+				const store = useFilesStore()
+				store.selectedFileId = -1
+				store.files[-1] = {
+					id: -1,
+					name: 'Envelope',
+					files: [
+						{ id: -7, nodeId: '9007199254740993', name: 'first.pdf' },
+						{ id: -22, nodeId: 22, name: 'second.pdf' },
+					],
+					signers: [{ email: 'signer@example.com' }],
+					signatureFlow: 'parallel',
+				}
+				axiosMock.mockResolvedValue({
+					data: { ocs: { data: { id: 12, nodeId: 'real-node', signatureFlow: 'parallel', signers: [] } } },
+				})
+
+				await store.saveOrUpdateSignatureRequest({})
+
+				const config = axiosMock.mock.calls[0][0]
+				expect(config.data.files).toEqual([{ nodeId: '9007199254740993' }, { nodeId: 22 }])
+			})
+
+			it('does not send the empty node id tab.ts falls back to when the node has none', async () => {
+				const store = useFilesStore()
+				store.files[-1] = {
+					id: -1,
+					nodeId: '',
+					name: 'unknown.pdf',
+					signers: [{ email: 'signer@example.com' }],
+					signatureFlow: 'parallel',
+				}
+				store.selectedFileId = -1
+				axiosMock.mockResolvedValue({
+					data: { ocs: { data: { id: 12, nodeId: 12, signatureFlow: 'parallel', signers: [] } } },
+				})
+
+				await store.saveOrUpdateSignatureRequest({})
+
+				const config = axiosMock.mock.calls[0][0]
+				expect(config.data.file).toBeNull()
+			})
+
 			it('serializes envelope files with nodeId-based references for creation flows', async () => {
 				const store = useFilesStore()
 				store.selectedFileId = -1
diff --git a/src/tests/tab.spec.ts b/src/tests/tab.spec.ts
index 1e033964ef..08886d1ea0 100644
--- a/src/tests/tab.spec.ts
+++ b/src/tests/tab.spec.ts
@@ -137,6 +137,33 @@ describe('tab.ts', () => {
 		})
 	})
 
+	it('enabled() keeps the string node id of a PDF whose node has no numeric fileid', async () => {
+		await loadTabModule('complete')
+		const tabConfig = getRegisteredTabConfig<{
+			enabled: (context: { node: Record<string, unknown> }) => boolean
+		}>()
+
+		// `@nextcloud/files` Node: `id` is always a string and `fileid` is
+		// undefined when the id does not fit a JavaScript number (#8363).
+		const enabled = tabConfig.enabled({
+			node: {
+				id: '9007199254740993',
+				fileid: undefined,
+				basename: 'copy of contract.pdf',
+				dirname: '/Documents',
+				type: 'file',
+				mime: 'application/pdf',
+			},
+		})
+
+		expect(enabled).toBe(true)
+		expect(window.OCA.Libresign.fileInfo).toMatchObject({
+			id: '9007199254740993',
+			name: 'copy of contract.pdf',
+			path: '/Documents',
+		})
+	})
+
 	it('lazy mounts Vue only when custom element is connected and unmounts on disconnect', async () => {
 		await loadTabModule('complete')
 
diff --git a/src/types/openapi/openapi-full.ts b/src/types/openapi/openapi-full.ts
index 82300ace97..4d5be161ec 100644
--- a/src/types/openapi/openapi-full.ts
+++ b/src/types/openapi/openapi-full.ts
@@ -1780,8 +1780,7 @@ export type components = {
         };
         NewFile: {
             base64?: string;
-            /** Format: int64 */
-            nodeId?: number;
+            nodeId?: number | string;
             path?: string;
             url?: string;
             name?: string;
@@ -4878,7 +4877,7 @@ export interface operations {
                      */
                     settings?: components["schemas"]["FolderSettings"];
                     /**
-                     * @description File object. Supports nodeId, url, base64 or path.
+                     * @description File object. Supports nodeId (a non-negative integer or its canonical decimal string, as Nextcloud node ids can exceed a JavaScript number), url, base64 or path.
                      * @default []
                      */
                     file?: components["schemas"]["NewFile"];
@@ -4957,7 +4956,7 @@ export interface operations {
                     uuid?: string | null;
                     /** @description Visible elements on document */
                     visibleElements?: components["schemas"]["VisibleElement"][] | null;
-                    /** @description File object. Supports nodeId, url, base64 or path when creating a new request. */
+                    /** @description File object. Supports nodeId (a non-negative integer or its canonical decimal string), url, base64 or path when creating a new request. */
                     file?: components["schemas"]["NewFile"];
                     /**
                      * Format: int64
diff --git a/src/types/openapi/openapi.ts b/src/types/openapi/openapi.ts
index eea99c2963..fadaab3a8a 100644
--- a/src/types/openapi/openapi.ts
+++ b/src/types/openapi/openapi.ts
@@ -1360,8 +1360,7 @@ export type components = {
         };
         NewFile: {
             base64?: string;
-            /** Format: int64 */
-            nodeId?: number;
+            nodeId?: number | string;
             path?: string;
             url?: string;
             name?: string;
@@ -4417,7 +4416,7 @@ export interface operations {
                      */
                     settings?: components["schemas"]["FolderSettings"];
                     /**
-                     * @description File object. Supports nodeId, url, base64 or path.
+                     * @description File object. Supports nodeId (a non-negative integer or its canonical decimal string, as Nextcloud node ids can exceed a JavaScript number), url, base64 or path.
                      * @default []
                      */
                     file?: components["schemas"]["NewFile"];
@@ -4496,7 +4495,7 @@ export interface operations {
                     uuid?: string | null;
                     /** @description Visible elements on document */
                     visibleElements?: components["schemas"]["VisibleElement"][] | null;
-                    /** @description File object. Supports nodeId, url, base64 or path when creating a new request. */
+                    /** @description File object. Supports nodeId (a non-negative integer or its canonical decimal string), url, base64 or path when creating a new request. */
                     file?: components["schemas"]["NewFile"];
                     /**
                      * Format: int64
diff --git a/tests/php/Unit/Controller/RequestSignatureControllerTest.php b/tests/php/Unit/Controller/RequestSignatureControllerTest.php
index 60b436ceaa..605de17bc9 100644
--- a/tests/php/Unit/Controller/RequestSignatureControllerTest.php
+++ b/tests/php/Unit/Controller/RequestSignatureControllerTest.php
@@ -12,14 +12,18 @@
 use OCA\Libresign\Controller\RequestSignatureController;
 use OCA\Libresign\Db\File as FileEntity;
 use OCA\Libresign\Db\FileMapper;
+use OCA\Libresign\Db\SignRequestMapper;
 use OCA\Libresign\Exception\LibresignException;
 use OCA\Libresign\Service\File\FileListService;
+use OCA\Libresign\Service\FolderService;
 use OCA\Libresign\Service\RequestSignatureService;
 use OCA\Libresign\Service\RequestSignatureWorkflowService;
+use OCA\Libresign\Service\Validation\FileInputValidator;
 use OCA\Libresign\Service\Validation\SignerValidator;
 use OCA\Libresign\Service\Validation\SigningRequestValidator;
 use OCA\Libresign\Service\Validation\VisibleElementValidator;
 use OCP\AppFramework\Http;
+use OCP\Files\IMimeTypeDetector;
 use OCP\IL10N;
 use OCP\IRequest;
 use OCP\IUser;
@@ -63,6 +67,13 @@ protected function setUp(): void {
 			$this->signerValidator,
 			$this->visibleElementValidator,
 			$this->fileMapper,
+			new FileInputValidator(
+				$this->l10n,
+				$this->createMock(SignRequestMapper::class),
+				$this->fileMapper,
+				$this->createMock(IMimeTypeDetector::class),
+				$this->createMock(FolderService::class),
+			),
 		);
 
 		$this->controller = new RequestSignatureController(
diff --git a/tests/php/Unit/Service/IdDocsServiceTest.php b/tests/php/Unit/Service/IdDocsServiceTest.php
index 7b94e64372..5937744e29 100644
--- a/tests/php/Unit/Service/IdDocsServiceTest.php
+++ b/tests/php/Unit/Service/IdDocsServiceTest.php
@@ -15,6 +15,7 @@
 use OCA\Libresign\Db\IdentifyMethodMapper;
 use OCA\Libresign\Db\SignRequest;
 use OCA\Libresign\Db\SignRequestMapper;
+use OCA\Libresign\Exception\LibresignException;
 use OCA\Libresign\Service\IdDocsService;
 use OCA\Libresign\Service\RequestSignatureService;
 use OCA\Libresign\Service\Validation\FileInputValidator;
@@ -241,6 +242,71 @@ public function testAddFilesToDocumentFolderStoresFilesUnderTheOwnerOfTheSignedF
 		);
 	}
 
+	/**
+	 * The `file` entry is the same HTTP payload as the signature request's,
+	 * so its node id is normalized at this boundary too and reaches
+	 * saveFile() as an int.
+	 */
+	public function testAddFilesToDocumentFolderNormalizesTheNodeIdOfEachFile(): void {
+		$signRequest = new SignRequest();
+		$signRequest->setId(55);
+		$signRequest->setFileId(10);
+		$this->fileMapper->method('getById')->willThrowException(new \OCP\AppFramework\Db\DoesNotExistException('no'));
+		$this->fileTypeMapper->method('getTypes')
+			->willReturn(['IDENTIFICATION' => ['type' => 'IDENTIFICATION']]);
+		$this->fileInputValidator->expects($this->once())
+			->method('normalizeNodeId')
+			->with(['nodeId' => '9007199254740993'], FileInputValidator::TYPE_ACCOUNT_DOCUMENT)
+			->willReturn(['nodeId' => 9007199254740993]);
+
+		$savedFile = new \OCA\Libresign\Db\File();
+		$savedFile->setId(77);
+		$this->requestSignatureService->expects($this->once())
+			->method('saveFile')
+			->with($this->callback(function (array $data): bool {
+				$this->assertSame(9007199254740993, $data['file']['nodeId']);
+				return true;
+			}))
+			->willReturn($savedFile);
+
+		$service = $this->getIdDocsService();
+		$service->addFilesToDocumentFolder(
+			[['type' => 'IDENTIFICATION', 'name' => 'id-front.pdf', 'file' => ['nodeId' => '9007199254740993']]],
+			$signRequest,
+		);
+	}
+
+	public function testAddIdDocsReportsAnInvalidNodeIdWithTheIndexOfTheFile(): void {
+		$user = $this->createMock(IUser::class);
+		$this->fileInputValidator->expects($this->exactly(2))
+			->method('normalizeNodeId')
+			->with($this->anything(), FileInputValidator::TYPE_ACCOUNT_DOCUMENT)
+			->willReturnCallback(static function (array $file): array {
+				if (($file['nodeId'] ?? null) === 'temp-node') {
+					throw new LibresignException('File type: Account document. Invalid fileID.');
+				}
+				return $file;
+			});
+		$this->requestSignatureService->expects($this->never())->method('saveFile');
+
+		$service = $this->getIdDocsService();
+		try {
+			$service->addIdDocs(
+				[
+					['type' => 'IDENTIFICATION', 'file' => ['base64' => 'ZmFrZQ==']],
+					['type' => 'IDENTIFICATION', 'file' => ['nodeId' => 'temp-node']],
+				],
+				$user,
+			);
+			$this->fail('An invalid node id must be rejected');
+		} catch (LibresignException $e) {
+			$this->assertSame(
+				['type' => 'danger', 'file' => 1, 'message' => 'File type: Account document. Invalid fileID.'],
+				json_decode($e->getMessage(), true),
+			);
+		}
+	}
+
 	public function testAddFilesToDocumentFolderWithoutResolvableOwnerKeepsCurrentBehaviour(): void {
 		$signRequest = new SignRequest();
 		$signRequest->setId(55);
diff --git a/tests/php/Unit/Service/RequestSignatureServiceTest.php b/tests/php/Unit/Service/RequestSignatureServiceTest.php
index 231813bb80..8a9cb3396f 100644
--- a/tests/php/Unit/Service/RequestSignatureServiceTest.php
+++ b/tests/php/Unit/Service/RequestSignatureServiceTest.php
@@ -195,6 +195,39 @@ private function getService(array $methods = []): RequestSignatureService|MockOb
 		);
 	}
 
+	/**
+	 * saveFile() receives the node id already normalized to int by the
+	 * workflow boundary and looks the existing LibreSign file up with it.
+	 */
+	public function testSaveFileReusesTheFileRegisteredForTheNodeId(): void {
+		$service = $this->getService();
+
+		$existing = new \OCA\Libresign\Db\File();
+		$existing->setId(7);
+		$existing->setNodeId(9007199254740993);
+		$this->fileMapper->expects($this->once())
+			->method('getByNodeId')
+			->with(9007199254740993)
+			->willReturn($existing);
+		$this->filePolicyApplier->expects($this->once())
+			->method('syncAllPolicies')
+			->with($existing, $this->anything());
+		$this->fileStatusService->expects($this->once())
+			->method('updateFileStatusIfUpgrade')
+			->with($existing, 1)
+			->willReturn($existing);
+		$this->fileService->expects($this->never())->method('getNodeFromData');
+
+		$result = $service->saveFile([
+			'file' => ['nodeId' => 9007199254740993],
+			'name' => 'contract',
+			'status' => 1,
+			'userManager' => $this->user,
+		]);
+
+		$this->assertSame($existing, $result);
+	}
+
 	public function testSaveFilesUsesSaveForSingleFile(): void {
 		$service = $this->getService(['save']);
 
diff --git a/tests/php/Unit/Service/RequestSignatureWorkflowServiceTest.php b/tests/php/Unit/Service/RequestSignatureWorkflowServiceTest.php
index f4916e3ac2..03aadebd32 100644
--- a/tests/php/Unit/Service/RequestSignatureWorkflowServiceTest.php
+++ b/tests/php/Unit/Service/RequestSignatureWorkflowServiceTest.php
@@ -10,13 +10,16 @@
 
 use OCA\Libresign\Db\File as FileEntity;
 use OCA\Libresign\Db\FileMapper;
+use OCA\Libresign\Db\SignRequestMapper;
 use OCA\Libresign\Exception\LibresignException;
+use OCA\Libresign\Service\FolderService;
 use OCA\Libresign\Service\RequestSignatureService;
 use OCA\Libresign\Service\RequestSignatureWorkflowService;
 use OCA\Libresign\Service\Validation\FileInputValidator;
 use OCA\Libresign\Service\Validation\SignerValidator;
 use OCA\Libresign\Service\Validation\SigningRequestValidator;
 use OCA\Libresign\Service\Validation\VisibleElementValidator;
+use OCP\Files\IMimeTypeDetector;
 use OCP\IL10N;
 use OCP\IUser;
 use PHPUnit\Framework\Attributes\DataProvider;
@@ -52,6 +55,13 @@ protected function setUp(): void {
 			$this->signerValidator,
 			$this->visibleElementValidator,
 			$this->fileMapper,
+			new FileInputValidator(
+				$this->l10n,
+				$this->createMock(SignRequestMapper::class),
+				$this->fileMapper,
+				$this->createMock(IMimeTypeDetector::class),
+				$this->createMock(FolderService::class),
+			),
 		);
 	}
 
@@ -155,6 +165,137 @@ public function testCreateRequestUsesEnvelopeSaveFilesAndReturnsProvidedChildren
 		$this->assertSame([$child], $result['children']);
 	}
 
+	/**
+	 * Regression #8363: the Files sidebar sends the node id as a string. The
+	 * workflow is the boundary between the HTTP payload and the services, so
+	 * everything after it must already see an int.
+	 */
+	public function testCreateRequestNormalizesTheStringNodeIdOnceAtTheBoundary(): void {
+		$fileEntity = new FileEntity();
+		$fileEntity->setId(9);
+
+		$this->requestSignatureService->expects($this->once())
+			->method('validateNewRequestToFile')
+			->with($this->callback(static fn (array $payload): bool => $payload['file']['nodeId'] === 9007199254740993));
+		$this->requestSignatureService->expects($this->once())
+			->method('save')
+			->with($this->callback(static fn (array $payload): bool => $payload['file']['nodeId'] === 9007199254740993))
+			->willReturn($fileEntity);
+
+		$result = $this->service->createRequest(
+			$this->user,
+			['nodeId' => '9007199254740993'],
+			[],
+			'copy of contract.pdf',
+			[],
+			[['identifyMethods' => [['method' => 'email', 'value' => 'user@example.test']]]],
+			1,
+			null,
+		);
+
+		$this->assertSame($fileEntity, $result['file']);
+	}
+
+	public function testCreateRequestKeepsAnIntegerNodeIdAsItIs(): void {
+		$fileEntity = new FileEntity();
+		$fileEntity->setId(9);
+
+		$this->requestSignatureService->expects($this->once())
+			->method('validateNewRequestToFile')
+			->with($this->callback(static fn (array $payload): bool => $payload['file']['nodeId'] === 11));
+		$this->requestSignatureService->expects($this->once())
+			->method('save')
+			->with($this->callback(static fn (array $payload): bool => $payload['file']['nodeId'] === 11))
+			->willReturn($fileEntity);
+
+		$this->service->createRequest(
+			$this->user,
+			['nodeId' => 11],
+			[],
+			'contract.pdf',
+			[],
+			[['identifyMethods' => [['method' => 'email', 'value' => 'user@example.test']]]],
+			1,
+			null,
+		);
+	}
+
+	public function testCreateRequestNormalizesTheNodeIdOfEachEnvelopeFile(): void {
+		$envelope = new FileEntity();
+		$envelope->setId(30);
+		$envelope->setNodeType('envelope');
+
+		$this->requestSignatureService->expects($this->once())
+			->method('validateNewRequestToFile')
+			->with($this->callback(static fn (array $payload): bool => $payload['files'][0]['nodeId'] === 9007199254740993
+				&& $payload['files'][1]['nodeId'] === 22
+				&& $payload['files'][2] === ['base64' => 'abc', 'name' => 'part-c.pdf']));
+		$this->requestSignatureService->expects($this->once())
+			->method('saveFiles')
+			->with($this->callback(static fn (array $payload): bool => $payload['files'][0]['nodeId'] === 9007199254740993
+				&& $payload['files'][1]['nodeId'] === 22))
+			->willReturn(['file' => $envelope, 'children' => []]);
+
+		$this->service->createRequest(
+			$this->user,
+			[],
+			[
+				['nodeId' => '9007199254740993', 'name' => 'part-a.pdf'],
+				['nodeId' => 22, 'name' => 'part-b.pdf'],
+				['base64' => 'abc', 'name' => 'part-c.pdf'],
+			],
+			'Envelope',
+			[],
+			[['identifyMethods' => [['method' => 'email', 'value' => 'user@example.test']]]],
+			0,
+			null,
+		);
+	}
+
+	public function testCreateRequestRejectsAnInvalidNodeIdBeforeAnyService(): void {
+		$this->requestSignatureService->expects($this->never())->method('validateNewRequestToFile');
+		$this->requestSignatureService->expects($this->never())->method('save');
+
+		$this->expectException(LibresignException::class);
+		$this->expectExceptionMessage('Invalid fileID');
+
+		$this->service->createRequest(
+			$this->user,
+			['nodeId' => 'temp-node'],
+			[],
+			'contract.pdf',
+			[],
+			[['identifyMethods' => [['method' => 'email', 'value' => 'user@example.test']]]],
+			1,
+			null,
+		);
+	}
+
+	public function testUpdateExistingRequestNormalizesTheStringNodeId(): void {
+		$fileEntity = new FileEntity();
+		$fileEntity->setId(21);
+		$fileEntity->setParentFileId(20);
+
+		$this->signingRequestValidator->expects($this->once())
+			->method('validateExistingFile')
+			->with($this->callback(static fn (array $payload): bool => $payload['file']['nodeId'] === 9007199254740993));
+		$this->requestSignatureService->expects($this->once())
+			->method('save')
+			->with($this->callback(static fn (array $payload): bool => $payload['file']['nodeId'] === 9007199254740993))
+			->willReturn($fileEntity);
+
+		$result = $this->service->updateExistingRequest(
+			$this->user,
+			[['identifyMethods' => [['method' => 'email', 'value' => 'user@example.test']]]],
+			'uuid-21',
+			null,
+			['nodeId' => '9007199254740993'],
+			null,
+		);
+
+		$this->assertSame($fileEntity, $result['file']);
+	}
+
 	public function testUpdateExistingRequestValidatesAndLoadsEnvelopeChildren(): void {
 		$fileEntity = new FileEntity();
 		$fileEntity->setId(21);
diff --git a/tests/php/Unit/Service/Validation/FileInputValidatorTest.php b/tests/php/Unit/Service/Validation/FileInputValidatorTest.php
index 733d9f3d8f..330ed0e533 100644
--- a/tests/php/Unit/Service/Validation/FileInputValidatorTest.php
+++ b/tests/php/Unit/Service/Validation/FileInputValidatorTest.php
@@ -47,6 +47,57 @@ public function setUp(): void {
 		);
 	}
 
+	/**
+	 * The Files app sends node ids as strings (`Node.id` of `@nextcloud/files`)
+	 * and, since Nextcloud 33, they can exceed a JavaScript number. The API
+	 * accepts an int or a string of digits and the services only see an int.
+	 */
+	#[DataProvider('nodeIdNormalizationCases')]
+	public function testNormalizeNodeId(array $file, array $expected): void {
+		$this->assertSame($expected, $this->validator->normalizeNodeId($file));
+	}
+
+	public static function nodeIdNormalizationCases(): array {
+		return [
+			'int stays int' => [['nodeId' => 42, 'name' => 'a.pdf'], ['nodeId' => 42, 'name' => 'a.pdf']],
+			'zero stays int' => [['nodeId' => 0], ['nodeId' => 0]],
+			'canonical decimal string becomes int' => [['nodeId' => '42'], ['nodeId' => 42]],
+			'zero as string' => [['nodeId' => '0'], ['nodeId' => 0]],
+			'node id above Number.MAX_SAFE_INTEGER' => [['nodeId' => '9007199254740993'], ['nodeId' => 9007199254740993]],
+			'64-bit node id' => [['nodeId' => '9223372036854775807'], ['nodeId' => PHP_INT_MAX]],
+			'null is absent' => [['nodeId' => null], ['nodeId' => null]],
+			'without nodeId' => [['base64' => 'abc'], ['base64' => 'abc']],
+		];
+	}
+
+	/**
+	 * The boundary either hands an int to the services or stops here: an
+	 * invalid value must not reach a later cast that could read it differently.
+	 */
+	#[DataProvider('invalidNodeIdCases')]
+	public function testNormalizeNodeIdRejectsAnythingElse(mixed $nodeId): void {
+		$this->expectException(LibresignException::class);
+		$this->expectExceptionMessage('Invalid fileID');
+
+		$this->validator->normalizeNodeId(['nodeId' => $nodeId]);
+	}
+
+	public static function invalidNodeIdCases(): array {
+		return [
+			'negative int' => [-1],
+			'string above PHP_INT_MAX' => ['9223372036854775808'],
+			'non numeric string' => ['temp-node'],
+			'signed string' => ['-42'],
+			'leading zeros' => ['0042'],
+			'decimal separator' => ['42.0'],
+			'surrounding spaces' => [' 42 '],
+			'empty string' => [''],
+			'float' => [42.0],
+			'bool' => [true],
+			'array' => [['42']],
+		];
+	}
+
 	#[DataProvider('mimeTypeCases')]
 	public function testValidatesMimeTypeForFileRole(string $mimeType, int $type, bool $valid): void {
 		if (!$valid) {