fix(Google Vertex Chat Model Node): Preserve Gemini thought signature on single tool calls (#34797)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: yehorkardash <yehor.kardash@n8n.io>

package.json

@@ -223,7 +223,8 @@
       "vuedraggable@4.1.0": "patches/vuedraggable@4.1.0.patch",
       "assert@2.1.0": "patches/assert@2.1.0.patch",
       "lodash@4.18.1": "patches/lodash@4.18.1.patch",
-      "@langchain/openai@1.4.4": "patches/@langchain__openai@1.4.4.patch"
+      "@langchain/openai@1.4.4": "patches/@langchain__openai@1.4.4.patch",
+      "@langchain/google-common@2.1.24": "patches/@langchain__google-common@2.1.24.patch"
     }
   }
 }

packages/@n8n/nodes-langchain/nodes/llms/LmChatGoogleVertex/LmChatGoogleVertex.node.ts

@@ -15,11 +15,12 @@ import {
 	type SupplyData,
 	type ILoadOptionsFunctions,
 	type JsonObject,
+	type NodeError,
 	NodeOperationError,
 	validateNodeParameters,
 } from 'n8n-workflow';
 
-import { makeErrorFromStatus } from './error-handling';
+import { extractGoogleErrorMessage, makeErrorFromStatus } from './error-handling';
 import { getAdditionalOptions } from '../gemini-common/additional-options';
 import {
 	getVertexEndpoint,
@@ -27,6 +28,14 @@ import {
 	vertexLocationField,
 } from '../gemini-common/vertex-location';
 
+function errorDescriptionMapper(error: NodeError) {
+	if (error.description?.includes('properties: should be non-empty for OBJECT type')) {
+		return 'Google Vertex requires at least one <a href="https://docs.n8n.io/advanced-ai/examples/using-the-fromai-function/" target="_blank">dynamic parameter</a> when using tools';
+	}
+
+	return error.description ?? 'Unknown error';
+}
+
 export class LmChatGoogleVertex implements INodeType {
 	description: INodeTypeDescription = {
 		displayName: 'Google Vertex Chat Model',
@@ -198,11 +207,12 @@ export class LmChatGoogleVertex implements INodeType {
 				temperature: options.temperature,
 				maxOutputTokens: options.maxOutputTokens,
 				safetySettings,
-				callbacks: [new N8nLlmTracing(this)],
+				callbacks: [new N8nLlmTracing(this, { errorDescriptionMapper })],
 				// Handle ChatVertexAI invocation errors to provide better error messages
 				onFailedAttempt: makeN8nLlmFailedAttemptHandler(this, (error: any) => {
 					// eslint-disable-next-line @typescript-eslint/no-unsafe-member-access
-					const customError = makeErrorFromStatus(Number(error?.response?.status), {
+					const status = Number(error?.response?.status);
+					const customError = makeErrorFromStatus(status, {
 						modelName,
 					});
 
@@ -210,6 +220,18 @@ export class LmChatGoogleVertex implements INodeType {
 						throw new NodeOperationError(this.getNode(), error as JsonObject, customError);
 					}
 
+					if (status === 400) {
+						// Surface Google's error detail; a bare rethrow would hide it behind a
+						// generic "Bad request" wrapper
+						throw new NodeOperationError(this.getNode(), error as JsonObject, {
+							message: 'Bad request - please check your parameters',
+							description:
+								extractGoogleErrorMessage(error as { message?: string }) ??
+								// eslint-disable-next-line @typescript-eslint/no-unsafe-member-access
+								(typeof error?.message === 'string' ? (error.message as string) : undefined),
+						});
+					}
+
 					throw error;
 				}),
 			};

packages/@n8n/nodes-langchain/nodes/llms/LmChatGoogleVertex/error-handling.ts

@@ -23,3 +23,40 @@ export function makeErrorFromStatus(statusCode: number, context?: ErrorContext):
 
 	return errorMessages[statusCode];
 }
+
+interface GoogleErrorLike {
+	message?: string;
+	response?: { status?: number; data?: unknown };
+}
+
+function readGoogleErrorMessage(data: unknown): string | undefined {
+	// Google error bodies look like `{ "error": { "message": "..." } }`, sometimes array-wrapped
+	const entry: unknown = Array.isArray(data) ? data[0] : data;
+	if (entry && typeof entry === 'object' && 'error' in entry) {
+		const innerError = (entry as { error?: { message?: unknown } }).error;
+		if (innerError && typeof innerError.message === 'string') return innerError.message;
+	}
+	return undefined;
+}
+
+/**
+ * Extracts the human-readable Google API error message from a failed request, so it can be
+ * surfaced to the user instead of a generic status-code wrapper.
+ */
+export function extractGoogleErrorMessage(error: GoogleErrorLike): string | undefined {
+	const fromData = readGoogleErrorMessage(error.response?.data);
+	if (fromData) return fromData;
+
+	// @langchain/google-common embeds the raw body in the error message:
+	// "Google request failed with status code <n>: <body>"
+	const match = error.message?.match(/status code \d+: ([\s\S]+)/);
+	if (match) {
+		try {
+			return readGoogleErrorMessage(JSON.parse(match[1])) ?? match[1];
+		} catch {
+			return match[1];
+		}
+	}
+
+	return undefined;
+}

packages/@n8n/nodes-langchain/nodes/llms/LmChatGoogleVertex/test/LmChatGoogleVertex.test.ts

@@ -201,4 +201,81 @@ describe('LmChatGoogleVertex - Thinking Budget', () => {
 			);
 		});
 	});
+
+	describe('supplyData - failed attempt error handling', () => {
+		const getCustomErrorHandler = async () => {
+			const mockContext = setupMockContext();
+
+			mockContext.getNodeParameter = vi.fn().mockImplementation((paramName: string) => {
+				if (paramName === 'modelName') return 'gemini-3.1-flash-lite';
+				if (paramName === 'projectId') return 'test-project';
+				if (paramName === 'options') return {};
+				if (paramName === 'options.safetySettings.values') return null;
+				return undefined;
+			});
+
+			await lmChatGoogleVertex.supplyData.call(mockContext, 0);
+
+			return mockedMakeN8nLlmFailedAttemptHandler.mock.calls[0][1]!;
+		};
+
+		it('should map 403 to a friendly unauthorized error', async () => {
+			const handler = await getCustomErrorHandler();
+
+			expect(() => handler({ response: { status: 403 } })).toThrowError(
+				'Unauthorized for this project',
+			);
+		});
+
+		it('should surface the Google error detail on 400', async () => {
+			const handler = await getCustomErrorHandler();
+			const error = {
+				message: 'Google request failed with status code 400',
+				response: {
+					status: 400,
+					data: { error: { message: 'Function call is missing a thought_signature' } },
+				},
+			};
+
+			try {
+				handler(error);
+				expect.unreachable('handler should throw');
+			} catch (e) {
+				expect(e).toMatchObject({
+					message: 'Bad request - please check your parameters',
+					description: 'Function call is missing a thought_signature',
+				});
+			}
+		});
+
+		it('should fall back to the raw error message on 400 when no Google detail is found', async () => {
+			const handler = await getCustomErrorHandler();
+			const error = {
+				message: 'Request had invalid authentication scopes',
+				response: { status: 400 },
+			};
+
+			try {
+				handler(error);
+				expect.unreachable('handler should throw');
+			} catch (e) {
+				expect(e).toMatchObject({
+					message: 'Bad request - please check your parameters',
+					description: 'Request had invalid authentication scopes',
+				});
+			}
+		});
+
+		it('should rethrow the original error for unmapped statuses', async () => {
+			const handler = await getCustomErrorHandler();
+			const error = { message: 'boom', response: { status: 500 } };
+
+			try {
+				handler(error);
+				expect.unreachable('handler should throw');
+			} catch (e) {
+				expect(e).toBe(error);
+			}
+		});
+	});
 });

packages/@n8n/nodes-langchain/nodes/llms/LmChatGoogleVertex/test/error-handling.test.ts

@@ -0,0 +1,67 @@
+import { extractGoogleErrorMessage, makeErrorFromStatus } from '../error-handling';
+
+describe('makeErrorFromStatus', () => {
+	it('should map 403 to an unauthorized error', () => {
+		expect(makeErrorFromStatus(403)).toEqual({
+			message: 'Unauthorized for this project',
+			description:
+				'Check your Google Cloud project ID, that your credential has access to that project and that billing is enabled',
+		});
+	});
+
+	it('should map 404 with the model name', () => {
+		expect(makeErrorFromStatus(404, { modelName: 'gemini-3.1-flash-lite' })).toEqual({
+			message: "No model found called 'gemini-3.1-flash-lite'",
+		});
+	});
+
+	it('should return undefined for unmapped statuses', () => {
+		expect(makeErrorFromStatus(400)).toBeUndefined();
+		expect(makeErrorFromStatus(500)).toBeUndefined();
+	});
+});
+
+describe('extractGoogleErrorMessage', () => {
+	it('should read the message from response data', () => {
+		const error = {
+			message: 'Google request failed with status code 400',
+			response: {
+				status: 400,
+				data: { error: { code: 400, message: 'Function call is missing a thought_signature' } },
+			},
+		};
+		expect(extractGoogleErrorMessage(error)).toBe('Function call is missing a thought_signature');
+	});
+
+	it('should read the message from array-wrapped response data', () => {
+		const error = {
+			response: {
+				status: 400,
+				data: [{ error: { message: 'properties: should be non-empty for OBJECT type' } }],
+			},
+		};
+		expect(extractGoogleErrorMessage(error)).toBe(
+			'properties: should be non-empty for OBJECT type',
+		);
+	});
+
+	it('should parse the body embedded in the error message', () => {
+		const error = {
+			message:
+				'Google request failed with status code 400: {"error":{"message":"Function call is missing a thought_signature","status":"INVALID_ARGUMENT"}}',
+		};
+		expect(extractGoogleErrorMessage(error)).toBe('Function call is missing a thought_signature');
+	});
+
+	it('should fall back to the raw embedded body when it is not JSON', () => {
+		const error = {
+			message: 'Google request failed with status code 400: something went wrong',
+		};
+		expect(extractGoogleErrorMessage(error)).toBe('something went wrong');
+	});
+
+	it('should return undefined when no detail is available', () => {
+		expect(extractGoogleErrorMessage({ message: 'Bad request' })).toBeUndefined();
+		expect(extractGoogleErrorMessage({})).toBeUndefined();
+	});
+});

packages/@n8n/nodes-langchain/utils/agent-execution/buildSteps.ts

@@ -191,21 +191,30 @@ interface ProcessedToolResponse {
  * The structure matches what @langchain/google-common expects:
  * - `__gemini_function_call_thought_signatures__`: maps the first tool call ID to the signature
  * - `tool_calls`: array of tool call descriptors
- * - `signatures`: array aligned to parts [textPart, functionCall_1, ...], with the signature
- *   only on the first function call (per Google's docs)
+ * - `signatures`: array positionally aligned to the Gemini request parts, with the signature
+ *   only on the first function call (per Google's docs). @langchain/google-common applies it
+ *   only when `signatures.length` equals the part count, so the array must account for whether
+ *   the message content produces a leading text part.
  *
  * @param toolCalls - Tool calls to include
  * @param thoughtSignature - The Gemini thought signature
+ * @param hasLeadingTextPart - Whether the message content serializes to a text part before the
+ *   function-call parts
  * @returns additional_kwargs object for AIMessage
  */
 function buildGeminiAdditionalKwargs(
 	toolCalls: Array<{ id: string; name: string; args: IDataObject }>,
 	thoughtSignature: string,
+	hasLeadingTextPart: boolean,
 ): Record<string, unknown> {
-	const signatures: string[] = ['', thoughtSignature];
-	for (let i = 2; i <= toolCalls.length; i++) {
-		signatures.push('');
-	}
+	const signatures = [
+		// Reserve the first position for the leading text part, which has no signature
+		...(hasLeadingTextPart ? [''] : []),
+		// Gemini attaches the thought signature only to the first function call
+		thoughtSignature,
+		// Reserve positions for the remaining unsigned function calls
+		...toolCalls.slice(1).map(() => ''),
+	];
 
 	return {
 		__gemini_function_call_thought_signatures__: {
@@ -242,6 +251,8 @@ function buildIndividualAIMessage(
 		? buildGeminiAdditionalKwargs(
 				[{ id: toolId, name: toolName, args: toolInput }],
 				providerMetadata.thoughtSignature,
+				// Empty content produces no text part, so the function call is the first part
+				content !== null && content.length > 0,
 			)
 		: {};
 	// DeepSeek's thinking mode requires reasoning_content to be echoed back on the
@@ -289,7 +300,8 @@ function buildSharedGeminiAIMessage(
 	return new AIMessage({
 		content: `Calling tools: ${toolNames}`,
 		tool_calls: allToolCalls,
-		additional_kwargs: buildGeminiAdditionalKwargs(allToolCalls, thoughtSignature),
+		// String content serializes to a text part preceding the function-call parts
+		additional_kwargs: buildGeminiAdditionalKwargs(allToolCalls, thoughtSignature, true),
 	});
 }
 

packages/@n8n/nodes-langchain/utils/agent-execution/test/buildSteps.test.ts

@@ -1519,6 +1519,10 @@ describe('buildSteps', () => {
 			expect(message.additional_kwargs.__gemini_function_call_thought_signatures__).toEqual({
 				call_123: 'gemini_thought_sig_abc123',
 			});
+			// Content is empty, so the function call is the only Gemini request part; the
+			// signatures array must align with the parts or google-common drops it entirely
+			expect(message.content).toEqual([]);
+			expect(message.additional_kwargs.signatures).toEqual(['gemini_thought_sig_abc123']);
 		});
 
 		it('should group parallel tool calls into shared AIMessage with Gemini signature', () => {

patches/@langchain__google-common@2.1.24.patch

@@ -0,0 +1,28 @@
+diff --git a/dist/auth.cjs b/dist/auth.cjs
+index 37cb150e17c88285b79d62883715b33dab80ac6a..6c73f2a6f4d6e05c97bf1dd8c6334bb6ef4e7638 100644
+--- a/dist/auth.cjs
++++ b/dist/auth.cjs
+@@ -54,6 +54,9 @@ var GoogleAbstractedFetchClient = class {
+ 						body = JSON.stringify(err.response.data);
+ 					} catch {}
+ 				}
++				// gaxios (responseType "stream") parses the error body into err.message
++				// while leaving response.data unset; fall back so the detail isn't lost
++				if (body === undefined && typeof err?.message === "string" && err.message.length > 0) body = err.message;
+ 				this._throwRequestError(status, body, err?.response ?? { status }, context);
+ 			}
+ 			throw fetchError;
+diff --git a/dist/auth.js b/dist/auth.js
+index b3f1afa39a53dfb2b4e7020df01e17f54996dacc..111cb0005d210addf190036be9e191597f72da91 100644
+--- a/dist/auth.js
++++ b/dist/auth.js
+@@ -54,6 +54,9 @@ var GoogleAbstractedFetchClient = class {
+ 						body = JSON.stringify(err.response.data);
+ 					} catch {}
+ 				}
++				// gaxios (responseType "stream") parses the error body into err.message
++				// while leaving response.data unset; fall back so the detail isn't lost
++				if (body === undefined && typeof err?.message === "string" && err.message.length > 0) body = err.message;
+ 				this._throwRequestError(status, body, err?.response ?? { status }, context);
+ 			}
+ 			throw fetchError;

pnpm-lock.yaml

@@ -782,6 +782,9 @@ overrides:
   '@babel/traverse': ^7.23.2
 
 patchedDependencies:
+  '@langchain/google-common@2.1.24':
+    hash: b50436a6c7b0a82d54472dff331ba1b42ab8b816995f83295eef2ee4b482a87d
+    path: patches/@langchain__google-common@2.1.24.patch
   '@langchain/openai@1.4.4':
     hash: 0160a23d307d3bb4aedce23be0d1df4224738bee9184dd4b56bc03eaa37c9a87
     path: patches/@langchain__openai@1.4.4.patch
@@ -3117,7 +3120,7 @@ importers:
         version: 1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(@smithy/signature-v4@5.3.5)(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))
       '@langchain/google-common':
         specifier: 2.1.24
-        version: 2.1.24(@langchain/core@1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10)))
+        version: 2.1.24(patch_hash=b50436a6c7b0a82d54472dff331ba1b42ab8b816995f83295eef2ee4b482a87d)(@langchain/core@1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10)))
       '@langchain/google-genai':
         specifier: 2.1.24
         version: 2.1.24(@langchain/core@1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10)))
@@ -26599,14 +26602,14 @@ snapshots:
       - openai
       - ws
 
-  '@langchain/google-common@2.1.24(@langchain/core@1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10)))':
+  '@langchain/google-common@2.1.24(patch_hash=b50436a6c7b0a82d54472dff331ba1b42ab8b816995f83295eef2ee4b482a87d)(@langchain/core@1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10)))':
     dependencies:
       '@langchain/core': 1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(@smithy/signature-v4@5.3.5)(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))
       uuid: 13.0.1
 
   '@langchain/google-gauth@2.1.24(@langchain/core@1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10)))':
     dependencies:
-      '@langchain/google-common': 2.1.24(@langchain/core@1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10)))
+      '@langchain/google-common': 2.1.24(patch_hash=b50436a6c7b0a82d54472dff331ba1b42ab8b816995f83295eef2ee4b482a87d)(@langchain/core@1.2.0(@opentelemetry/api@1.9.0)(@opentelemetry/exporter-trace-otlp-proto@0.217.0(@opentelemetry/api@1.9.0))(@opentelemetry/sdk-trace-base@2.7.1(@opentelemetry/api@1.9.0))(openai@6.46.0(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10))(zod@3.25.67))(ws@8.21.0(bufferutil@4.0.9)(utf-8-validate@5.0.10)))
       google-auth-library: 10.1.0
     transitivePeerDependencies:
       - '@langchain/core'