diff --git a/apps/server/src/modules/tool/common/service/common-tool-validation.service.spec.ts b/apps/server/src/modules/tool/common/service/common-tool-validation.service.spec.ts
index 43a2aba1a39..d3ca547a854 100644
--- a/apps/server/src/modules/tool/common/service/common-tool-validation.service.spec.ts
+++ b/apps/server/src/modules/tool/common/service/common-tool-validation.service.spec.ts
@@ -597,6 +597,7 @@ describe('CommonToolValidationService', () => {
 				const setup = () => {
 					const undefinedRegex: CustomParameter = customParameterFactory.build({
 						name: 'undefinedRegex',
+						isOptional: false,
 						scope: CustomParameterScope.SCHOOL,
 						type: CustomParameterType.STRING,
 						regex: undefined,
@@ -629,6 +630,7 @@ describe('CommonToolValidationService', () => {
 				const setup = () => {
 					const validRegex: CustomParameter = customParameterFactory.build({
 						name: 'validRegex',
+						isOptional: false,
 						scope: CustomParameterScope.SCHOOL,
 						type: CustomParameterType.STRING,
 						regex: '[x]',
@@ -661,6 +663,7 @@ describe('CommonToolValidationService', () => {
 				const setup = () => {
 					const validRegex: CustomParameter = customParameterFactory.build({
 						name: 'validRegex',
+						isOptional: false,
 						scope: CustomParameterScope.SCHOOL,
 						type: CustomParameterType.STRING,
 						regex: '[x]',
@@ -688,6 +691,39 @@ describe('CommonToolValidationService', () => {
 					expect(func).toThrowError('tool_param_value_regex');
 				});
 			});
+
+			describe('when parameter is optional and a regex is given, but the param value is undefined', () => {
+				const setup = () => {
+					const optionalRegex: CustomParameter = customParameterFactory.build({
+						name: 'optionalRegex',
+						isOptional: true,
+						scope: CustomParameterScope.SCHOOL,
+						type: CustomParameterType.STRING,
+						regex: '[x]',
+					});
+					const { externalTool, schoolExternalTool } = createTools(
+						{
+							parameters: [optionalRegex],
+						},
+						{
+							parameters: [{ name: 'optionalRegex', value: undefined }],
+						}
+					);
+
+					return {
+						externalTool,
+						schoolExternalTool,
+					};
+				};
+
+				it('should return without error', () => {
+					const { externalTool, schoolExternalTool } = setup();
+
+					const func = () => service.checkCustomParameterEntries(externalTool, schoolExternalTool);
+
+					expect(func).not.toThrowError('tool_param_value_regex');
+				});
+			});
 		});
 	});
 });
diff --git a/apps/server/src/modules/tool/common/service/common-tool-validation.service.ts b/apps/server/src/modules/tool/common/service/common-tool-validation.service.ts
index 9d315a97f34..3ee9ee7d465 100644
--- a/apps/server/src/modules/tool/common/service/common-tool-validation.service.ts
+++ b/apps/server/src/modules/tool/common/service/common-tool-validation.service.ts
@@ -87,7 +87,7 @@ export class CommonToolValidationService {
 	}
 
 	private checkParameterRegex(foundEntry: CustomParameterEntry, param: CustomParameter): void {
-		if (param.regex && !new RegExp(param.regex).test(foundEntry.value ?? '')) {
+		if (foundEntry.value !== undefined && param.regex && !new RegExp(param.regex).test(foundEntry.value ?? '')) {
 			throw new ValidationError(
 				`tool_param_value_regex: The given entry for the parameter with name ${foundEntry.name} does not fit the regex.`
 			);
diff --git a/apps/server/src/modules/tool/external-tool/service/external-tool-validation.service.ts b/apps/server/src/modules/tool/external-tool/service/external-tool-validation.service.ts
index 434e7fac86e..bf768a83a3e 100644
--- a/apps/server/src/modules/tool/external-tool/service/external-tool-validation.service.ts
+++ b/apps/server/src/modules/tool/external-tool/service/external-tool-validation.service.ts
@@ -1,6 +1,5 @@
-import { Inject, Injectable } from '@nestjs/common';
+import { Injectable } from '@nestjs/common';
 import { ValidationError } from '@shared/common';
-import { IToolFeatures, ToolFeatures } from '../../tool-config';
 import { ExternalTool } from '../domain';
 import { ExternalToolLogoService } from './external-tool-logo.service';
 import { ExternalToolParameterValidationService } from './external-tool-parameter-validation.service';
@@ -11,7 +10,6 @@ export class ExternalToolValidationService {
 	constructor(
 		private readonly externalToolService: ExternalToolService,
 		private readonly externalToolParameterValidationService: ExternalToolParameterValidationService,
-		@Inject(ToolFeatures) private readonly toolFeatures: IToolFeatures,
 		private readonly externalToolLogoService: ExternalToolLogoService
 	) {}