diff --git a/apps/server/src/api-integration/__test__/integration.utils.test.ts b/apps/server/src/api-integration/__test__/integration.utils.test.ts
index 4b9c6a1d6..ea639877b 100644
--- a/apps/server/src/api-integration/__test__/integration.utils.test.ts
+++ b/apps/server/src/api-integration/__test__/integration.utils.test.ts
@@ -1,35 +1,52 @@
-import { EntryCustomFields, OntimeEvent } from 'ontime-types';
+import { CustomFields, EntryCustomFields, OntimeDelay, OntimeEvent } from 'ontime-types';
 import { isValidChangeProperty } from '../integration.utils.js';
 
 describe('isValidChangeProperty()', () => {
-  test('correct value and property', () => {
+  test('allows changing a valid event property with valid value', () => {
     const testEvent = {
       id: 'test',
       duration: 111,
     } as OntimeEvent;
 
-    expect(isValidChangeProperty(testEvent, 'duration', 123)).toBeTruthy();
+    expect(isValidChangeProperty(testEvent, 'duration', 123, {})).toBeTruthy();
   });
 
-  test('correct property and undefined value', () => {
+  test('forbids changing a valid event property with undefined value', () => {
     const testEvent = {
       id: 'test',
       duration: 111,
     } as OntimeEvent;
 
-    expect(isValidChangeProperty(testEvent, 'duration', undefined)).toBeFalsy();
+    expect(isValidChangeProperty(testEvent, 'duration', undefined, {})).toBeFalsy();
   });
 
-  test('missing property and undefined value', () => {
+  test('forbids changing a non-existing property', () => {
     const testEvent = {
       id: 'test',
       duration: 111,
     } as OntimeEvent;
 
-    expect(isValidChangeProperty(testEvent, 'missing', 123)).toBeFalsy();
+    expect(isValidChangeProperty(testEvent, 'missing', 123, {})).toBeFalsy();
   });
 
-  test('non existing custom value', () => {
+  test('forbids changing customValues in event without custom fields', () => {
+    const testDelay = {
+      id: 'test',
+      duration: 111,
+    } as OntimeDelay;
+
+    const testCustomFields = {
+      test: {
+        type: 'text',
+        colour: '#ffffff',
+        label: 'Test Field',
+      },
+    } satisfies CustomFields;
+
+    expect(isValidChangeProperty(testDelay, 'custom:test', 123, testCustomFields)).toBeFalsy();
+  });
+
+  test('forbids changing a non-existing custom value', () => {
     const testEvent = {
       id: 'test',
       duration: 111,
@@ -38,10 +55,10 @@ describe('isValidChangeProperty()', () => {
       } as EntryCustomFields,
     } as OntimeEvent;
 
-    expect(isValidChangeProperty(testEvent, 'custom:test', 123)).toBeFalsy();
+    expect(isValidChangeProperty(testEvent, 'custom:test', 123, {})).toBeFalsy();
   });
 
-  test('existing custom value', () => {
+  test('allows changing an existing custom value', () => {
     const testEvent = {
       id: 'test',
       duration: 111,
@@ -50,10 +67,18 @@ describe('isValidChangeProperty()', () => {
       } as EntryCustomFields,
     } as OntimeEvent;
 
-    expect(isValidChangeProperty(testEvent, 'custom:test', 123)).toBeTruthy();
+    const testCustomFields = {
+      test: {
+        type: 'text',
+        colour: '#ffffff',
+        label: 'Test Field',
+      },
+    } satisfies CustomFields;
+
+    expect(isValidChangeProperty(testEvent, 'custom:test', 123, testCustomFields)).toBeTruthy();
   });
 
-  test('empty custom definition', () => {
+  test('forbids changing a custom field with a missing value', () => {
     const testEvent = {
       id: 'test',
       duration: 111,
@@ -62,10 +87,18 @@ describe('isValidChangeProperty()', () => {
       } as EntryCustomFields,
     } as OntimeEvent;
 
-    expect(isValidChangeProperty(testEvent, 'custom:', 123)).toBeFalsy();
+    const testCustomFields = {
+      test: {
+        type: 'text',
+        colour: '#ffffff',
+        label: 'Test Field',
+      },
+    } satisfies CustomFields;
+
+    expect(isValidChangeProperty(testEvent, 'custom:', 123, testCustomFields)).toBeFalsy();
   });
 
-  test('build-in in custom', () => {
+  test('forbids references to prototype properties in custom object', () => {
     const testEvent = {
       id: 'test',
       duration: 111,
@@ -74,10 +107,18 @@ describe('isValidChangeProperty()', () => {
       } as EntryCustomFields,
     } as OntimeEvent;
 
-    expect(isValidChangeProperty(testEvent, 'custom:toString', 123)).toBeFalsy();
+    const testCustomFields = {
+      test: {
+        type: 'text',
+        colour: '#ffffff',
+        label: 'Test Field',
+      },
+    } satisfies CustomFields;
+
+    expect(isValidChangeProperty(testEvent, 'custom:toString', 123, testCustomFields)).toBeFalsy();
   });
 
-  test('build-in in top object', () => {
+  test('forbids references to prototype properties in object', () => {
     const testEvent = {
       id: 'test',
       duration: 111,
@@ -86,6 +127,14 @@ describe('isValidChangeProperty()', () => {
       } as EntryCustomFields,
     } as OntimeEvent;
 
-    expect(isValidChangeProperty(testEvent, 'toString', 123)).toBeFalsy();
+    const testCustomFields = {
+      test: {
+        type: 'text',
+        colour: '#ffffff',
+        label: 'Test Field',
+      },
+    } satisfies CustomFields;
+
+    expect(isValidChangeProperty(testEvent, 'toString', 123, testCustomFields)).toBeFalsy();
   });
 });
diff --git a/apps/server/src/api-integration/integration.controller.ts b/apps/server/src/api-integration/integration.controller.ts
index 08ca3a044..9d9f47393 100644
--- a/apps/server/src/api-integration/integration.controller.ts
+++ b/apps/server/src/api-integration/integration.controller.ts
@@ -23,7 +23,7 @@ import { socket } from '../adapters/WebsocketAdapter.js';
 import { coerceEnum } from '../utils/coerceType.js';
 import { editEntry } from '../api-data/rundown/rundown.service.js';
 import { willCauseRegeneration } from '../api-data/rundown/rundown.utils.js';
-import { getCurrentRundown } from '../api-data/rundown/rundown.dao.js';
+import { getCurrentRundown, getProjectCustomFields } from '../api-data/rundown/rundown.dao.js';
 
 let lastRequest: Date | null = null;
 
@@ -65,6 +65,7 @@ const actionHandlers: Record<ApiActionTag, ActionHandler> = {
     }
 
     const { entries } = getCurrentRundown();
+    const customFields = getProjectCustomFields();
 
     const targetEntry = entries[id];
     if (!targetEntry) {
@@ -77,7 +78,7 @@ const actionHandlers: Record<ApiActionTag, ActionHandler> = {
     let shouldThrottle = false;
 
     Object.entries(data).forEach(([property, value]) => {
-      if (!isValidChangeProperty(targetEntry, property, value)) {
+      if (!isValidChangeProperty(targetEntry, property, value, customFields)) {
         throw new Error('Invalid property or value');
       }
       const newObjectProperty = parseProperty(property, value);
diff --git a/apps/server/src/api-integration/integration.utils.ts b/apps/server/src/api-integration/integration.utils.ts
index d2bb401c3..d33b1702b 100644
--- a/apps/server/src/api-integration/integration.utils.ts
+++ b/apps/server/src/api-integration/integration.utils.ts
@@ -1,8 +1,7 @@
-import { EndAction, OntimeEntry, TimeStrategy, TimerType, isKeyOfType } from 'ontime-types';
+import { CustomFields, EndAction, OntimeEntry, TimeStrategy, TimerType, isKeyOfType } from 'ontime-types';
 import { maxDuration } from 'ontime-utils';
 
 import { coerceBoolean, coerceColour, coerceEnum, coerceNumber, coerceString } from '../utils/coerceType.js';
-import { getDataProvider } from '../classes/data-provider/DataProvider.js';
 
 /**
  *
@@ -44,13 +43,14 @@ const propertyConversion: Record<string, (value: unknown) => unknown> = {
   timeEnd: (value: unknown) => clampDuration(coerceNumber(value)),
 };
 
+/**
+ * coerces a property of an Ontime Entry
+ * @throws if the value does not conform to the expected type
+ */
 export function parseProperty(property: string, value: unknown) {
   if (property.startsWith('custom:')) {
+    // custom fields have been validated when used here
     const customKey = property.split(':')[1];
-    const customFields = getDataProvider().getCustomFields();
-    if (!(customKey in customFields)) {
-      throw new Error(`Custom field ${customKey} not found`);
-    }
     const parserFn = propertyConversion.custom;
     return { custom: { [customKey]: parserFn(value) } };
   }
@@ -61,13 +61,21 @@ export function parseProperty(property: string, value: unknown) {
   return { [property]: parserFn(value) };
 }
 
-export function isValidChangeProperty(target: OntimeEntry, property: string, value: unknown): boolean {
+/**
+ * Checks whether a valid property - value pair are applicable to an entry
+ */
+export function isValidChangeProperty(
+  target: OntimeEntry,
+  property: string,
+  value: unknown,
+  customFields: CustomFields,
+): boolean {
   if (typeof property !== 'string') return false;
   if (value === undefined) return false;
-  if (property.startsWith('custom:') && 'custom' in target) {
+  if (property.startsWith('custom:') && Object.hasOwn(target, 'custom')) {
     const customProperty = property.slice('custom:'.length);
     if (!customProperty) return false;
-    return Object.hasOwn(target.custom, customProperty);
+    return Object.hasOwn(customFields, customProperty);
   }
   return Object.hasOwn(target, property);
 }