diff --git a/apps/server/src/api-data/rundown/rundown.dao.ts b/apps/server/src/api-data/rundown/rundown.dao.ts
index ef42edafe..b0f6828e4 100644
--- a/apps/server/src/api-data/rundown/rundown.dao.ts
+++ b/apps/server/src/api-data/rundown/rundown.dao.ts
@@ -27,6 +27,7 @@ import {
   isPlayableEvent,
 } from 'ontime-types';
 import { addToRundown, createGroup, customFieldLabelToKey, getInsertAfterId, insertAtIndex } from 'ontime-utils';
+import type { DeepReadonly } from 'ts-essentials';
 
 import { getDataProvider } from '../../classes/data-provider/DataProvider.js';
 import { consoleError } from '../../utils/console.js';
@@ -80,9 +81,16 @@ export const getRundownMetadata = (): Readonly<RundownMetadata> => rundownMetada
 export const getProjectCustomFields = (): Readonly<CustomFields> => projectCustomFields;
 export const getEntryWithId = (entryId: EntryId): OntimeEntry | undefined => cachedRundown.entries[entryId];
 
-type Transaction = {
+/**
+ * @param R the type callers see for `rundown` - a plain, mutable `Rundown` when the
+ * transaction was opened with `mutableRundown: true`, otherwise a `DeepReadonly<Rundown>`
+ * so that accidentally mutating an entry (or an order array) on a non-mutable transaction
+ * - which would silently corrupt the live cache without going through commit() - is a
+ * compile-time error instead of a runtime bug.
+ */
+type Transaction<R> = {
   customFields: CustomFields;
-  rundown: Rundown;
+  rundown: R;
 
   commit: (shouldProcess?: boolean) => Promise<{
     rundown: Readonly<Rundown>;
@@ -103,7 +111,13 @@ type TransactionOptions = {
   rundownId?: string;
 };
 
-export function createTransaction(options: TransactionOptions): Transaction {
+export function createTransaction(options: TransactionOptions & { mutableRundown: true }): Transaction<Rundown>;
+export function createTransaction(
+  options: TransactionOptions & { mutableRundown?: false },
+): Transaction<DeepReadonly<Rundown>>;
+export function createTransaction(
+  options: TransactionOptions,
+): Transaction<Rundown> | Transaction<DeepReadonly<Rundown>> {
   const targetId = options.rundownId ?? cachedRundown.id;
   const isLoaded = targetId === cachedRundown.id;
   const sourceRundown: Rundown = isLoaded ? cachedRundown : (getDataProvider().getRundown(targetId) as Rundown);