diff --git a/libraries/overleaf-editor-core/lib/operation/text_operation.js b/libraries/overleaf-editor-core/lib/operation/text_operation.js
index e153f0c3ec..76a738c428 100644
--- a/libraries/overleaf-editor-core/lib/operation/text_operation.js
+++ b/libraries/overleaf-editor-core/lib/operation/text_operation.js
@@ -45,9 +45,13 @@ class TextOperation extends EditOperation {
   /**
    * Length of the longest file that we'll attempt to edit, in characters.
    *
+   * Backend OT/history processing limit. Incoming uploads/edits are separately
+   * capped at 2MB via `max_doc_length`; this higher ceiling lets legacy updates
+   * with larger OT results flow through history instead of throwing TooLongError.
+   *
    * @type {number}
    */
-  static MAX_STRING_LENGTH = 2 * Math.pow(1024, 2)
+  static MAX_STRING_LENGTH = 3 * Math.pow(1024, 2)
   static UnprocessableError = UnprocessableError
   static ApplyError = ApplyError
   static InvalidInsertionError = InvalidInsertionError
diff --git a/services/web/frontend/js/features/source-editor/extensions/realtime.ts b/services/web/frontend/js/features/source-editor/extensions/realtime.ts
index 99d60262c5..4932c70aab 100644
--- a/services/web/frontend/js/features/source-editor/extensions/realtime.ts
+++ b/services/web/frontend/js/features/source-editor/extensions/realtime.ts
@@ -332,6 +332,11 @@ class HistoryOTAdapter {
 
   handleUpdateFromCM(transactions: readonly Transaction[]) {
     for (const transaction of transactions) {
+      // TODO: unlike the sharejs adapter above, this only checks visible
+      // length. When history-ot is enabled, add the retained tracked-deletes
+      // length here too (as #20052 did for sharejs) — otherwise total stored
+      // content (visible + tracked deletes) can exceed the backend cap
+      // (TextOperation.MAX_STRING_LENGTH) and fail with a 422.
       if (
         this.maxDocLength &&
         transaction.changes.newLength >= this.maxDocLength