📦 refactor: Request Message Sanitization for Smaller Final Response (#10792)

* refactor: implement sanitizeFileForTransmit and sanitizeMessageForTransmit functions for smaller payload to client transmission

* refactor: enhance sanitizeMessageForTransmit to preserve empty files array and avoid mutating original message

* refactor: update sanitizeMessageForTransmit to ensure immutability of files array and improve test clarity

packages/api/src/utils/message.ts

@@ -0,0 +1,68 @@
+import type { TFile, TMessage } from 'librechat-data-provider';
+
+/** Fields to strip from files before client transmission */
+const FILE_STRIP_FIELDS = ['text', '_id', '__v'] as const;
+
+/** Fields to strip from messages before client transmission */
+const MESSAGE_STRIP_FIELDS = ['fileContext'] as const;
+
+/**
+ * Strips large/unnecessary fields from a file object before transmitting to client.
+ * Use this within existing loops when building file arrays to avoid extra iterations.
+ *
+ * @param file - The file object to sanitize
+ * @returns A new file object without the stripped fields
+ *
+ * @example
+ * // Use in existing file processing loop:
+ * for (const attachment of client.options.attachments) {
+ *   if (messageFiles.has(attachment.file_id)) {
+ *     userMessage.files.push(sanitizeFileForTransmit(attachment));
+ *   }
+ * }
+ */
+export function sanitizeFileForTransmit<T extends Partial<TFile>>(
+  file: T,
+): Omit<T, (typeof FILE_STRIP_FIELDS)[number]> {
+  const sanitized = { ...file };
+  for (const field of FILE_STRIP_FIELDS) {
+    delete sanitized[field as keyof typeof sanitized];
+  }
+  return sanitized;
+}
+
+/**
+ * Sanitizes a message object before transmitting to client.
+ * Removes large fields like `fileContext` and strips `text` from embedded files.
+ *
+ * @param message - The message object to sanitize
+ * @returns A new message object safe for client transmission
+ *
+ * @example
+ * sendEvent(res, {
+ *   final: true,
+ *   requestMessage: sanitizeMessageForTransmit(userMessage),
+ *   responseMessage: response,
+ * });
+ */
+export function sanitizeMessageForTransmit<T extends Partial<TMessage>>(
+  message: T,
+): Omit<T, (typeof MESSAGE_STRIP_FIELDS)[number]> {
+  if (!message) {
+    return message as Omit<T, (typeof MESSAGE_STRIP_FIELDS)[number]>;
+  }
+
+  const sanitized = { ...message };
+
+  // Remove message-level fields
+  for (const field of MESSAGE_STRIP_FIELDS) {
+    delete sanitized[field as keyof typeof sanitized];
+  }
+
+  // Always create a new array when files exist to maintain full immutability
+  if (Array.isArray(sanitized.files)) {
+    sanitized.files = sanitized.files.map((file) => sanitizeFileForTransmit(file));
+  }
+
+  return sanitized;
+}