diff --git a/frontend/src/components/features/chat/chat-interface.tsx b/frontend/src/components/features/chat/chat-interface.tsx
index 9cb72864b3..23c7dc9992 100644
--- a/frontend/src/components/features/chat/chat-interface.tsx
+++ b/frontend/src/components/features/chat/chat-interface.tsx
@@ -32,6 +32,7 @@ import { ErrorMessageBanner } from "./error-message-banner";
 import { shouldRenderEvent } from "./event-content-helpers/should-render-event";
 import { useUploadFiles } from "#/hooks/mutation/use-upload-files";
 import { useConfig } from "#/hooks/query/use-config";
+import { validateFiles } from "#/utils/file-validation";
 
 function getEntryPoint(
   hasRepository: boolean | null,
@@ -92,9 +93,12 @@ export function ChatInterface() {
 
   const handleSendMessage = async (
     content: string,
-    images: File[],
-    files: File[],
+    originalImages: File[],
+    originalFiles: File[],
   ) => {
+    // Create mutable copies of the arrays
+    const images = [...originalImages];
+    const files = [...originalFiles];
     if (events.length === 0) {
       posthog.capture("initial_query_submitted", {
         entry_point: getEntryPoint(
@@ -110,6 +114,16 @@ export function ChatInterface() {
         current_message_length: content.length,
       });
     }
+
+    // Validate file sizes before any processing
+    const allFiles = [...images, ...files];
+    const validation = validateFiles(allFiles);
+
+    if (!validation.isValid) {
+      displayErrorToast(`Error: ${validation.errorMessage}`);
+      return; // Stop processing if validation fails
+    }
+
     const promises = images.map((image) => convertImageToBase64(image));
     const imageUrls = await Promise.all(promises);
 
diff --git a/frontend/src/components/features/chat/interactive-chat-box.tsx b/frontend/src/components/features/chat/interactive-chat-box.tsx
index d9428f4347..e99d20d04d 100644
--- a/frontend/src/components/features/chat/interactive-chat-box.tsx
+++ b/frontend/src/components/features/chat/interactive-chat-box.tsx
@@ -5,6 +5,8 @@ import { ImageCarousel } from "../images/image-carousel";
 import { UploadImageInput } from "../images/upload-image-input";
 import { FileList } from "../files/file-list";
 import { isFileImage } from "#/utils/is-file-image";
+import { displayErrorToast } from "#/utils/custom-toast-handlers";
+import { validateFiles } from "#/utils/file-validation";
 
 interface InteractiveChatBoxProps {
   isDisabled?: boolean;
@@ -27,14 +29,20 @@ export function InteractiveChatBox({
   const [files, setFiles] = React.useState<File[]>([]);
 
   const handleUpload = (selectedFiles: File[]) => {
-    setFiles((prevFiles) => [
-      ...prevFiles,
-      ...selectedFiles.filter((f) => !isFileImage(f)),
-    ]);
-    setImages((prevImages) => [
-      ...prevImages,
-      ...selectedFiles.filter((f) => isFileImage(f)),
-    ]);
+    // Validate files before adding them
+    const validation = validateFiles(selectedFiles, [...images, ...files]);
+
+    if (!validation.isValid) {
+      displayErrorToast(`Error: ${validation.errorMessage}`);
+      return; // Don't add any files if validation fails
+    }
+
+    // Filter valid files by type
+    const validFiles = selectedFiles.filter((f) => !isFileImage(f));
+    const validImages = selectedFiles.filter((f) => isFileImage(f));
+
+    setFiles((prevFiles) => [...prevFiles, ...validFiles]);
+    setImages((prevImages) => [...prevImages, ...validImages]);
   };
 
   const removeElementByIndex = (array: Array<File>, index: number) => {
diff --git a/frontend/src/utils/file-validation.ts b/frontend/src/utils/file-validation.ts
new file mode 100644
index 0000000000..490912d506
--- /dev/null
+++ b/frontend/src/utils/file-validation.ts
@@ -0,0 +1,70 @@
+export const MAX_FILE_SIZE = 3 * 1024 * 1024; // 3MB maximum file size
+export const MAX_TOTAL_SIZE = 3 * 1024 * 1024; // 3MB maximum total size for all files combined
+
+export interface FileValidationResult {
+  isValid: boolean;
+  errorMessage?: string;
+  oversizedFiles?: string[];
+}
+
+/**
+ * Validates individual file sizes
+ */
+export function validateIndividualFileSizes(
+  files: File[],
+): FileValidationResult {
+  const oversizedFiles = files.filter((file) => file.size > MAX_FILE_SIZE);
+
+  if (oversizedFiles.length > 0) {
+    const fileNames = oversizedFiles.map((f) => f.name);
+    return {
+      isValid: false,
+      errorMessage: `Files exceeding 3MB are not allowed: ${fileNames.join(", ")}`,
+      oversizedFiles: fileNames,
+    };
+  }
+
+  return { isValid: true };
+}
+
+/**
+ * Validates total file size including existing files
+ */
+export function validateTotalFileSize(
+  newFiles: File[],
+  existingFiles: File[] = [],
+): FileValidationResult {
+  const currentTotalSize = existingFiles.reduce(
+    (sum, file) => sum + file.size,
+    0,
+  );
+  const newFilesSize = newFiles.reduce((sum, file) => sum + file.size, 0);
+  const totalSize = currentTotalSize + newFilesSize;
+
+  if (totalSize > MAX_TOTAL_SIZE) {
+    const totalSizeMB = (totalSize / (1024 * 1024)).toFixed(1);
+    return {
+      isValid: false,
+      errorMessage: `Total file size would be ${totalSizeMB}MB, exceeding the 3MB limit. Please select fewer or smaller files.`,
+    };
+  }
+
+  return { isValid: true };
+}
+
+/**
+ * Validates both individual and total file sizes
+ */
+export function validateFiles(
+  newFiles: File[],
+  existingFiles: File[] = [],
+): FileValidationResult {
+  // First check individual file sizes
+  const individualValidation = validateIndividualFileSizes(newFiles);
+  if (!individualValidation.isValid) {
+    return individualValidation;
+  }
+
+  // Then check total size
+  return validateTotalFileSize(newFiles, existingFiles);
+}
diff --git a/openhands/server/shared.py b/openhands/server/shared.py
index b62157983c..715cd0e630 100644
--- a/openhands/server/shared.py
+++ b/openhands/server/shared.py
@@ -43,7 +43,11 @@ if redis_host:
 
 
 sio = socketio.AsyncServer(
-    async_mode='asgi', cors_allowed_origins='*', client_manager=client_manager
+    async_mode='asgi',
+    cors_allowed_origins='*',
+    client_manager=client_manager,
+    # Increase buffer size to 4MB (to handle 3MB files with base64 overhead)
+    max_http_buffer_size=4 * 1024 * 1024,
 )
 
 MonitoringListenerImpl = get_impl(