diff --git a/apps/web/src/components/ai-elements/prompt-input.test.tsx b/apps/web/src/components/ai-elements/prompt-input.test.tsx
index 1aafcdffd..838e54ece 100644
--- a/apps/web/src/components/ai-elements/prompt-input.test.tsx
+++ b/apps/web/src/components/ai-elements/prompt-input.test.tsx
@@ -151,6 +151,34 @@ describe('PromptInput', () => {
     expect(textarea).toHaveFocus();
   });
 
+  it('reports unsupported files in a mixed selection', () => {
+    const onError = vi.fn();
+    const prompt = (
+      <PromptInput accept=".txt" onSubmit={() => {}} onError={onError}>
+        <AttachmentCount />
+      </PromptInput>
+    );
+    const { container } = render(prompt);
+
+    fireEvent.change(container.querySelector('input[type="file"]')!, {
+      target: {
+        files: [
+          new File(['notes'], 'notes.txt', { type: 'text/plain' }),
+          new File(['unsupported'], 'attachment.bin', {
+            type: 'application/octet-stream',
+          }),
+        ],
+      },
+    });
+
+    expect(screen.getByTestId('attachment-count')).toHaveTextContent('1');
+    expect(onError).toHaveBeenCalledWith({
+      code: 'accept',
+      message:
+        'Some files were not added because their types are not supported.',
+    });
+  });
+
   it('accepts attachments that match extension-based filters', () => {
     const { container } = render(
       <PromptInput accept=".md,.pdf" onSubmit={() => {}}>
diff --git a/apps/web/src/components/ai-elements/prompt-input.tsx b/apps/web/src/components/ai-elements/prompt-input.tsx
index 3fa573a8b..a7d056bd3 100644
--- a/apps/web/src/components/ai-elements/prompt-input.tsx
+++ b/apps/web/src/components/ai-elements/prompt-input.tsx
@@ -347,13 +347,15 @@ export const PromptInput = ({
       const incoming = Array.from(fileList);
       const accepted = incoming.filter((f) => matchesAccept(f));
 
-      if (incoming.length && accepted.length === 0) {
+      if (incoming.length > accepted.length) {
         onError?.({
           code: 'accept',
-          message: 'No files match the accepted types.',
+          message: accepted.length
+            ? 'Some files were not added because their types are not supported.'
+            : 'No files match the accepted types.',
         });
 
-        return;
+        if (accepted.length === 0) return;
       }
 
       const withinSize = (f: File) =>
@@ -424,13 +426,15 @@ export const PromptInput = ({
       const incoming = Array.from(fileList);
       const accepted = incoming.filter((f) => matchesAccept(f));
 
-      if (incoming.length && accepted.length === 0) {
+      if (incoming.length > accepted.length) {
         onError?.({
           code: 'accept',
-          message: 'No files match the accepted types.',
+          message: accepted.length
+            ? 'Some files were not added because their types are not supported.'
+            : 'No files match the accepted types.',
         });
 
-        return;
+        if (accepted.length === 0) return;
       }
 
       const withinSize = (f: File) =>
diff --git a/apps/web/src/components/tasks/TaskPromptInput.client.test.tsx b/apps/web/src/components/tasks/TaskPromptInput.client.test.tsx
index f84c92eac..2e8f267e5 100644
--- a/apps/web/src/components/tasks/TaskPromptInput.client.test.tsx
+++ b/apps/web/src/components/tasks/TaskPromptInput.client.test.tsx
@@ -1,4 +1,5 @@
 import { useState } from 'react';
+import { toast } from 'sonner';
 import {
   createEvent,
   fireEvent,
@@ -47,6 +48,30 @@ function KeyboardPrompt({
 }
 
 describe('TaskPromptInput', () => {
+  it('reports rejected attachments without clearing the prompt', () => {
+    const errorToast = vi.spyOn(toast, 'error').mockReturnValue(0);
+    try {
+      const { container } = renderPromptInput();
+      fireEvent.change(container.querySelector('input[type="file"]')!, {
+        target: {
+          files: [
+            new File(['unsupported'], 'attachment.bin', {
+              type: 'application/octet-stream',
+            }),
+          ],
+        },
+      });
+
+      expect(errorToast).toHaveBeenCalledWith(
+        'No files match the accepted types.',
+      );
+      expect(screen.getByRole('textbox')).toHaveValue('Fix the login bug');
+      expect(screen.queryByText('attachment.bin')).not.toBeInTheDocument();
+    } finally {
+      errorToast.mockRestore();
+    }
+  });
+
   it('explains why the send button is disabled when hovering it', async () => {
     const reason = 'Create an environment before starting a task.';
 
diff --git a/apps/web/src/components/tasks/TaskPromptInput.tsx b/apps/web/src/components/tasks/TaskPromptInput.tsx
index 02d14119e..bbfcc27ba 100644
--- a/apps/web/src/components/tasks/TaskPromptInput.tsx
+++ b/apps/web/src/components/tasks/TaskPromptInput.tsx
@@ -1,4 +1,5 @@
 import type { ReactNode } from 'react';
+import { toast } from 'sonner';
 import { BasicTooltip, SendHorizontal } from '@/components/system';
 
 import {
@@ -162,6 +163,7 @@ export function TaskPromptInput({
       <PromptInputRoot
         key={promptKey}
         onSubmit={onSubmit}
+        onError={(error) => toast.error(error.message)}
         clearOnSubmit={false}
         accept={ROOMOTE_FILE_ATTACHMENT_ACCEPT}
         multiple