Bug Description

File upload fails with error:

blob:http://localhost:2026/xxx:1 Failed to load resource: net::ERR_FAILED
Failed to fetch file xxx.docx: TypeError: Failed to fetch

Code Location

frontend/src/core/threads/hooks.ts:263

const response = await fetch(fileUIPart.url);  // blob URL
const blob = await response.blob();            // fails

Full Error Stack

blob:http://localhost:2026/93e20197-a4ac-4e68-a2ee-2b483873e14e:1 Failed to load resource: net::ERR_FAILED
1e7b60cd27d87c04.js:59 Failed to fetch file test.docx: TypeError: Failed to fetch
  at 1e7b60cd27d87c04.js:59:33014
  at async Promise.all (index 0)
  at async 1e7b60cd27d87c04.js:59:33160
1e7b60cd27d87c04.js:59 Failed to upload files: Error: Failed to prepare 1 attachment(s) for upload. Please retry.

Environment

• OS: macOS (Apple Silicon)
• Browser: Chrome 146
• Node.js: v24.14.0
• Docker: 29.3.1
• DeerFlow Version: 0f1b023 (latest main)
• Run Mode: Docker (both make docker-start and make up)

Reproduction Steps

1. Start DeerFlow: make up or make docker-start
2. Open http://localhost:2026 in Chrome
3. Create a new chat
4. Click attachment button and select any file (tried .docx, .txt, .jpg)
5. Error: "Failed to prepare 1 attachment(s) for upload. Please retry."

Expected Behavior

File should be uploaded successfully.

Actual Behavior

Blob URL created by browser fails to fetch, upload fails.

Additional Context

• Issue persists in both development and production modes
• Issue persists with English file names
• Issue persists after browser cache clear
• Issue persists in incognito mode

Possible Cause

The fetch(fileUIPart.url) call on blob URLs is failing. This might be related to:

• Browser security restrictions on blob URLs
• Timing issues with blob URL lifecycle
• React component re-rendering invalidating blob URLs

Suggested Fix

Consider storing the actual File object instead of re-fetching from blob URL, or use FileReader to read the file data directly.