How to Fix Claude Cloudflare Error 520

Cloudflare Error 520 ("Web server is returning an unknown error") occurs when Claude's origin servers at Anthropic send an invalid, empty, or unexpected response back to Cloudflare's reverse proxy. Because Cloudflare cannot interpret the response, it halts the connection and displays the 520 error page.

While this error often points to temporary backend instability on Anthropic's side, it can also be triggered by localized network issues, bloated browser cookies, or interfering browser extensions. Follow this step-by-step troubleshooting guide to resolve the error and restore your access to Claude.

1. Perform a Hard Refresh to Bypass Local Cache A standard browser refresh reuses cached files, which may preserve the bad connection state. A hard refresh forces your browser to request the entire webpage directly from the server.

• Windows / Linux: Press Ctrl + F5, or press Ctrl + Shift + R.
• Mac: Press Cmd + Shift + R.
• Safari (Mac): Hold the Option key and click the Reload icon in the address bar.

If the page loads successfully after a hard refresh, the issue was likely a temporary routing glitch at the CDN edge.

2. Clear Claude-Specific Cookies and Site Storage Cloudflare Error 520 can be caused by "header bloat." If your browser's cookies and local storage for `claude.ai` grow too large, the origin server may reject the request headers, causing Cloudflare to throw a 520 error.

1. Open Chrome, Edge, or Brave and navigate to claude.ai.
2. Click the padlock or settings icon on the far left of the URL address bar.
3. Select Site settings (or Cookies and site data > Manage cookies and site data).
4. Click Delete or Clear data next to all entries associated with claude.ai and anthropic.com.
5. Restart your browser and attempt to sign back into Claude.

3. Disable Conflicting Extensions and VPNs Certain ad blockers, privacy extensions, and VPNs modify HTTP request headers in a way that triggers Cloudflare's security policies, leading to an unexpected 520 response.

1. Open a private or incognito window (Ctrl + Shift + N or Cmd + Shift + N). This disables most extensions by default.
2. Log in to Claude within the private window.
3. If Claude works, return to your normal browser window and disable browser extensions one by one—starting with ad-blockers, script-blockers, and anti-fingerprinting extensions—until you locate the culprit.
4. If you are using a VPN, disconnect from it or switch to a different protocol (e.g., OpenVPN to WireGuard) to change how your traffic is encapsulated.

4. Check for Localized Routing Issues (Flush DNS) If your ISP's DNS records are outdated, your traffic might be routed through an unstable or overloaded Cloudflare edge server. Flushing your DNS cache forces your operating system to fetch updated IP mapping records.

Windows: 1. Press the **Start** key, type `cmd`, and open the **Command Prompt** as administrator. 2. Type `ipconfig /flushdns` and press **Enter**.

macOS: 1. Open **Terminal** via Spotlight Search. 2. Type `sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder` and press **Enter**. 3. Enter your system password when prompted.

5. Use the Console Interface as a Temporary Backup When the main chat interface at `claude.ai` is experiencing 520 errors due to a frontend web server outage, the developer-oriented Anthropic Console often remains operational because it uses a different routing path.

1. Navigate to console.anthropic.com.
2. Sign in with your existing Claude credentials.
3. Go to the Workbench tab.
4. You can prompt Claude models directly from this interface using your account's API credits or free trial tier while the web UI recovers.

When to Escalate If you have completed all the steps and the Error 520 persists for more than 30 minutes, the problem lies entirely on Anthropic's origin infrastructure. Check the official `status.anthropic.com` page. If no outage is listed but the error persists across multiple devices and networks, contact Anthropic Support by clicking the "Help" button on their support homepage or emailing `support@anthropic.com` with a screenshot of the Cloudflare Ray ID printed at the bottom of the error page.