Fix Claude 'Failed to Send Message' Red Error Banner

Why Does the "Failed to Send Message" Error Occur?

The "Failed to send message" error, accompanied by a prominent red banner at the bottom of the Claude.ai screen, signals a complete breakdown in the communication pipeline between your local interface and Anthropic's processing backend.

This specific error usually triggers when: * The WebSocket handshake drops mid-transmission. * Your authentication token has expired or desynced. * The size of your prompt payload (including file attachments) exceeds network gateway transmission thresholds. * Aggressive web filters, VPN configurations, or firewalls block the API requests behind the user interface.

Below are the diagnostic steps to identify the precise point of failure and clear the error banner.

How to Resolve the Claude Red Error Banner

1. Perform a Clean Session Reset An expired or corrupted authentication session is the most common cause of the red error banner. This happens frequently if you leave a Claude tab open overnight or switch networks while a tab is active.

1. Click your profile icon in the bottom-left corner of the Claude dashboard and select Sign Out.
2. Close all open browser tabs pointing to claude.ai.
3. Clear your browser's application cache specifically for the Claude domain.
4. Open a fresh browser tab, go to https://claude.ai, sign back in, and start a new conversation.

2. Check and Optimize File Attachments If the red banner only appears when you upload files (such as PDFs, CSVs, or images), the file size, formatting, or extraction process may be crashing the request parser.

• Reduce File Sizes: Ensure files are well below the max upload size. Split large PDFs into smaller sections of 10-20 pages each.
• Check File Encoding: Ensure text files are standard UTF-8 encoded. Corrupt binary files or password-protected PDFs will fail to process and prompt the red error banner.
• Clear Attachment Queue: Remove any pending file attachments, type a simple text-only message, and hit send. If that works, the issue lies entirely with the files you tried to upload.

3. Verify WebSocket and HTTPS Port Configuration Claude relies heavily on modern security configurations and persistent streaming channels. If your network blocks standard communication ports or fails to process chunked transfers, transmissions will abort.

• Check Port 443: Ensure your network allows uninterrupted outbound traffic on Port 443 (HTTPS) and Port 80.
• Disable Ad Blockers and Security Suites: Security software like McAfee, Norton, or browser shields (like Brave's Shield or uBlock Origin) can interpret streaming data as a potential security risk and cut the connection. Whitelist *.claude.ai and *.anthropic.com inside these tools.

4. Resolve Multi-Tab and Session Conflicts Running Claude in multiple browser windows or across several devices simultaneously can desynchronize session state tracking, causing older sessions to be rejected by the server.

1. Close Claude on all secondary devices (phones, tablets, other laptops).
2. Keep only one browser tab active on your main computer.
3. Refresh the page to synchronize your current state with the server.

Troubleshooting WebSocket and Network Blocks

If the red banner persists, test if your current internet connection is blocking the necessary WebSocket protocols.

1. Open your browser's Developer Tools (right-click anywhere and select Inspect, then go to the Console tab).
2. Attempt to send a message to Claude.
3. Watch for red error messages in the Console log. If you see errors containing WebSocket connection failed or ERR_CONNECTION_CLOSED, your router, firewall, or ISP is actively terminating the connection.
4. To bypass this, temporarily switch to a mobile network hotspot to confirm if the block is local to your router or primary internet connection.

When to Escalate to Claude Support

If you have ruled out local network blocks, cleared your session cache, verified the files being uploaded are clean, and the red banner still appears on every single simple text prompt across multiple devices, open a support ticket. Reach out through the Anthropic Help Center at support.anthropic.com. Be sure to take a screenshot of the developer console errors showing the failed network requests, as this will help support agents diagnose your routing path or account-level lockouts immediately.