[BUG] claude.ai Web – Platform-wide instability, chat 404s and send failures in normal browser sessions

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Since this morning (2026-09-10) after a normal PC restart, claude.ai shows severe instability in regular browser chat sessions — not limited to Claude Code or MCP features.

Three distinct failure modes observed and documented with screenshots:

1. Chat URL returns 404 "Seite nicht gefunden" — a valid, existing chat URL (claude.ai/chat/[uuid]) renders a blank page with "Page not found" error, while the "We are experiencing technical difficulties" banner is shown simultaneously.

1. "Senden fehlgeschlagen" (Send failed) — messages fail to send within an active session. A "Wiederholen" (Retry) link appears but the session remains broken.

1. Persistent "We are experiencing technical difficulties. Some functionality may be temporarily unavailable." banner — visible across multiple sessions and tabs.

All three issues occur in standard chat sessions without any Claude Code, MCP, or Routines involvement.

What Should Happen?

Chat URLs should resolve correctly. Messages should send without failure. No platform error banners should appear during normal usage.

Error Messages/Logs

- "Seite nicht gefunden / Page not found" on valid chat URLs
- "Senden fehlgeschlagen" in active chat sessions
- "We are experiencing technical difficulties. Some functionality may be temporarily unavailable."

Steps to Reproduce

1. Open claude.ai in Chrome on Windows 11
2. Open an existing chat via its direct URL
3. Attempt to send a message in any session

Issue appears immediately and persists across multiple tabs and sessions.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Unknown — claude.ai web platform, no version number visible to end users.

Claude Code Version

N/A — This bug affects the claude.ai web platform in a standard browser session, not Claude Code CLI.

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

Plan: Claude Max
Browser: Google Chrome (latest)
OS: Windows 11 Pro, Version 25H2, Build 26200.9445
Issue appears to be server-side — persists across multiple browser tabs and sessions simultaneously.

<img width="927" height="856" alt="Image" src="https://github.com/user-attachments/assets/a169a318-7e31-4346-963a-518bd614c250" />
<img width="1092" height="1847" alt="Image" src="https://github.com/user-attachments/assets/aa21d0be-877b-48da-84ce-aaa3c24efeef" />
<img width="972" height="1730" alt="Image" src="https://github.com/user-attachments/assets/f6fa1cef-7581-4139-b610-4acf5d68625f" />
<img width="927" height="1290" alt="Image" src="https://github.com/user-attachments/assets/4ea7b2ab-4fd3-4c25-87eb-871f67170743" />
<img width="1326" height="1698" alt="Image" src="https://github.com/user-attachments/assets/90002df6-4a5b-488b-bfda-23ed6ddf8d01" />