[BUG]

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?

Environment

Platform: Claude Code Web (claude.ai/code)
Subscription: Max 20x
Browser: Chrome Version 143.0.7499.170 (Official Build) (arm64)
OS: 15.6.1 (24G90)
Date: December 27, 2025

What's Wrong?
Claude Code web sessions fail when pasting structured markdown prompts that have worked reliably for months. This appears to be a recent regression.
Behavior:

Starting a new session at claude.ai/code
Pasting a structured prompt (~300-400 lines, markdown with embedded code blocks)
Session immediately fails with "Session failed. Try again"
Retrying produces the same result

Key observations:

Short, simple prompts (e.g., "recommend areas to improve our tests") work fine
The exact same structured prompts worked without issue for months prior
Issue reproduces on both web browser and desktop app
Old session URLs fail (expected), but new sessions with the same prompt format also fail
Standard claude.ai chat works fine — this is isolated to Claude Code web sessions

Expected Behavior
Session should start and accept the prompt, as it has for months with identical formatting.

What I've Tried

Starting fresh sessions (not reloading old session URLs)
Testing on web and desktop app
Pasting shorter portions of the prompt
Multiple attempts over 30+ minutes
Verified no active outages on status.anthropic.com

Additional Context
This is blocking a production development workflow. The prompt structure includes:

Markdown headers
Code blocks (TypeScript)
Checklists
Tables

No special characters, no unusual formatting — just standard markdown that has been accepted without issue until today.
Contacted support and received an AI bot response with CLI troubleshooting steps (/doctor, /compact) that don't apply to web sessions. Requesting engineering investigation.

What Should Happen?

Expected Behavior

Navigate to claude.ai/code
Paste structured markdown prompt into the session
Session starts and Claude Code begins processing the prompt
Claude executes the instructions (runs recon commands, creates files, etc.)

As I have done for months.

Error Messages/Logs

Session creation failed. Please try again.

Steps to Reproduce

Go to claude.ai/code
Start a new session
Paste a structured markdown prompt (~300-400 lines containing headers, TypeScript code blocks, tables, and checklists)
Observe: "Session failed. Try again"
Retry with a short simple prompt (e.g., "hello" or "recommend areas to improve tests")
Observe: Session starts successfully

Note on Reproduction
I cannot share the exact prompt (proprietary), but the structure is standard markdown — nothing exotic. The same prompts have worked daily for months. Happy to test with an Anthropic engineer if needed to help isolate the issue.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

claude.ai/code

Platform

Other

Operating System

macOS

Terminal/Shell

Other

Additional Information

_No response_