claude-sonnet-4-5-20250929

Preflight Checklist

• [x] I have searched existing issues for similar behavior reports
• [x] This report does NOT contain sensitive information (API keys, passwords, etc.)

Type of Behavior Issue

Other unexpected behavior

What You Asked Claude to Do

• Issue: Claude violated XML comment rules by using -- inside comments despite knowing this is invalid
• Impact: Created invalid build script requiring debugging and manual fixes, wasting developer time
• Context: Working with Apache Ant XML build scripts
• Expected: Claude should follow fundamental XML parsing rules it already knows
• Actual: Generated invalid XML with nested comment syntax

What Claude Actually Did

Claude violated XML comment rules by using -- inside comments despite knowing this is invalid

Expected Behavior

This wasn't a capability gap - I knew the rule and was careless. But that's arguably worse because:

• Users rely on me being careful and attentive
• Basic mistakes waste more time than complex errors
• If I'm not being careful with fundamentals, what else might I miss?

Files Affected

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

_No response_

Claude Model

Sonnet

Relevant Conversation

Impact

Critical - Data loss or corrupted project

Claude Code Version

claude-sonnet-4-5-20250929

Platform

Anthropic API

Additional Context

_No response_