Claude Code implements speculative fixes without verifying root cause

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?

When debugging, Claude Code guesses the most likely root cause and immediately starts coding a fix without verifying. In my case, I reported an overlay not displaying on a job card. Claude read related code, hypothesized a company name mismatch, and began rewriting the API's matching logic — without ever checking the actual data to confirm the mismatch existed. The assumed root cause may have been completely wrong.

What Should Happen?

When the root cause isn't deterministic from code reading alone, Claude should gather evidence first — add diagnostic logging, check server logs, inspect actual runtime data — then present findings before proposing a fix. Evidence-based debugging should be the default behavior, not something developers need to configure via prompt instructions.

Error Messages/Logs

No error — this is a behavioral issue. Claude reads code, forms a hypothesis, and starts implementing a fix without diagnostic evidence.
Example transcript:

User: "Help me see why the applied overlay isn't displaying on this job card"
Claude: [reads code] "The issue is likely a company name mismatch between the card DOM and the stored applied list. Let me rewrite the matching logic to normalize company names..."
User: "Wait — were you able to identify the root cause?"
Claude: "Honestly, no — I was making an educated guess."

Claude had not checked: the database, server logs, API responses, or actual runtime data before proposing and beginning to implement a fix.

Steps to Reproduce

Steps to Reproduce:

1. Work in a codebase with multiple interconnected files (e.g., UI components, service layers, API endpoints, database queries)
2. Report a bug with multiple plausible causes: "The applied overlay isn't displaying over the job card"
3. Claude reads ~6 related files across the stack (adapter, service, API endpoint, DB query logic)
4. Claude identifies a plausible cause (company name mismatch between DOM extraction and stored data) and immediately begins rewriting the API matching logic
5. When asked "Were you able to identify the root cause?" Claude responds: "Honestly, no — I was making an educated guess"

What happened: Claude read enough code to form a plausible hypothesis and treated it as confirmed. It began implementing a significant change (adding company name normalization to an API endpoint) without checking the database, server logs, API responses, or any runtime data.

Expected: Claude should recognize when a root cause is uncertain and gather evidence before coding a fix.
Note: This behavior is more likely in larger codebases where pattern-matching produces a "likely" cause. Simple 2-file examples may not reliably reproduce it.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.34

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

Workaround: Adding these instructions to .claude/feedback/ and MEMORY.md reduces (but doesn't eliminate) the behavior:

"NEVER guess the root cause and jump to coding a fix. Always:

1. Add logging / check actual data first
2. Confirm root cause with evidence
3. Only then propose and implement a fix"

Environment:

Model: Claude Opus 4.6 (claude-opus-4-6)
Claude Code: v2.1.34
OS: Windows 11

This behavior appears more pronounced with:

• Larger multi-file codebases (more patterns to latch onto)
• Bugs where one cause "looks obvious" from code reading
• Longer conversation context where momentum builds toward a fix