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brian austin
brian austin

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Claude Code debugging: read the error, don't guess

#claudecode #debugging #programming #devtools

Claude Code debugging: read the error, don't guess

Most Claude Code sessions that go wrong follow the same pattern: Claude makes a change, something breaks, and instead of reading the actual error message, it starts guessing — trying five different "fixes" in sequence, each one drifting further from the real problem.

Here's how to stop that from happening.

The core debugging loop

Claude Code works best when you enforce a strict read-then-act discipline:

error occurs → read full error → identify root cause → one targeted fix → verify
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Not:

error occurs → try something → try something else → try something else again
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To enforce this in your CLAUDE.md:

## Debugging rules
- When a command fails, read the FULL error output before attempting any fix
- State the root cause explicitly before making any change
- Make ONE change to fix ONE problem — no shotgun approaches
- After fixing, run the same failing command to verify
- If three attempts fail, STOP and ask for guidance
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Capturing real errors

Claude Code sometimes truncates error output when it's too long. Force full capture:

# Instead of just running the command
npm test

# Capture everything
npm test 2>&1 | tee /tmp/test-output.txt
cat /tmp/test-output.txt | tail -50
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Or tell Claude explicitly in your prompt:

Run `npm test 2>&1 | tail -100` and read the FULL output before doing anything
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The "state the cause" technique

Before Claude makes any fix, require it to state what it believes the root cause is:

Run the tests. Before making any changes, write:
"ROOT CAUSE: [one sentence describing the exact problem]"
Then make exactly one change to fix that cause.
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This forces Claude to think before acting. If it can't state the cause clearly, it doesn't actually understand the problem.

Example of bad debugging:

> Tests are failing

Claude: Let me try updating the import... still failing. Let me try changing 
the mock... still failing. Let me try reinstalling dependencies...
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Example of good debugging:

> Run the tests and state the root cause before fixing

Claude: ROOT CAUSE: The mock for `fetchUser` returns undefined but the code 
expects an object with `.id` property. Fixing the mock return value.
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Common Claude Code debugging failures

1. Fixing symptoms instead of causes

Error: TypeError: Cannot read property 'map' of undefined

Bad fix: Add || [] everywhere

Good fix: Find where the data is supposed to come from and fix that source

Add to CLAUDE.md:

- Do not add null checks to silence TypeErrors — find why the value is null
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2. Test output truncation

Claude sees the first 50 lines of test output and assumes it understands all failures.

# Add this to your .claude/commands/debug.md
npm test -- --verbose 2>&1 | tee /tmp/full-test.txt
wc -l /tmp/full-test.txt  # Report total lines
cat /tmp/full-test.txt | grep -A 5 'FAIL\|Error:'
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3. Environment assumption errors

Claude assumes the environment matches what it knows — but your .env might be missing a variable, or a service might be down.

Force environment checks first:

## Before debugging any service error
1. Check all required env vars are set: `env | grep -E 'DATABASE|API|SECRET'`
2. Check dependent services are running: `curl -f http://localhost:PORT/health`
3. Check logs for the last 20 lines: `tail -20 /var/log/app.log`
Only then attempt a code fix.
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4. Circular debugging

Claude fixes problem A, which breaks B, then fixes B, which breaks A again.

Detect this in your CLAUDE.md:

- If you fix the same error twice, STOP. The problem is architectural.
- Write a one-paragraph explanation of the full flow before attempting another fix.
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The five-minute debugging protocol

For any bug that Claude can't fix in three attempts:

1. cat the error file completely
2. List every file that was changed in the last 10 minutes: 
   `git diff --name-only HEAD`
3. Pick the most likely culprit
4. `git diff HEAD -- [that file]`
5. If still unclear, `git stash` and verify the baseline works
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Add this as a custom slash command at .claude/commands/five-minute-debug.md:

Run the five-minute debugging protocol:
1. Show the full error: `cat /tmp/last-error.txt 2>/dev/null || echo 'no error file'`
2. Show recent changes: `git diff --name-only HEAD~1`
3. State which change most likely caused the current error
4. Show the diff for that file: `git diff HEAD~1 -- [file]`
5. Propose ONE targeted fix
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Rate limits during debugging

Long debugging sessions burn through Claude's rate limits fast — especially when Claude is looping on the same problem.

The most effective fix: proxy your ANTHROPIC_BASE_URL to a service that removes rate limits between your Claude Code session and Anthropic.

export ANTHROPIC_BASE_URL=https://simplylouie.com
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SimplyLouie is ✌️$2/month and removes the rate limit interruptions that break debugging flow mid-session. When Claude is deep into a debugging trace, a rate limit error kills the entire context thread.

TL;DR

  • Capture full error output before any fix
  • Make Claude state root cause explicitly
  • One change per attempt
  • After three failures, do a git diff and start fresh
  • Proxy ANTHROPIC_BASE_URL to avoid rate limit interruptions during long debug sessions

The pattern is always: understand first, then fix. Claude Code is powerful enough to solve most problems correctly on the first attempt — if you give it the full picture.

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