Prevent Claude Code Syntax Errors Cascading

Claude Code edits files one after another. When it introduces a syntax error in file #1, it doesn't notice. It moves to file #2, which imports file #1. Now file #2 breaks. By the time it stops, 30 files have cascading errors.

How Syntax Errors Cascade

The 30-file cascade

Claude refactors a shared utility file. It removes a closing brace. The file is syntactically broken, but Claude moves on to the next file that imports it. Each file it touches inherits the problem. The developer returns to find 30+ files with errors traced back to one missing }.

The JSON config corruption

Claude edits a tsconfig.json and leaves a trailing comma. Every TypeScript file in the project now fails to compile. Claude spends 20 minutes "fixing" type errors that don't actually exist.

The Python import chain

Claude introduces an indentation error in a base module. All 15 files that import it start throwing IndentationError. Claude tries to fix each file individually instead of the root cause.

Why This Keeps Happening

Claude doesn't run a syntax check after editing a file. It trusts its own output. When it makes a mistake — a missing bracket, wrong indentation, unclosed string — it proceeds to the next task. The error propagates silently.

The Fix: PostToolUse Syntax Check

syntax-check hook — validates every file immediately after editing:

#!/bin/bash
# Runs after every Write/Edit tool use
INPUT=$(cat)
FILE=$(echo "$INPUT" | jq -r '.tool_input.file_path // empty')

[ -z "$FILE" ] && exit 0

case "$FILE" in
  *.py)  python3 -c "import ast; ast.parse(open('$FILE').read())" 2>&1 ;;
  *.js)  node --check "$FILE" 2>&1 ;;
  *.ts)  npx tsc --noEmit "$FILE" 2>&1 ;;
  *.json) python3 -m json.tool "$FILE" > /dev/null 2>&1 ;;
  *.sh)  bash -n "$FILE" 2>&1 ;;
esac

if [ $? -ne 0 ]; then
  echo "SYNTAX ERROR in $FILE — fix before continuing" >&2
  exit 2
fi

This hook runs after every file edit. If the file has a syntax error, Claude is forced to fix it before touching anything else.

Languages Supported

Language	Check Method
Python	`python3 -c "import ast; ast.parse(...)"`
JavaScript	`node --check`
TypeScript	`npx tsc --noEmit`
JSON	`python3 -m json.tool`
Bash	`bash -n`
Go, Rust, Ruby	Supported via `--install-example`

Install All 8 Safety Hooks in 10 Seconds

npx cc-safe-setup

Catches syntax errors, blocks rm -rf, prevents force-push, stops secret leaks. 9,200+ tests 655 examples

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