Claude Code hooks: how to intercept every tool call before it runs

One of the most powerful — and least documented — features revealed in the Claude Code source is the hooks system. You can intercept every single tool call Claude makes, before it executes.

This means you can:

• Auto-approve certain commands (no more hitting Enter 40 times)

• Block dangerous operations entirely

• Log every file Claude touches

• Inject context before tool execution

Here's how it works.

The hooks directory

Create a .claude/hooks/ directory in your project:

mkdir -p .claude/hooks

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Hooks are shell scripts that Claude executes at specific lifecycle points.

PreToolUse hook

This runs before any tool call. The tool name and arguments are passed as environment variables:

# .claude/hooks/PreToolUse.sh #!/bin/bash

# .claude/hooks/PreToolUse.sh #!/bin/bash

Auto-approve file reads — stop asking me every time

if [ "$TOOL_NAME" = "Read" ]; then exit 0 # 0 = approve fi

Block rm -rf entirely

if [ "$TOOL_NAME" = "Bash" ] && echo "$TOOL_INPUT" | grep -q 'rm -rf'; then echo "Blocked: rm -rf is not allowed" >&2 exit 1 # 1 = reject fi

Everything else: default behavior

exit 0`

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Make it executable:

chmod +x .claude/hooks/PreToolUse.sh

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PostToolUse hook

This runs after a tool executes. Use it for logging or side effects:

# .claude/hooks/PostToolUse.sh #!/bin/bash

# .claude/hooks/PostToolUse.sh #!/bin/bash

Log every file write to a change log

if [ "$TOOL_NAME" = "Write" ]; then echo "$(date -u +%Y-%m-%dT%H:%M:%SZ) WROTE: $TOOL_INPUT_PATH" >> .claude/changes.log fi

Log every bash command Claude runs

if [ "$TOOL_NAME" = "Bash" ]; then echo "$(date -u +%Y-%m-%dT%H:%M:%SZ) BASH: $TOOL_INPUT" >> .claude/commands.log fi

exit 0`

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Available hook types

From the source, there are four hook points:

Hook When it fires

PreToolUse Before any tool call

PostToolUse After any tool call

Notification When Claude sends a notification

Stop When Claude finishes a task

Available environment variables

All hooks receive these env vars:

$TOOL_NAME # "Read", "Write", "Bash", "Edit", etc. $TOOL_INPUT # Full JSON of the tool arguments $TOOL_INPUT_PATH # For file tools: the file path $TOOL_OUTPUT # PostToolUse only: what the tool returned $SESSION_ID # Current Claude Code session

$TOOL_NAME # "Read", "Write", "Bash", "Edit", etc. $TOOL_INPUT # Full JSON of the tool arguments $TOOL_INPUT_PATH # For file tools: the file path $TOOL_OUTPUT # PostToolUse only: what the tool returned $SESSION_ID # Current Claude Code session

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Real-world use case: auto-approve safe operations

The most common use case is eliminating repetitive permission prompts:

# .claude/hooks/PreToolUse.sh #!/bin/bash

# .claude/hooks/PreToolUse.sh #!/bin/bash

Auto-approve all reads

if [ "$TOOL_NAME" = "Read" ]; then exit 0; fi

Auto-approve writes to test/temp directories

if [ "$TOOL_NAME" = "Write" ]; then if echo "$TOOL_INPUT_PATH" | grep -qE '^(tests?/|tmp/|/tmp/)'; then exit 0 fi fi

Auto-approve git status/log/diff (read-only git)

if [ "$TOOL_NAME" = "Bash" ]; then if echo "$TOOL_INPUT" | grep -qE '^git (status|log|diff|show)'; then exit 0 fi fi

Default: require manual approval

exit 2 # 2 = ask human`

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Use case: Stop hook for task completion

# .claude/hooks/Stop.sh #!/bin/bash

# .claude/hooks/Stop.sh #!/bin/bash

Notify when Claude finishes

echo "✅ Claude Code task complete" | notify-send "Claude Code" 2>/dev/null || true

Run your test suite automatically after every Claude session

if [ -f package.json ]; then npm test --silent 2>&1 | tail -5 fi

exit 0`

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Register hooks in settings.json

You can also register hooks in .claude/settings.json instead of the hooks directory:

{  "hooks": {  "PreToolUse": [  {  "matcher": "Bash",  "hooks": [  {  "type": "command",  "command": "bash .claude/hooks/PreToolUse.sh"  }  ]  }  ]  } }

{  "hooks": {  "PreToolUse": [  {  "matcher": "Bash",  "hooks": [  {  "type": "command",  "command": "bash .claude/hooks/PreToolUse.sh"  }  ]  }  ]  } }

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The matcher field filters by tool name — you can have different hooks for different tools.

Combine with CLAUDE.md for full control

Hooks handle the mechanical side. CLAUDE.md handles the behavioral side. Together:

# CLAUDE.md

Tool behavior

• You do not need to ask permission for file reads
• You do not need to ask permission for writes to tests/ or tmp/
• Always run the test suite after making changes
• Never run rm -rf (it is blocked at the hook level)`

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Now Claude knows it can proceed on reads/writes, and the hook enforces the rm -rf block even if Claude forgets.

Complete starter hooks setup

Copy this into .claude/hooks/PreToolUse.sh:

#!/bin/bash

Claude Code PreToolUse hook — safe auto-approvals

TOOL=$TOOL_NAME INPUT=$TOOL_INPUT PATH_=$TOOL_INPUT_PATH_

=== ALWAYS APPROVE ===

[[ "$TOOL" == "Read" ]] && exit 0 [[ "$TOOL" == "LS" ]] && exit 0 [[ "$TOOL" == "Glob" ]] && exit 0 [[ "$TOOL" == "Grep" ]] && exit 0

=== ALWAYS BLOCK ===

if [[ "$TOOL" == "Bash" ]]; then echo "$INPUT" | grep -qE 'rm -rf|DROP TABLE|DELETE FROM' && { echo "Blocked dangerous command" >&2 exit 1 } fi

=== DEFAULT ===

exit 0 # approve everything else`

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The hooks system turns Claude Code from a supervised assistant into a trusted autonomous agent — you define the safety boundaries once, and it runs within them without interruption.

If you're running Claude on a flat-rate API (not pay-per-token), this becomes even more valuable — you can let it run long autonomous sessions without worrying about runaway costs.

I use SimplyLouie as my ANTHROPIC_BASE_URL for exactly this: $2/month flat rate, no token anxiety, hooks handling the safety layer.

Drop your hook configs in the comments — curious what permission rules others are running.