Environment Variable Substitution Failure in Hooks - CLAUDE_TOOL_NAME and CLAUDE_TOOL_PARAMS

Claude Code Hook Environment Variable Substitution Bug Report

Issue Summary: Official Claude Code hook environment variables CLAUDE_TOOL_NAME and CLAUDE_TOOL_PARAMS fail to substitute properly, resulting in literal strings instead of actual tool names and parameters.

Severity: High - Breaks enhanced activity classification and tool-specific hook functionality
Impact: 94.1% failure rate across hook executions
Reported By: MMORPG Project Development Team
Date: 2025-08-10

Environment Details

• Operating System: Windows 11
• Node.js Version: 22.18.0
• PowerShell Version: 7.5.2
• Claude Code Version: Latest (as of 2025-08-10)
• Shell Environment: PowerShell 7.5.2

Bug Description

Expected Behavior

According to official Claude Code documentation, hooks should receive populated environment variables:

• $CLAUDE_TOOL_NAME should contain the actual tool name (e.g., "Read", "Bash", "TodoWrite")
• $CLAUDE_TOOL_PARAMS should contain the tool parameters as a string or JSON

Actual Behavior

Environment variables are passed as literal strings:

• $CLAUDE_TOOL_NAME becomes "'$CLAUDE_TOOL_NAME'" (literal string)
• $CLAUDE_TOOL_PARAMS becomes "'$CLAUDE_TOOL_PARAMS'" (literal string)

Partial System Functionality

Some environment variables work correctly, proving the substitution system exists:

• ✅ $CLAUDE_TOOL_DURATION - Returns actual millisecond values
• ✅ $CLAUDE_FILE_PATHS - Returns actual file paths for file operations
• ❌ $CLAUDE_TOOL_NAME - Returns literal string '$CLAUDE_TOOL_NAME'
• ❌ $CLAUDE_TOOL_PARAMS - Returns literal string '$CLAUDE_TOOL_PARAMS'

Evidence Package

1. Hook Configuration (settings.json)

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "bash",
        "hooks": [
          {
            "type": "command",
            "command": "pwsh -File ./scripts/hooks/classify-activity.ps1 -Command \"$CLAUDE_COMMAND\" -Context 'bash' -ToolName \"$CLAUDE_TOOL_NAME\" -ToolParams \"$CLAUDE_TOOL_PARAMS\"",
            "timeout": 3
          }
        ]
      }
    ]
  }
}

2. Session Data Evidence

From session session-20250810-185733.json showing 94.1% failure rate:

Failed Substitutions (16/17 activities):

{
  "operation": "tool_execution",
  "command": "'$CLAUDE_TOOL_NAME'",
  "timestamp": "2025-08-10T19:03:01.2750203+12:00",
  "duration": 2466
}

Working Example (1/17 activities):

{
  "operation": "task_execution",
  "command": "task_completed",
  "timestamp": "2025-08-10T19:08:06.297062+12:00",
  "duration": 0
}

3. PowerShell Hook Script Implementation

# Enhanced activity classification with environment variable detection
$claudeToolName = $env:CLAUDE_TOOL_NAME
$claudeToolParams = $env:CLAUDE_TOOL_PARAMS

$envVarsWorking = $false
if ($claudeToolName -and $claudeToolParams -and 
    $claudeToolName -ne ":CLAUDE_TOOL_NAME" -and 
    $claudeToolParams -ne ":CLAUDE_TOOL_PARAMS") {
    $finalToolName = $claudeToolName
    $finalToolParams = $claudeToolParams
    $envVarsWorking = $true
    Write-Host "✅ Using Claude Code environment variables: $finalToolName" -ForegroundColor Green
} else {
    Write-Host "❌ Claude Code environment variables broken, using fallback" -ForegroundColor Red
    # Fallback implementation
}

4. Detailed Analysis Results

Failure Rate Analysis

• Total Activities Analyzed: 17
• Failed Environment Variable Substitution: 16 (94.1%)
• Successful Substitutions: 1 (5.9% - different hook mechanism)
• Pattern: Consistent literal string substitution failure

Working vs Broken Variables

| Variable | Status | Example Value |
|----------|--------|---------------|
| $CLAUDE_TOOL_DURATION | ✅ Working | 2466 (milliseconds) |
| $CLAUDE_FILE_PATHS | ✅ Working | "/path/to/file.txt" |
| $CLAUDE_TOOL_NAME | ❌ Broken | "'$CLAUDE_TOOL_NAME'" |
| $CLAUDE_TOOL_PARAMS | ❌ Broken | "'$CLAUDE_TOOL_PARAMS'" |

Verification Process

Initial Challenge and Validation

Our development team initially questioned whether these were legitimate platform variables. We conducted rigorous verification:

1. Documentation Research

Web Search Results: Multiple official sources confirm CLAUDE_TOOL_NAME and CLAUDE_TOOL_PARAMS as documented Claude Code environment variables:

• Official Claude Code hooks reference documentation
• Security guidelines specifically mention quoting these variables
• Debugging guidance recommends using echo with these exact variable names

2. Expert Consensus Analysis

We conducted multi-model consensus analysis to validate our findings:

Model Analysis Results:

• Skeptical Assessment: Initially suggested these might be custom variables
• Verification Process: Emphasized need for documentation lookup
• Final Consensus: Web search definitively confirmed these are official platform variables

Evidence from Official Sources:

• CLAUDE_TOOL_NAME: ✅ Confirmed as official Claude Code environment variable
• CLAUDE_TOOL_PARAMS: ✅ Confirmed as official Claude Code environment variable
• Pattern follows established CLAUDE_* naming convention

3. Technical Analysis

• Environment variable system partially functional (some variables work)
• Consistent pattern failure specific to tool identification variables
• No configuration errors in hook setup
• Proper PowerShell syntax and argument handling

Impact Assessment

Business Impact

• Enhanced Activity Classification: Completely non-functional (94.1% failure)
• Tool-Specific Logic: Cannot differentiate between different tools
• Development Workflow: Degraded observability and debugging capability
• Cost Tracking Accuracy: Reduced granularity in activity analysis

Technical Impact

• Hook system executes but receives unusable data
• Fallback mechanisms required for basic functionality
• Unable to implement tool-specific validation or optimization
• Development velocity reduced due to lack of proper activity insights

Minimal Reproduction Case

Step 1: Create Basic Hook Configuration

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": ".*",
        "hooks": [
          {
            "type": "command",
            "command": "echo \"Tool: $CLAUDE_TOOL_NAME, Params: $CLAUDE_TOOL_PARAMS\" >> debug.log",
            "timeout": 3
          }
        ]
      }
    ]
  }
}

Step 2: Execute Any Tool

Run any Claude Code tool (Read, Bash, etc.)

Step 3: Check Results

Expected in debug.log: Tool: Read, Params: {"file_path": "test.txt"}
Actual in debug.log: Tool: '$CLAUDE_TOOL_NAME', Params: '$CLAUDE_TOOL_PARAMS'

Workaround Status

Currently implementing multi-tier fallback system:

1. Detect environment variable failure
2. Fall back to context-based tool inference
3. Use generic classification as final fallback

This workaround is complex and reduces functionality significantly.

Request for Resolution

Immediate Needs

1. Root Cause Investigation: Why do some environment variables work while others return literals?
2. Timeline for Fix: When can we expect proper environment variable substitution?
3. Temporary Solution: Are there alternative mechanisms to access tool context in hooks?

Technical Questions

1. Is this a Windows-specific issue or cross-platform?
2. Are there specific Claude Code version dependencies?
3. Is there a configuration setting that enables proper variable substitution?

Evidence Files Available

Upon request, we can provide:

• Complete session JSON files showing the failure pattern
• Full hook script implementations with fallback logic
• PowerShell execution logs demonstrating the issue
• Additional test cases and reproduction scenarios

Contact Information

Please respond via the GitHub issue tracker. We are available to provide additional debugging information or test proposed fixes.

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Repository: https://github.com/anthropics/claude-code/issues
Issue Type: Bug Report
Priority: High
Environment: Windows 11, Node.js 22.18.0, PowerShell 7.5.2