Hook Output Mechanisms: Context Cost Analysis

Report Date: 2025-10-25 Author: Claude Code Analysis Scope: Claude Code plugin hook output fields and token costs

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1. Context Cost Analysis

feedback Field

• Token Cost: 0 tokens ✅
• Added to Claude's Context: NO
• Shown to User: YES (in UI)
• Visible in Transcript (Ctrl-R): YES (unless suppressOutput: true)

Usage: Display information to users without consuming context tokens.

hookSpecificOutput.additionalContext Field

• Token Cost: Full text length (~4 chars per token) ⚠️
• Added to Claude's Context: YES
• Shown to User: NO (intended), YES (bug #9455)
• Visible in Transcript (Ctrl-R): Depends on suppressOutput

Usage: Add persistent knowledge Claude needs across the session.

suppressOutput Field

• Token Cost: 0 tokens ✅
• Added to Claude's Context: NO
• Shown to User: Controlled by this flag
• Visible in Transcript (Ctrl-R): NO when true

Usage: Hide hook output from transcript mode (Ctrl-R view).

continue Field

• Token Cost: 0 tokens ✅
• Purpose: Control whether Claude continues processing
• Values: true (continue), false (stop with optional stopReason)

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2. Zero-Context Pattern

Objective: Show Contextune commands in UI without consuming tokens.

{
  "continue": true,
  "feedback": "💡 Contextune Commands:\n  /contextune:config - Configure detection\n  /contextune:stats - View statistics\n  /contextune:verify - Verify detection\n\nOr type naturally - I'll detect your intent!",
  "suppressOutput": false
}

Results: - Token cost: 0 tokens - User sees: Full command list with descriptions - Claude sees: Nothing (no context added) - Transcript visibility: YES (useful for user reference)

When to Use: - Command lists at session start - UI notifications and hints - Status messages - Non-critical information

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3. Implementation Guide

SessionStart Hook: Zero-Context Command List

File: /Users/shakes/DevProjects/contextune/hooks/session_start.js

#!/usr/bin/env node

const fs = require('fs');
const path = require('path');

function getContextuneCommands() {
  const pluginRoot = process.env.CLAUDE_PLUGIN_ROOT || path.join(__dirname, '..');
  const commandsDir = path.join(pluginRoot, 'commands');

  if (!fs.existsSync(commandsDir)) {
    return [];
  }

  const commands = [];
  const files = fs.readdirSync(commandsDir);

  for (const file of files) {
    if (file.endsWith('.md')) {
      try {
        const content = fs.readFileSync(path.join(commandsDir, file), 'utf8');
        const frontmatterMatch = content.match(/^---\n([\s\S]*?)\n---/);

        if (frontmatterMatch) {
          const frontmatter = frontmatterMatch[1];
          const nameMatch = frontmatter.match(/name:\s*(.+)/);
          const descMatch = frontmatter.match(/description:\s*(.+)/);

          if (nameMatch) {
            commands.push({
              name: `/${nameMatch[1].trim()}`,
              description: descMatch ? descMatch[1].trim() : ''
            });
          }
        }
      } catch (err) {
        // Ignore parsing errors
      }
    }
  }

  return commands;
}

function formatCommandList(commands) {
  if (commands.length === 0) {
    return '💡 Contextune is ready! Type naturally and I\'ll detect commands.';
  }

  const lines = ['💡 Contextune Commands Available:', ''];

  for (const cmd of commands) {
    lines.push(`  ${cmd.name}`);
    if (cmd.description) {
      lines.push(`    ${cmd.description}`);
    }
  }

  lines.push('', 'Or just type naturally - I\'ll detect your intent!');
  return lines.join('\n');
}

function main() {
  try {
    const commands = getContextuneCommands();
    const message = formatCommandList(commands);

    // ZERO-CONTEXT PATTERN: UI message only, no token cost
    const output = {
      continue: true,
      feedback: message,
      suppressOutput: false
    };

    console.log(JSON.stringify(output));
    process.exit(0);

  } catch (err) {
    // Fail silently - don't block session start
    console.error('SessionStart hook error:', err.message);
    process.exit(0);
  }
}

if (require.main === module) {
  main();
}

Hook Registration

File: /Users/shakes/DevProjects/contextune/hooks/hooks.json

{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "*",
        "hooks": [
          {
            "type": "command",
            "command": "node ${CLAUDE_PLUGIN_ROOT}/hooks/session_start.js",
            "timeout": 1000,
            "description": "Show Contextune commands (zero context cost)"
          }
        ]
      }
    ],
    "UserPromptSubmit": [
      {
        "matcher": "*",
        "hooks": [
          {
            "type": "command",
            "command": "uv run ${CLAUDE_PLUGIN_ROOT}/hooks/user_prompt_submit.py",
            "timeout": 5000,
            "description": "Contextune intent detection"
          }
        ]
      }
    ]
  }
}

Performance Characteristics

Hook Type	Pattern	Token Cost	Latency	Use Case
SessionStart	Zero-context	0 tokens	<1ms	Command list UI
SessionStart	Context injection	100-500 tokens	<1ms	Config/state
UserPromptSubmit	Context injection	~20 tokens/match	2-50ms	Detected commands

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4. Multi-Hook Coordination

Sequential Execution Order

{
  "hooks": {
    "SessionStart": [{
      "hooks": [
        {"command": "hook1.js"},  // Runs first
        {"command": "hook2.js"}   // Runs second
      ]
    }]
  }
}

Context Combination Rules

1. Multiple additionalContext: Concatenated in execution order
2. Multiple feedback: Implementation-dependent (possible bug)
3. Each hook: Independent suppressOutput control

Best Practice: - ONE hook for context injection (persistent knowledge) - SEPARATE hook for UI feedback (zero-context)

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5. Token Cost Examples

Example 1: Minimal Command List (0 tokens)

{
  "continue": true,
  "feedback": "💡 Contextune: 3 commands available. Type naturally!",
  "suppressOutput": false
}

Token cost: 0 (feedback not added to context)

Example 2: Detailed Command List (0 tokens)

{
  "continue": true,
  "feedback": "💡 Contextune Commands:\n  /contextune:config - Configure detection settings\n  /contextune:stats - View usage statistics\n  /contextune:verify - Verify detected command\n\nOr type naturally - I'll detect your intent!",
  "suppressOutput": false
}

Token cost: 0 (still zero - feedback is UI-only)

Example 3: Context Injection (~100 tokens)

{
  "continue": true,
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "[Contextune Configuration]\n- Custom patterns: 5 commands\n- Detection tiers: keyword, Model2Vec, semantic\n- Min confidence: 0.7\n- Fallback: prompt user if < 0.5"
  },
  "feedback": "💡 Contextune loaded with custom config",
  "suppressOutput": true
}

Token cost: ~100 tokens (additionalContext added to Claude's context)

Example 4: Both Patterns (~100 tokens)

{
  "continue": true,
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "[Contextune: 10 custom patterns loaded]"
  },
  "feedback": "💡 Contextune ready with your custom patterns!\n\nCommands:\n  /contextune:config\n  /contextune:stats\n\nType naturally for intent detection.",
  "suppressOutput": false
}

Token cost: ~15 tokens (only additionalContext counts)

---

6. Recommendations

For Contextune Plugin

SessionStart Hook - Use zero-context pattern:

// RECOMMENDED: Show commands without context cost
{
  "continue": true,
  "feedback": formatCommandList(commands),
  "suppressOutput": false
}

Rationale: - Users see available commands immediately - Zero token overhead (important for long sessions) - Commands detected via UserPromptSubmit hook anyway - Claude doesn't need to "know" commands in advance

UserPromptSubmit Hook - Keep context injection:

// KEEP: Inform Claude of detected command
{
  "continue": true,
  "hookSpecificOutput": {
    "hookEventName": "UserPromptSubmit",
    "additionalContext": `[Contextune detected: ${match.command}]`
  },
  "feedback": `💡 Suggested: ${match.command}`,
  "suppressOutput": false
}

Rationale: - Claude needs to know detected command (~20 tokens) - User sees suggestion in UI (helpful feedback) - Total cost per session: 20-100 tokens (acceptable)

General Guidelines

Use feedback (0 tokens) for: - ✅ Command lists and menus - ✅ Status notifications - ✅ UI hints and tips - ✅ Help text and documentation - ✅ Diagnostic information

Use additionalContext (token cost) for: - ✅ Configuration Claude needs to remember - ✅ Detected commands/intents - ✅ Project state and context - ✅ Custom rules and patterns - ✅ Session-specific knowledge

Use suppressOutput: true for: - ✅ Context injection (reduce UI noise) - ✅ Internal diagnostics - ✅ Background processing - ✅ When feedback is redundant

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7. Known Issues & Bugs

Issue #9455: additionalContext Shown to User

Expected: additionalContext should be invisible to user, only added to Claude's context

Actual: Sometimes displayed at session start

Workaround: Use suppressOutput: true to minimize visibility

Impact: Cosmetic (doesn't affect functionality)

Status: Under investigation by Anthropic

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Conclusion

Key Findings:

1. feedback has ZERO context cost - use liberally for UI
2. additionalContext has FULL token cost - use sparingly for knowledge
3. Zero-context pattern enables rich UI without token overhead
4. Multi-hook coordination allows separation of concerns

Contextune Implementation:

• SessionStart: Zero-context command list (0 tokens)
• UserPromptSubmit: Context injection for matches (~20 tokens/match)
• Total overhead: Minimal (20-100 tokens per session)

Performance: Excellent UX with minimal token cost.

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Working Examples

See implementation examples: - /Users/shakes/DevProjects/contextune/examples/session_start_zero_context.js - /Users/shakes/DevProjects/contextune/examples/session_start_with_context.js

Full analysis: - /Users/shakes/DevProjects/contextune/docs/hook-output-analysis.md