Hook Deduplication Guide

Overview

This skill guides you through implementing robust deduplication for Claude Code hooks, using content-based hashing instead of session-based tracking. Prevents duplicate insights from being stored while allowing multiple unique insights per session.

Based on 1 insight:

• Hook Deduplication Session Management (hooks-and-events, 2025-11-03)

Key Capabilities:

• Content-based deduplication using SHA256 hashes
• Session-independent duplicate detection
• Efficient hash storage with rotation
• State management best practices

When to Use This Skill

Trigger Phrases:

• "implement hook deduplication"
• "prevent duplicate insights in hooks"
• "content-based deduplication for hooks"
• "hook state management patterns"

Use Cases:

• Developing new Claude Code hooks that store data
• Refactoring hooks to prevent duplicates
• Implementing efficient state management for hooks
• Debugging duplicate data issues in hooks

Do NOT use when:

• Creating hooks that don't store data (read-only hooks)
• Session-based deduplication is actually desired
• Hook doesn't run frequently enough to need deduplication

Response Style

Educational and practical - explain the why behind content-based vs. session-based deduplication, then guide implementation with code examples.

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Workflow

Phase 1: Choose Deduplication Strategy

Purpose: Determine whether content-based or session-based deduplication is appropriate.

Steps:

1. Assess hook behavior:

   • How often does the hook run? (per message, per session, per event)
   • What data is being stored? (insights, logs, metrics)
   • Is the same content likely to appear across sessions?

2. Evaluate deduplication needs:

   • Content-based: Use when the same insight/data might appear in different sessions
     • Example: Extract-explanatory-insights hook (same insight might appear in multiple conversations)
   • Session-based: Use when duplicates should only be prevented within a session
     • Example: Error logging (same error in different sessions should be logged)

3. Recommend strategy:

   • For insights/lessons-learned: Content-based (SHA256 hashing)
   • For session logs/events: Session-based (session ID tracking)
   • For unique events: No deduplication needed

Output: Clear recommendation on deduplication strategy.

Common Issues:

• Unsure which to use: Default to content-based for data that's meant to be unique (insights, documentation)
• Performance concerns: Content-based hashing is fast (<1ms for typical content)

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Phase 2: Implement Content-Based Deduplication

Purpose: Set up SHA256 hash-based deduplication with state management.

Steps:

1. Create state directory:

   mkdir -p ~/.claude/state/hook-state/
   

2. Initialize hash storage file:

   HASH_FILE="$HOME/.claude/state/hook-state/content-hashes.txt"
   touch "$HASH_FILE"
   

3. Implement hash generation:

   # Generate SHA256 hash of content
   compute_content_hash() {
     local content="$1"
     echo -n "$content" | sha256sum | awk '{print $1}'
   }
   

4. Check for duplicates:

   # Returns 0 if content is new, 1 if duplicate
   is_duplicate() {
     local content="$1"
     local content_hash=$(compute_content_hash "$content")
   
     if grep -Fxq "$content_hash" "$HASH_FILE"; then
       return 1  # Duplicate found
     else
       return 0  # New content
     fi
   }
   

5. Store hash after processing:

   store_content_hash() {
     local content="$1"
     local content_hash=$(compute_content_hash "$content")
     echo "$content_hash" >> "$HASH_FILE"
   }
   

6. Integrate into hook:

   # In your hook script
   content="extracted insight or data"
   
   if is_duplicate "$content"; then
     # Skip - duplicate content
     echo "Duplicate detected, skipping..." >&2
     exit 0
   fi
   
   # Process new content
   process_content "$content"
   
   # Store hash to prevent future duplicates
   store_content_hash "$content"
   

Output: Working content-based deduplication in your hook.

Common Issues:

• Hash file grows too large: Implement rotation (see Phase 3)
• False positives: Ensure content normalization (whitespace, formatting)

---

Phase 3: Implement Hash Rotation

Purpose: Prevent hash file from growing indefinitely.

Steps:

1. Set rotation limit:

   MAX_HASHES=10000  # Keep last 10,000 hashes
   

2. Implement rotation logic:

   rotate_hash_file() {
     local hash_file="$1"
     local max_hashes="${2:-10000}"
   
     # Count current hashes
     local current_count=$(wc -l < "$hash_file")
   
     # Rotate if needed
     if [ "$current_count" -gt "$max_hashes" ]; then
       tail -n "$max_hashes" "$hash_file" > "${hash_file}.tmp"
       mv "${hash_file}.tmp" "$hash_file"
       echo "Rotated hash file: kept last $max_hashes hashes" >&2
     fi
   }
   

3. Call rotation periodically:

   # After storing new hash
   store_content_hash "$content"
   rotate_hash_file "$HASH_FILE" 10000
   

Output: Self-maintaining hash storage with bounded size.

Common Issues:

• Rotation too aggressive: Increase MAX_HASHES
• Rotation too infrequent: Consider checking count before every append

---

Phase 4: Testing and Validation

Purpose: Verify deduplication works correctly.

Steps:

1. Test duplicate detection:

   # First run - should process
   echo "Test insight" | your_hook.sh
   # Check: Content was processed
   
   # Second run - should skip
   echo "Test insight" | your_hook.sh
   # Check: Duplicate detected message
   

2. Test multiple unique items:

   echo "Insight 1" | your_hook.sh  # Processed
   echo "Insight 2" | your_hook.sh  # Processed
   echo "Insight 3" | your_hook.sh  # Processed
   echo "Insight 1" | your_hook.sh  # Skipped (duplicate)
   

3. Verify hash file:

   cat ~/.claude/state/hook-state/content-hashes.txt
   # Should show 3 unique hashes (not 4)
   

4. Test rotation:

   # Generate more than MAX_HASHES entries
   for i in {1..10500}; do
     echo "Insight $i" | your_hook.sh
   done
   
   # Verify file size bounded
   wc -l ~/.claude/state/hook-state/content-hashes.txt
   # Should be ~10000, not 10500
   

Output: Confirmed working deduplication with proper rotation.

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Reference Materials

• Original Insight - Full context on hook deduplication patterns

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Important Reminders

• Use content-based deduplication for insights/documentation - prevents duplicates across sessions
• Use session-based deduplication for logs/events - same event in different sessions is meaningful
• Normalize content before hashing - whitespace differences shouldn't create false negatives
• Implement rotation - prevent unbounded hash file growth
• Hash storage location: ~/.claude/state/hook-state/ (not project-specific)
• SHA256 is fast - no performance concerns for typical hook data
• Test both paths - verify both new content and duplicates work correctly

Warnings:

• ⚠️ Do not use session ID alone - prevents same insight in different sessions from being stored
• ⚠️ Do not skip rotation - hash file will grow indefinitely
• ⚠️ Do not hash before normalization - formatting changes will cause false negatives

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Best Practices

1. Choose the Right Strategy: Content-based for unique data, session-based for session-specific events
2. Normalize Before Hashing: Strip whitespace, lowercase if appropriate, consistent formatting
3. Efficient Storage: Use grep -Fxq for fast hash lookups (fixed-string, line-match, quiet)
4. Bounded Growth: Implement rotation to prevent file bloat
5. Clear Logging: Log when duplicates are detected for debugging
6. State Location: Use ~/.claude/state/hook-state/ for cross-project state

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Troubleshooting

Duplicates not being detected

Symptoms: Same content processed multiple times

Solution:

1. Check hash file exists and is writable
2. Verify store_content_hash is called after processing
3. Check content normalization (whitespace differences)
4. Verify grep command uses -Fxq flags

Prevention: Test deduplication immediately after implementation

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Hash file growing too large

Symptoms: Hash file exceeds MAX_HASHES significantly

Solution:

1. Verify rotate_hash_file is called
2. Check MAX_HASHES value is reasonable
3. Manually rotate if needed: tail -n 10000 hashes.txt > hashes.tmp && mv hashes.tmp hashes.txt

Prevention: Call rotation after every hash storage

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False positives (new content marked as duplicate)

Symptoms: Different content being skipped

Solution:

1. Check for hash collisions (extremely unlikely with SHA256)
2. Verify content is actually different
3. Check normalization isn't too aggressive
4. Review recent hashes in file

Prevention: Use consistent normalization, test with diverse content

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Next Steps

After implementing deduplication:

1. Monitor hash file growth over time
2. Tune MAX_HASHES based on usage patterns
3. Consider adding metrics (duplicates prevented, storage size)
4. Share pattern with team for other hooks

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Metadata

Source Insights:

• Session: abc123-session-id
• Date: 2025-11-03
• Category: hooks-and-events
• File: docs/lessons-learned/hooks-and-events/2025-11-03-hook-deduplication.md

Skill Version: 0.1.0 Generated: 2025-11-16 Last Updated: 2025-11-16