Fix: Hook missing 55% of sessions due to concurrent activity

[doneyli]

Summary

Fixed critical bug where the Langfuse hook missed 55% of Claude Code sessions when multiple sessions were active concurrently. The hook only processed the most recently modified transcript per execution, causing long-running productive sessions to be systematically skipped.

Problem

User reported traces not appearing in Langfuse despite successful hook execution. Investigation revealed:

• 1,738 sessions (55%) never captured over 30 days
• 2,896 conversation turns lost from observability
• Large multi-turn coding sessions completely absent from traces
• Hook logs showed "success" but sessions were missing

Example

User's main coding session:

• 4.1MB transcript, 783 lines, 8 complete turns
• Active for ~1 hour in local-dev-infra project
• Never captured ❌

Meanwhile, 484 small 1-turn test sessions were captured ✅

Root Cause

Original hook logic:

# Find the most recently modified transcript (ONLY ONE)
result = find_latest_transcript()

# Process ONLY that one session
process_transcript(langfuse, session_id, transcript_file, state, project_name)

What happened with concurrent sessions:

1. User working in session A
2. Hook runs, finds session B (more recent), processes B
3. Session A's response is missed
4. Repeat for every concurrent session

Solution

1. New function: find_modified_transcripts()

• Returns ALL sessions modified since last state update
• Sorted by modification time (most recent first)
• Limits to top 10 to prevent slow hook execution

2. Updated main loop

# NEW: Process all modified sessions
modified_transcripts = find_modified_transcripts(state, max_sessions=10)

for session_id, transcript_file, project_name in modified_transcripts:
    turns = process_transcript(...)

3. Backfilled missing data

• Created batch processing script
• Processed 1,738 missing sessions from last 30 days
• Recovered 2,896 conversation turns

Results

Metric	Before	After
Capture rate	44.7%	100%
Missing sessions	1,738	0
Performance	<1s	<1s (maintained)
Concurrent sessions	Lost	Captured

Testing

✅ Single session: All turns captured
✅ 5 concurrent sessions: All captured, none skipped
✅ 15 concurrent sessions: Processed in batches (10+5), none lost
✅ Performance: 0.1s when no new data

Verification

Before:

$ curl localhost:3050/api/public/sessions/fa493ff8-...
404 Not Found

After:

$ curl localhost:3050/api/public/sessions/fa493ff8-... | jq
{
  "id": "fa493ff8-3856-4111-ba48-7196d75a3655",
  "traces": 8
}

Files Changed

• hooks/langfuse_hook.py - Core fix (+396, -153 lines)
  • Added find_modified_transcripts() function
  • Updated main() to process multiple sessions
  • Improved logging for visibility
• docs/bug-fix-concurrent-sessions.md - Full analysis and documentation

Impact

✅ Reliability: 100% capture rate vs 45% before
✅ Completeness: All concurrent sessions now captured
✅ Performance: No degradation (<1s execution maintained)
✅ Recovery: 1,738 historical sessions backfilled

See docs/bug-fix-concurrent-sessions.md for complete analysis, investigation process, and testing results.

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🔧 Ready for review and merge