Stats cache loses historical data after v2.1.27 update - firstSessionDate incorrectly reset

Bug Description

After updating to Claude Code v2.1.27, the /stats command shows incorrect data. The stats-cache.json file has firstSessionDate set to a recent date (Dec 24, 2025) instead of the actual first session date (Sept 29, 2025), causing ~84 days of historical activity to be missing from stats.

Environment

• Claude Code Version: 2.1.27
• OS: macOS (Darwin 25.2.0)
• Platform: darwin arm64

Steps to Reproduce

1. Have been using Claude Code since Sept 2025 with extensive history
2. Update to v2.1.27 (from v2.1.25)
3. Run /stats command
4. Observe incorrect/missing historical data

Expected Behavior

• Stats should show all historical data from history.jsonl
• firstSessionDate should reflect the actual first session (Sept 29, 2025)
• Active days count should include all days with activity (~122 days)

Actual Behavior

• Stats only show data from Dec 24, 2025 onwards
• firstSessionDate in stats-cache.json is incorrectly set to 2025-12-24
• 84 days of activity before Dec 24 are missing from the cache
• Total sessions shows 344 instead of actual count

Investigation Findings

Raw data (history.jsonl) is intact:

First entry: 2025-09-29 11:04:04
Last entry: 2026-01-31 00:17:58
Total entries: 8,805
Unique active days: 123
Days before Dec 24: 84

Cache file (stats-cache.json) is incorrect:

{
  "firstSessionDate": "2025-12-24T19:28:55.109Z",  // WRONG - should be Sept 29
  "totalSessions": 344,  // Missing many sessions
  "dailyActivity": [...]  // Only 32 entries, missing 84+ days
}

Timeline:

| Time | Event |
|------|-------|
| Jan 29 13:11 | v2.1.25 installed |
| Jan 30 15:18 | v2.1.27 installed |
| Jan 30 21:28 | stats-cache.json regenerated (data loss occurs) |

Workaround Attempted

Deleting ~/.claude/stats-cache.json and running /stats again may regenerate the cache, but if the bug is in the computation logic, it will reproduce the same incorrect data.

Additional Context

The history.jsonl file contains complete data - this appears to be a bug in how the stats cache is computed/regenerated from the history file, specifically in determining firstSessionDate and aggregating dailyActivity.

---
🤖 Generated with Claude Code