Cross-Session Summary Contamination in Claude Code

[phpmypython]

Description

Claude Code incorrectly writes summaries from previous sessions into the current session's JSONL file when starting a new conversation in the same directory.

Steps to Reproduce

1. Have a conversation in a project directory (Session A)
2. Close Claude Code before summaries are generated for Session A
3. Later (hours or days), start a new Claude Code session in the same directory (Session B)
4. Check Session B's JSONL file (~/.claude/projects/<project-name>/<session-id>.jsonl)
5. Observe that Session B's file contains summary entries from Session A

Expected Behavior

• Summaries should be written to their original session files
• Each session's JSONL file should only contain summaries for its own conversations

Actual Behavior

• On startup, Claude Code scans all session files in the current directory
• For any conversation chains lacking summaries, it generates them
• These summaries are incorrectly appended to the CURRENT session file instead of their original files
• This causes session files to contain unrelated summaries with leafUuid values pointing to messages in other sessions

Impact

• Confuses session history and makes it difficult to understand conversation flow
• Pollutes session files with unrelated content
• The leafUuid values in summaries reference messages that don't exist in the current session

Additional Context

• This appears to be caused by a delayed summary generation feature that runs on startup
• The global session singleton writes all generated summaries to the current session file
• Message UUIDs are reused across sessions (each appears exactly twice), which may be related to the session continuation feature

Environment

• Claude Code version: 1.0.34
• Platform: macOS Darwin 24.5.0

[phpmypython]

I've done some analysis on my own session files and can confirm this bug has widespread impact:

Analysis Results:

• Analyzed 20 session files in my ~/.claude/projects directory
• Found 26 misplaced summaries across 16 session files (80% of my sessions affected!)
• Almost every session had summaries belonging to other sessions
• Some sessions were both victims and perpetrators (containing wrong summaries while missing their own)

Notable example:
One of my sessions contains 10 summaries from a completely unrelated conversation, while the original session is missing exactly those 10 summaries.

Additional finding:
One summary was completely orphaned - its referenced UUID doesn't exist in any session file, suggesting the original session may have been deleted or the UUID reference is invalid.

This creates a chain effect where summaries are scattered across sessions based on which session happened to be active when Claude Code started and ran the delayed summary generation.

[github-actions]

Found 3 possible duplicate issues:

1. [BUG] Context bleeding between concurrent Claude Code sessions #2272
2. [BUG] Session Isolation Failure - Information Leaking Between Multiple Claude Code Sessions #1985
3. "Session Contamination: Incorrect Context Persistence Across Conversations" This title captures the key technical issue of: - Sessions bleeding into each other - Inappropriate context carryover - Loss of conversation isolation #2572

This issue will be automatically closed as a duplicate in 3 days.

• If your issue is a duplicate, please close it and 👍 the existing issue instead
• To prevent auto-closure, add a comment or 👎 this comment

🤖 Generated with Claude Code