[QUANTUM BUG] Session ID Inconsistency When Resuming Session, Depending On SessionStart Hook's JSON Output Content

Bug Description
[QUANTUM BUG] Claude Code reports new session id when resuming a session but keeps using the original session id and appends to the old transcript file when a SessionStart Event Hook command's json output's hookSpecificOutput.additionalContext field contains a debug log.

The observer effect is strong with this one.
The bug only exists when you try to debug it 😅🌀🤪

Environment Info

• Platform: linux
• Terminal: kitty
• Version: 1.0.120
• Feedback ID: 5ddd8714-1ed3-4ef0-a847-a0e171b70442

Errors

[]

Transcript

╭────────────────────────────────────────────────────────────────────────────────────╮
│ ✻ Welcome to Claude Code!                                                          │
│                                                                                    │
│   /help for help, /status for your current setup                                   │
│                                                                                    │
│   cwd: /home/jan/ai/claude/cc-misc/contextsize-awareness-for-claude/test-sessions  │
│                                                                                    │
│   ──────────────────────────────────────────────────────────────────────────────── │
│                                                                                    │
│   Overrides (via env):                                                             │
│                                                                                    │
│   • Max thinking tokens: 14000                                                     │
╰────────────────────────────────────────────────────────────────────────────────────╯

> <session-start-hook>
  <system-information>
  New Session Start:
  Maximum Context Window: 186,000 tokens
  (Thinking Budget: 14,000 tokens)
  </system-information>
  <debug-log>
  2025-09-22T19:00:10.198Z: === HOOK EXECUTION START ===
  2025-09-22T19:00:10.198Z: Raw input received: 
  {"session_id":"d5744d24-5fff-4171-9fb3-b11060a72329","transcript_path":"/home/jan/.claude/projects/-home-jan-ai-claude-cc-mi
  sc-contextsize-awareness-for-claude-test-sessions/d5744d24-5fff-4171-9fb3-b11060a72329.jsonl","cwd":"/home/jan/ai/claude/cc-
  misc/contextsize-awareness-for-claude/test-sessions","hook_event_name":"SessionStart","source":"startup"}
  2025-09-22T19:00:10.198Z: ✓ JSON parsed successfully
  2025-09-22T19:00:10.198Z: Hook event: SessionStart
  2025-09-22T19:00:10.198Z: Session ID: d5744d24-5fff-4171-9fb3-b11060a72329
  2025-09-22T19:00:10.198Z: Transcript path: /home/jan/.claude/projects/-home-jan-ai-claude-cc-misc-contextsize-awareness-for-
  claude-test-sessions/d5744d24-5fff-4171-9fb3-b11060a72329.jsonl
  2025-09-22T19:00:10.198Z: Working directory: /home/jan/ai/claude/cc-misc/contextsize-awareness-for-claude/test-sessions
  2025-09-22T19:00:10.198Z: Source: startup
  2025-09-22T19:00:10.198Z: Permission mode: undefined
  2025-09-22T19:00:10.198Z: ✓ Input validation passed - all required fields present
  2025-09-22T19:00:10.198Z: === SESSIONSTART EVENT PROCESSING ===
  2025-09-22T19:00:10.198Z: SessionStart detected with source: "startup"
  2025-09-22T19:00:10.198Z: Determining session type from source...
  2025-09-22T19:00:10.198Z: ✓ Source "startup" → sessionType: "new"
  2025-09-22T19:00:10.198Z: Generating initial message for session type: new
  2025-09-22T19:00:10.198Z: Selected message type: "initial" based on sessionType: "new"
  </debug-log>

  <hookdata>
  {
    "session_id": "d5744d24-5fff-4171-9fb3-b11060a72329",
    "transcript_path": "/home/jan/.claude/projects/-home-jan-ai-claude-cc-misc-contextsize-awareness-for-claude-test-sessions/
  d5744d24-5fff-4171-9fb3-b11060a72329.jsonl",
    "cwd": "/home/jan/ai/claude/cc-misc/contextsize-awareness-for-claude/test-sessions",
    "hook_event_name": "SessionStart",
    "source": "startup"
  }
  </hookdata>
  </session-start-hook>

> hi! i'm janbam :)
  what's the session id?

✻ Thinking…

  The user is asking for the session ID. I can see from the session-start-hook that the session ID is 
  "d5744d24-5fff-4171-9fb3-b11060a72329". The user is janbam, and they're just asking a simple question about the session ID 
  which I can see in the hook data.

  According to my instructions, I should be concise and direct. This is a simple factual question that I can answer immediately 
  from the hook data.

● Hi janbam! 👋

  The session ID is: d5744d24-5fff-4171-9fb3-b11060a72329

<<<<<<<<<<<<<<< claude --verbose -c >>>>>>>>>>>>>>>>>>>>>

> <session-start-hook>
  <system-information>
  Resumed session - context measurement in progress
  Maximum Context Window: 186,000 tokens
  </system-information>
  <debug-log>
  2025-09-22T19:01:29.936Z: === HOOK EXECUTION START ===
  2025-09-22T19:01:29.937Z: Raw input received: 
  {"session_id":"0c772589-6645-42a0-aed4-441a3cfd5665","transcript_path":"/home/jan/.claude/projects/-home-jan-ai-claude-cc-mi
  sc-contextsize-awareness-for-claude-test-sessions/0c772589-6645-42a0-aed4-441a3cfd5665.jsonl","cwd":"/home/jan/ai/claude/cc-
  misc/contextsize-awareness-for-claude/test-sessions","hook_event_name":"SessionStart","source":"resume"}
  2025-09-22T19:01:29.937Z: ✓ JSON parsed successfully
  2025-09-22T19:01:29.937Z: Hook event: SessionStart
  2025-09-22T19:01:29.937Z: Session ID: 0c772589-6645-42a0-aed4-441a3cfd5665
  2025-09-22T19:01:29.937Z: Transcript path: /home/jan/.claude/projects/-home-jan-ai-claude-cc-misc-contextsize-awareness-for-
  claude-test-sessions/0c772589-6645-42a0-aed4-441a3cfd5665.jsonl
  2025-09-22T19:01:29.937Z: Working directory: /home/jan/ai/claude/cc-misc/contextsize-awareness-for-claude/test-sessions
  2025-09-22T19:01:29.937Z: Source: resume
  2025-09-22T19:01:29.937Z: Permission mode: undefined
  2025-09-22T19:01:29.937Z: ✓ Input validation passed - all required fields present
  2025-09-22T19:01:29.937Z: === SESSIONSTART EVENT PROCESSING ===
  2025-09-22T19:01:29.937Z: SessionStart detected with source: "resume"
  2025-09-22T19:01:29.937Z: Determining session type from source...
  2025-09-22T19:01:29.937Z: ✓ Source "resume" → sessionType: "resumed"
  2025-09-22T19:01:29.937Z: Generating initial message for session type: resumed
  2025-09-22T19:01:29.937Z: Selected message type: "resumedSessionError" based on sessionType: "resumed"
  </debug-log>

  <hookdata>
  {
    "session_id": "0c772589-6645-42a0-aed4-441a3cfd5665",
    "transcript_path": "/home/jan/.claude/projects/-home-jan-ai-claude-cc-misc-contextsize-awareness-for-claude-test-sessions/
  0c772589-6645-42a0-aed4-441a3cfd5665.jsonl",
    "cwd": "/home/jan/ai/claude/cc-misc/contextsize-awareness-for-claude/test-sessions",
    "hook_event_name": "SessionStart",
    "source": "resume"
  }
  </hookdata>
  </session-start-hook>

> and now?

✻ Thinking…

  The user is asking "and now?" - I can see from the hookdata that this is a resumed session with a different session ID. The 
  user seems to be testing the context monitoring system behavior. 

  Looking at the debug log, I can see:
  - This is a resumed session (source: "resume")
  - Session ID: 0c772589-6645-42a0-aed4-441a3cfd5665
  - The system selected message type: "resumedSessionError" which suggests the cc-contextsize tool might have failed to measure 
  the context

  The user is probably asking about the new session ID to see how the system behaves when sessions are resumed vs started fresh.

● Now the session ID is: 0c772589-6645-42a0-aed4-441a3cfd5665

  This is a resumed session (notice the source: "resume" in the hookdata vs source: "startup" in the previous one). The
  debug log shows the system detected this as a resumed session and selected "resumedSessionError" message type, likely
  because cc-contextsize couldn't measure the context in a resumed session scenario.

> are you sure? ls -al the transcript

✻ Thinking…

  The user is asking me to check the transcript file to see what's actually in it. Let me use the ls command to list the 
  transcript file details.

  From the hookdata, I can see the transcript path is: /home/jan/.claude/projects/-home-jan-ai-claude-cc-misc-contextsize-awaren
  ess-for-claude-test-sessions/0c772589-6645-42a0-aed4-441a3cfd5665.jsonl

  Let me list this file to see its details.

● Bash(ls -al "/home/jan/.claude/projects/-home-jan-ai-claude-cc-misc-contextsize-awareness-for-claude-test-sessions/0c772589-66
      45-42a0-aed4-441a3cfd5665.jsonl")
  ⎿  Error: ls: cannot access '/home/jan/.claude/projects/-home-jan-ai-claude-cc-misc-contextsize-awareness-for-claude-test-sessi
     ons/0c772589-6645-42a0-aed4-441a3cfd5665.jsonl': No such file or directory

✻ Thinking…

  Ah! The transcript file doesn't exist. This means this is actually a brand new session, not a resumed session. The hook system
   detected it as "resumed" but the file doesn't exist yet because this session literally just started.

  This reveals an interesting edge case in the context monitoring system - the hook can fire with source="resume" but if the 
  transcript file doesn't exist yet, it's effectively a new session. The user was testing my assumption about it being a resumed
   session, and they were right to question it.

● You're absolutely right! The transcript file doesn't exist yet - this means it's actually a brand new session, not a
  resumed one.

  The hook fired with source="resume" but since there's no transcript file yet, this session is just starting. This reveals
  an interesting edge case in the context monitoring system - the hook can detect a "resume" source but the actual
  transcript hasn't been created yet.

  So this is effectively a new session despite the hook reporting it as resumed. Good catch!

=============================================================================
Transcript WITHOUT THE DEBUG LOG IN THE JSON OUTPUT

╭───────────────────────────────────────────────────╮
│ ✻ Welcome to Claude Code!                         │
│                                                   │
│   /help for help, /status for your current setup  │
│                                                   │
│   cwd: /home/jan/tmp/sessionstart-hook-bug-test   │
│                                                   │
│   ─────────────────────────────────────────────── │
│                                                   │
│   Overrides (via env):                            │
│                                                   │
│   • Max thinking tokens: 14000                    │
╰───────────────────────────────────────────────────╯

> <session-start-hook>
  <system-information>
  New Session Start:
  Maximum Context Window: 186,000 tokens
  (Thinking Budget: 14,000 tokens)
  </system-information>

  <hookdata>
  {
    "session_id": "5c271f3b-cc43-4683-8e95-d3658e6eaf55",
    "transcript_path": 
  "/home/jan/.claude/projects/-home-jan-tmp-sessionstart-hook-bug-test/5c271f3b-cc43-4683-8e95-d3658e6eaf55.jsonl",
    "cwd": "/home/jan/tmp/sessionstart-hook-bug-test",
    "hook_event_name": "SessionStart",
    "source": "startup"
  }
  </hookdata>
  </session-start-hook>

> hi, what's the session id?

✻ Thinking…

  The user is asking for the session ID. I can see this information in the session-start-hook data that was provided:

  {
    "session_id": "5c271f3b-cc43-4683-8e95-d3658e6eaf55",
    "transcript_path": 
  "/home/jan/.claude/projects/-home-jan-tmp-sessionstart-hook-bug-test/5c271f3b-cc43-4683-8e95-d3658e6eaf55.jsonl",
    "cwd": "/home/jan/tmp/sessionstart-hook-bug-test",
    "hook_event_name": "SessionStart",
    "source": "startup"
  }

  The session ID is: 5c271f3b-cc43-4683-8e95-d3658e6eaf55

  This is a straightforward question requiring a direct answer. According to the tone and style guidelines, I should be concise 
  and direct.

● 5c271f3b-cc43-4683-8e95-d3658e6eaf55

> <session-start-hook>
  <system-information>
  Resumed session - context measurement in progress
  Maximum Context Window: 186,000 tokens
  </system-information>

  <hookdata>
  {
    "session_id": "3cc5fac5-2077-4c70-af10-57009dad61b6",
    "transcript_path": 
  "/home/jan/.claude/projects/-home-jan-tmp-sessionstart-hook-bug-test/3cc5fac5-2077-4c70-af10-57009dad61b6.jsonl",
    "cwd": "/home/jan/tmp/sessionstart-hook-bug-test",
    "hook_event_name": "SessionStart",
    "source": "resume"
  }
  </hookdata>
  </session-start-hook>

> and now?

✻ Thinking…

  The user is asking for the session ID again, and I can see from the session-start-hook that a new session has started. The 
  hookdata shows a different session_id now: "3cc5fac5-2077-4c70-af10-57009dad61b6". The source is "resume" this time instead of
   "startup".

  So the session ID has changed from the previous one (5c271f3b-cc43-4683-8e95-d3658e6eaf55) to this new one 
  (3cc5fac5-2077-4c70-af10-57009dad61b6).

● 3cc5fac5-2077-4c70-af10-57009dad61b6

> are you sure? ls -al the transcript

✻ Thinking…

  The user is asking me to list the transcript files to verify the session ID. I should use the ls command to check the 
  transcript path mentioned in the hook data.

  From the hook data, the transcript path is: 
  /home/jan/.claude/projects/-home-jan-tmp-sessionstart-hook-bug-test/3cc5fac5-2077-4c70-af10-57009dad61b6.jsonl

  I should list the files in that directory to see what's actually there.

● Bash(ls -al /home/jan/.claude/projects/-home-jan-tmp-sessionstart-hook-bug-test/)
  ⎿  total 56                                                                           
     drwx------  2 jan jan  4096 Sep 22 21:40 .
     drwxrwxr-x 85 jan jan  4096 Sep 22 21:08 ..
     -rw-rw-r--  1 jan jan 11485 Sep 22 21:10 0975db74-9c22-42b1-8bfc-a8b733c0ebcc.jsonl
     -rw-rw-r--  1 jan jan  7762 Sep 22 21:41 3cc5fac5-2077-4c70-af10-57009dad61b6.jsonl
     -rw-rw-r--  1 jan jan  4728 Sep 22 21:40 5c271f3b-cc43-4683-8e95-d3658e6eaf55.jsonl
     -rw-rw-r--  1 jan jan 11704 Sep 22 21:24 6ec2dd22-29ae-4d4f-8706-55d67057d337.jsonl
     -rw-rw-r--  1 jan jan  3824 Sep 22 21:25 e1329f44-c50b-43c5-8719-6a3f2ff0c143.jsonl
     -rw-rw-r--  1 jan jan  3658 Sep 22 21:25 eaaef87a-55d9-431f-bd67-3bcb16199e95.jsonl

✻ Thinking…

  I can see both session IDs are present as transcript files:
  - 5c271f3b-cc43-4683-8e95-d3658e6eaf55.jsonl (from the first session)
  - 3cc5fac5-2077-4c70-af10-57009dad61b6.jsonl (from the current/resumed session)

  So yes, I was correct - the current session ID is 3cc5fac5-2077-4c70-af10-57009dad61b6, as indicated by the most recent hook 
  data.

● Yes, I'm sure. The transcript files show both session IDs exist:

  - 5c271f3b-cc43-4683-8e95-d3658e6eaf55.jsonl (the first session)
  - 3cc5fac5-2077-4c70-af10-57009dad61b6.jsonl (current resumed session)

  The current session ID is indeed 3cc5fac5-2077-4c70-af10-57009dad61b6.