[Feature Request] Seamless in-session upgrade — apply updates without exit/restart/resume workflow

Problem

When a new version of Claude Code is released and installed (via brew upgrade, npm update, or auto-update), the update only takes effect for new sessions. The currently running session continues using the old binary loaded in memory.

This creates a disruptive workflow:

1. User is in the middle of a productive session
2. User upgrades Claude Code (or auto-update runs)
3. To use the new version, user must:

• Exit the current session
• Start a new session
• Find and resume the previous session via --resume
• Accept the quality degradation that comes with resumption

The Deeper Problem: Quality Degradation on Resume

Even with --resume, there is documented quality loss. As noted in the DoltHub Claude Code Gotchas article:

"Claude Code is definitely dumber after the compaction. It doesn't know what files it was looking at and needs to re-read them. It will make mistakes you specifically corrected again earlier in the session."

This means the exit → upgrade → resume workflow doesn't just add friction — it actively degrades session quality because:

• Behavioral conditioning is lost: The AI forgets corrections and patterns learned during the session
• File awareness resets: Must re-read files it was already working with
• Repeated mistakes: Makes errors that were already corrected earlier
• Mental model disruption: Loses the accumulated understanding of the task

For sessions that have compacted multiple times, this degradation compounds — the AI has already lost some conditioning at each compaction, and a full restart/resume loses even more.

Who This Impacts Most

• Power users with long, complex sessions
• Sessions that have compacted multiple times
• Multi-file debugging or refactoring tasks
• Users who have spent time correcting and training the AI during the session
• Anyone who doesn't want to sacrifice session quality just to get a bug fix

Proposed Solution

Implement a graceful in-place update mechanism that preserves the running session's full state.

Option A: /upgrade Slash Command (Recommended)

User runs /upgrade
→ Claude Code serializes full session state (conversation, behavioral context, tool states, environment)
→ Process performs graceful restart with new binary
→ State is restored in-place
→ User continues with ZERO quality loss

This follows established patterns like:

• PM2 graceful restart using SIGINT with wait_ready
• nginx reload using SIGHUP
• systemd service reloads

Option B: Automatic Detection with Prompt

• Detect newer version on disk
• Prompt: "Version 2.0.70 available. Upgrade now? Session will be preserved."
• If accepted, perform graceful restart with state preservation

Technical Considerations

• Serialize not just conversation text but behavioral state where possible
• Handle in-flight tool operations gracefully
• Rollback mechanism if new binary fails to start
• Cross-version state compatibility (handle schema changes)

Expected Behavior

| Current Workflow | Proposed Workflow |
|------------------|-------------------|
| Upgrade requires exit + manual resume | Upgrade happens seamlessly in-place |
| Resume causes quality degradation | Full session quality preserved |
| AI forgets corrections and files | AI retains all learned context |
| User must track session IDs | No manual intervention needed |

Impact

• Eliminates the trade-off between "stay current" and "preserve session quality"
• Reduces friction for staying on latest version
• Improves experience for power users
• Makes auto-update truly seamless

Related Issues

• #12990 - Persistent session context across restarts
• #12774 - Session Continuity Improvements
• #6497 - Hot reload of agents and slash commands
• #1340 - Session resumption (closed, but quality issues remain)

References

• DoltHub: Claude Code Gotchas — Documents post-compaction quality loss
• PM2: Graceful Restart — Proven pattern for state-preserving restarts
• nginx: Graceful Reload — Signal-based reload pattern