Auto-save session state before compaction for seamless resume

Feature Request

Problem

When running long tasks (especially multi-phase workflows like GSD), context fills up and auto-compaction loses critical state. Users must manually /clear and re-establish context, which is error-prone and breaks flow. There's no built-in mechanism to preserve session state across compaction or clear boundaries.

Current Workarounds

I built a custom hook system to address this:

• PreCompact hook that auto-saves state to a handoff file before compaction
• PostToolUse context monitor that detects low context (35% remaining) and proactively saves state
• StatusLine bridge that shares context_window.remaining_percentage between hooks

This works but requires ~200 lines of custom JavaScript across 3 hook files plus settings configuration. Every user doing long-running tasks faces this same problem.

Proposed Solution

Built-in context state preservation, ideally with three levels:

1. Auto-save session state before compaction (minimum viable):

• When PreCompact fires for auto trigger, Claude Code writes a structured state file (modified files, recent actions, current task, key decisions)
• SessionStart with compact matcher automatically loads this state
• No user intervention needed — compaction becomes seamless

1. Proactive state snapshots at configurable thresholds (better):

• New setting: contextSaveThreshold: 35 (percentage remaining)
• When threshold is hit, state file is written automatically
• Agent receives context about the save so it can wrap up gracefully

1. Auto-clear-and-resume (ideal):

• When context hits a threshold, Claude Code automatically: saves state → clears → resumes with state loaded
• The user experiences a seamless continuation with a brief "Context refreshed" notification
• This is the equivalent of what IDEs do with memory management — invisible to the user

Additional Context

• Auto-compaction triggers at ~83.5% usage (16.5% remaining / ~33K buffer)
• Research shows quality degrades well before that threshold (context rot)
• Community consensus is to compact at 70% usage, but most users don't monitor this
• The PreCompact hook exists but has no decision control — it can only do side effects
• Hooks cannot inject /clear or /compact (no updatedPrompt field on UserPromptSubmit)
• The StatusLine is the only mechanism that receives context_window.remaining_percentage

Key Insight

The pieces are already there (PreCompact hook, SessionStart compact matcher, StatusLine metrics) but they're not connected into a cohesive auto-save-and-resume flow. Making this built-in would benefit every user doing tasks longer than a single context window.

Environment

• Claude Code on macOS (Apple Silicon)
• Using GSD workflow framework for multi-phase projects
• Typical sessions fill context in 30-60 minutes of active work