[FEATURE] Expose the effective auto-compact window in the statusLine JSON

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When an auto-compact window is set, for example with /autocompact 400k, the statusLine JSON does not reflect it. A status line therefore cannot show how close the conversation is to compaction.

context_window.context_window_size is always the model's full window, and the status line docs state that used_percentage "always measures against the model's full context window." With an auto-compact window set, that percentage no longer indicates when compaction will run.

Example: Opus on a 1M window with autoCompactWindow: 400000. At 392k tokens, the status line shows 39% used. Compaction is about 8k tokens away.

The value cannot be recovered reliably from outside the session, because it is per-session state. Each session resolves its window once at startup, from --autocompact, CLAUDE_CODE_AUTO_COMPACT_WINDOW, or autoCompactWindow in settings. After that, only /autocompact run in that same session changes it. The settings file is shared, though. When session A runs /autocompact, it rewrites autoCompactWindow in the user settings file, but session B keeps compacting at its startup value. A status line command sees only its stdin JSON, an environment fixed at launch, and files on disk. None of these holds the value session B actually uses.

Proposed Solution

Add the session's resolved value to context_window:

"context_window": {
  "context_window_size": 1000000,
  "auto_compact": {
    "enabled": true,
    "window": 400000,
    "threshold_tokens": 400000,
    "source": "settings"
  }
}

• window is the effective auto-compact window for this session, after all precedence rules and caps. It is the session's own value, not the current contents of the settings file.
• threshold_tokens is the token count at which auto-compaction triggers. It differs from window when CLAUDE_AUTOCOMPACT_PCT_OVERRIDE or a tuned default applies.
• source is env, flag, settings, or default. A status line can use it to mark an explicit override.
• When auto-compaction is off, the object is omitted or reports enabled: false.

A single auto_compact_window number would cover most of the need. The internal autocompact_state event for remote sessions appears to carry the same values (enabled, effective_window, threshold, source), so they may already be computed.

Alternative Solutions

I tried the options available to a status line today. Each one falls short:

• Read autoCompactWindow from ~/.claude/settings.json. This is my current workaround. It reads and parses the settings file on every render to reconstruct a value Claude Code already holds in memory. It is correct in a single session, but wrong in every other open session after one of them runs /autocompact. It cannot see --autocompact. It also has to copy Claude Code's resolution rules: env over flag over settings scopes, the 100k–1M clamp, the cap at the model window, and the tuned defaults. These rules can change in any release.
• Set CLAUDE_CODE_AUTO_COMPACT_WINDOW instead. The status line command inherits the variable, so the displayed value is accurate. But the variable is a hard override: while it is set, /autocompact and --autocompact stop working.
• Bridge the value with hooks. No hook input carries the auto-compact window. ConfigChange fires in every session when the settings file changes, and its input does not say which session made the change. CLAUDE_ENV_FILE is available only to a few hook events and feeds the Bash tool, not the status line. A bridge would need per-session state files and a cleanup job.
• Add the field through status line configuration. The statusLine setting accepts only type, command, padding, refreshInterval, and hideVimModeIndicator. There is no way to extend the payload.

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

1. I set /autocompact 400k so long sessions compact well before the 1M limit.
2. My status line shows context usage as a bar, a percentage, and used/max tokens, with colour thresholds near the limit.
3. I usually have several sessions open. When I change the window in one of them, the others keep their old value.
4. With this field, each session's bar, percentage, and thresholds measure against the window that session will actually compact at.

Additional Context

• Related: #90212 asks to change the auto-compact window at runtime. This request is about displaying it.
• #95368 makes the same argument for /focus state: it lives in the running process, and a separate status line process cannot read it. One mechanism for exposing session state could cover both.
• Similar request for other session state: #94299.
• Observed on Claude Code 2.1.280, macOS.