Claude Code tracks where the auto-compact window comes from, including per-model settings

What

Compaction is when Claude Code shortens the conversation so it fits in the model's working memory (the context window). The autoCompactWindow setting controls how full that memory gets before this happens.

• A per-model value is looked up at modelSettings.autoCompactWindow, so different models can have different windows.
• Claude Code now records the window actually in force and where it came from, once each time it changes. Possible sources are env, server, experiment, default, agent_definition, session_override, settings_per_model and settings_top_level. The code that works out the window was split up to allow this.
• Records of successful compaction gain autoCompactWindow, autoCompactWindowOrigin and armTrigger fields.
• A new helper can prepare a compaction in advance while Claude Code is idle (armTrigger "idle").
• The list of settings Claude Code reports as set now includes modelSettings.autoCompactWindow when any entry under modelSettings defines autoCompactWindow, alongside permissions.defaultMode and worktree.baseRef.

Why

Several places can set the auto-compact window, from environment variables to settings files to agent definitions. Recording which one won makes it possible to tell why compaction happens when it does, and confirms a per-model setting is recognised.