Feature Request: Let Claude trigger context compaction via tool call

Summary

Allow Claude to proactively trigger context compaction via a tool call (e.g., CompactContext), rather than relying solely on automatic threshold-based triggers or user-initiated /compact.

Motivation

Current compaction is either:

1. Automatic - fires at a fixed threshold (~78-80%), often at inopportune moments mid-task
2. User-initiated - /compact command

Neither leverages the fact that Claude itself knows best when compaction would be strategic. The model has awareness of:

• Whether it just finished a major task and context is full of now-irrelevant tool outputs
• Whether it's about to start a new phase of work that would benefit from a clean slate
• Whether the remaining context budget is tight for the upcoming task
• Whether critical context could be summarized without loss before starting something new

This would address pain points from several related issues where compaction fires at bad times (#21925, #18264, #18802) or users want more control over timing (#23711, #23419).

Proposed Behavior

• Expose a CompactContext tool (or similar) that Claude can call like any other tool
• Claude provides compaction instructions as a parameter (what to preserve, what to summarize, what to drop) - similar to how /compact accepts custom instructions
• The tool triggers compaction immediately and returns the new context state
• Could be gated behind a setting (allowAgentCompaction: true) for users who prefer current behavior

Example Use Cases

1. After completing a multi-file refactor with hundreds of lines of tool output, Claude recognizes the diffs are no longer needed and compacts before starting the next task
2. During a long planning session, Claude compacts after plan approval but before implementation begins
3. When context is at ~60% but the next task requires reading many large files, Claude preemptively compacts to make room
4. After a research phase with many web fetches, Claude summarizes findings and compacts before writing code

Relationship to Existing Issues

• #23261 (/compact-next) - complementary; that's user-scheduled, this is agent-initiated
• #23493 (low-context warning hook) - this would be a more capable alternative
• #23711 (configurable threshold) - this removes the need for threshold tuning entirely
• #21925 (compaction destroys workflow) - agent-controlled timing avoids mid-task disruption