compaction

Claude Code v2.1.296 lets a subagent auto-compact earlier than the main conversation. Set autoCompactWindow in the subagent's frontmatter or --agents JSON as a plain integer from 100000 to 1000000, and watch out for the 200k parsing trap, the CLAUDE_CODE_AUTO_COMPACT_WINDOW override, and the --agent session mode where the field does nothing.

A context window is everything the model can see in one request: system prompt, tool definitions, history, tool results, thinking and the reply. Here is what counts, what happens at the limit on claude-opus-5-5 and friends, and the six techniques agents use to work past it: deferred tools, context editing, compaction, memory, subagents and output filtering.