SpyBara
Go Premium

context-window.md 2026-10-22 22:58 UTC to 2026-10-23 22:59 UTC

This page contains 23 additions and 23 deletions.

2026
Thu 1 23:59 Tue 6 23:59 Wed 7 23:59 Thu 8 22:58 Wed 14 22:58 Tue 20 23:00 Thu 22 22:58 Fri 23 22:59 Tue 27 22:59

Explore the context window

An interactive simulation of how Claude Code's context window fills during a session. See what loads automatically, what each file read costs, and when rules and hooks fire.

Claude Code's context window holds everything Claude knows about your session: your instructions, the files it reads, its own responses, and content that never appears in your terminal. The timeline below plays a full session from startup to compaction: what loads before you type, what each file read, rule, and hook adds as Claude works, and how a subagent keeps large reads out of your context. See the written breakdown for the same content as a list.

What the timeline shows

The session walks through a realistic flow with representative token counts:

  • Before you type anything: CLAUDE.md, auto memory, MCP tool names, and skill descriptions all load into context. AGENTS.md files can load in place of CLAUDE.md. Your own setup may add more here, like an output style or text from --append-system-prompt.
  • As Claude works: each file read adds to context, path-scoped rules load automatically alongside matching files, and a PostToolUse hook fires after each edit.
  • The follow-up prompt: a subagent handles the research in its own separate context window, so the large file reads stay out of yours. Only the summary and a small metadata trailer come back.
  • At the end of the walkthrough: you run /compact, which replaces the conversation with a structured summary. Most startup content reloads automatically; the table below shows what happens to each mechanism.

What survives compaction

When a long session compacts, Claude Code summarizes the conversation history to fit the context window. As of v2.1.198, the summarization request inherits your session's extended thinking configuration, so it reasons with thinking enabled when your session has it enabled and stays off otherwise. Thinking affects only how the summary is produced; your session settings are unchanged afterward. What happens to each kind of content depends on how it was loaded:

Mechanism After compaction
System prompt and output style Both still apply
Project-root CLAUDE.md and unscoped rules Re-injected from disk
Auto memory Re-injected from disk
Skill descriptions Rebuilt from your current skills and re-injected. Requires Claude Code v2.1.304 or later
Git status snapshot If the session started with one, Claude Code reads a fresh one from your repository
The plan Claude wrote in plan mode Re-injected from disk
Rules with paths: frontmatter Claude Code reloads them on demand
Nested CLAUDE.md in subdirectories Claude Code reloads them on demand
Files Claude read or edited Claude Code re-reads up to five, most recently modified first
Invoked skill bodies Re-injected, capped at 5,000 tokens per skill and 25,000 tokens total; oldest dropped first
Background commands and background subagents Keep running. Claude Code reminds Claude which ones are still running so it doesn't start a duplicate
Context that hooks added earlier Summarized with the rest of the conversation
SessionStart hooks that match the compact source Claude Code runs them and adds their output to the compacted context

Right after compaction, Claude Code re-reads up to five of the files Claude has read or edited in the session, choosing the ones modified most recently. A file over 5,000 tokens comes back as a path reference without its content, shown as Referenced file instead of Read.

Path-scoped rules and nested CLAUDE.md files load into message history when Claude reads, writes, or edits their trigger file, so compaction summarizes them away with everything else. If a rule must persist across compaction, drop the paths: frontmatter or move it to the project-root CLAUDE.md.

Skill bodies are re-injected after compaction, but large skills are truncated to fit the per-skill cap, and the oldest invoked skills are dropped once the total budget is exceeded. Truncation keeps the start of the file, so put the most important instructions near the top of SKILL.md.

When your context fills up

Claude Code compacts automatically as you approach the limit, so a full context window doesn't end your session. The automatic pass works the same way as the /compact step in the timeline. See When context fills up for what it preserves. For when Claude Code also compacts a conversation you leave idle, see idle compaction.

You can also act before the automatic pass runs:

  • Compact with a focus: run /compact with instructions, like /compact focus on the auth bug fix, before starting a long new task. The summary keeps what you choose instead of what the automatic pass guesses is important.
  • Compact part of the conversation: run /rewind, select a message, and choose Summarize from here or Summarize up to here. See Rewind and summarize for what each option keeps and how to guide the summary.
  • Compact earlier: run /autocompact with a token count, like /autocompact 500k, to set how full the context window gets before the automatic pass runs. See Set the auto-compact window for accepted values and overrides.
  • Clear between tasks: run /clear when switching to unrelated work. Old conversation crowds out the files you need next and costs tokens on every message.
  • Delegate large reads: send research to a subagent so the file contents stay in its context window, not yours.

If you need a larger window rather than a smaller conversation, Fable models, Sonnet 5 and later, Haiku 5.5, Opus 4.6 and later, and Sonnet 4.6 support a 1 million token context window. See Extended context for availability by plan and how to select a [1m] model variant. Compaction works the same way at the larger limit.

The point where automatic compaction runs depends on your model and configuration. See Default auto-compact thresholds for the boundaries per model, and Correct the window for a gateway or custom model ID if Claude Code assumes the wrong window for your model ID, such as an LLM gateway alias.

Check your own session

The visualization uses representative numbers. To see your actual context usage at any point, run /context for a live breakdown by category with optimization suggestions, including which CLAUDE.md and auto memory files loaded. Run /memory to open and edit those files.

For deeper coverage of the features shown in the timeline, see these pages: