Explore the context window changedcontext-window

The first capture of this source. The page was already there, and this is what it said.

# 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.

export const ContextWindow = () => {
  const MAX = 200000;
  const STARTUP_END = 0.2;
  {}
  const EVENTS = useMemo(() => [{}, {
    t: 0.015,
    kind: 'auto',
    label: 'System prompt',
    tokens: 4200,
    color: '#6B6964',
    vis: 'hidden',
    desc: 'Core instructions for behavior, tool use, and response formatting. Always loaded first. You never see it.',
    link: null
  }, {
    t: 0.035,
    kind: 'auto',
    label: 'Auto memory (MEMORY.md)',
    tokens: 680,
    color: '#E8A45C',
    vis: 'hidden',
    desc: "Claude's notes to itself from previous sessions: build commands it learned, patterns it noticed, mistakes to avoid. The first 200 lines or 25KB, whichever comes first, are loaded into the conversation context.",
    link: '/en/memory#auto-memory'
  }, {
    t: 0.06,
    kind: 'auto',
    label: 'Environment info',
    tokens: 280,
    color: '#6B6964',
    vis: 'hidden',
    desc: 'Working directory, platform, shell, OS version, and whether this is a git repo. Git branch, status, and recent commits load as a separate block at the very end of the system prompt.',
    link: null
  }, {
    t: 0.08,
    kind: 'auto',
    label: 'MCP tools (deferred)',
    tokens: 120,
    color: '#9B7BC4',
    vis: 'hidden',
    desc: 'MCP tool names listed so Claude knows what is available. By default, full schemas stay deferred and Claude loads specific ones on demand via tool search when a task needs them. Set `ENABLE_TOOL_SEARCH=auto` to load schemas upfront when they fit within 10% of the context window, or `ENABLE_TOOL_SEARCH=false` to load everything.',
    link: '/en/mcp#scale-with-mcp-tool-search'
  }, {
    t: 0.1,
    kind: 'auto',
    label: 'Skill descriptions',
    tokens: 450,
    color: '#D4A843',
    vis: 'hidden',
    noSurviveCompact: true,
    desc: 'One-line descriptions of available skills so Claude knows what it can invoke. Full skill content loads only when Claude actually uses one. Skills with `disable-model-invocation: true` are not in this list. They stay completely out of context until you invoke them with `/name`. Unlike the rest of the startup content, this listing is not re-injected after `/compact`. Only skills you actually invoked get preserved.',
    link: '/en/skills'
  }, {
    t: 0.12,
    kind: 'auto',
    label: '~/.claude/CLAUDE.md',
    tokens: 320,
    color: '#6A9BCC',
    vis: 'hidden',
    desc: 'Your global preferences. Applies to every project. Loaded alongside project instructions at the start of every conversation.',
    link: '/en/memory#choose-where-to-put-claude-md-files'
  }, {
    t: 0.14,
    kind: 'auto',
    label: 'Project CLAUDE.md',
    tokens: 1800,
    color: '#6A9BCC',
    vis: 'hidden',
    desc: 'Project conventions, build commands, architecture notes. The most important file you can create. Lives in your project root, so your whole team gets the same instructions.',
    tip: 'Keep it under 200 lines. Move reference content to skills or path-scoped rules so it only loads when needed.',
    link: '/en/memory'
  }, {}, {
    t: 0.22,
    kind: 'user',
    label: 'Your prompt',
    tokens: 45,
    color: '#558A42',
    vis: 'full',
    desc: '"Fix the auth bug where users get 401 after token refresh"',
    link: null
  }, {}, {
    t: 0.28,
    kind: 'claude',
    label: 'Read src/api/auth.ts',
    tokens: 2400,
    color: '#8A8880',
    vis: 'brief',
    desc: 'Main auth file. You see "Read auth.ts" in your terminal, but the 2,400 tokens of file content only Claude sees.',
    tip: 'File reads dominate context usage. Be specific in prompts ("fix the bug in auth.ts") so Claude reads fewer files. For research-heavy tasks, use a subagent.',
    link: null
  }, {
    t: 0.32,
    kind: 'claude',
    label: 'Read src/lib/tokens.ts',
    tokens: 1100,
    color: '#8A8880',
    vis: 'brief',
    desc: 'Following imports to the token module. Shown as a one-liner in your terminal.',
    link: null
  }, {
    t: 0.35,
    kind: 'auto',
    label: 'Rule: api-conventions.md',
    tokens: 380,
    color: '#4A9B8E',
    vis: 'brief',
    desc: 'This rule in `.claude/rules/` has a `paths:` pattern matching `src/api/**`. It loaded automatically when Claude read a file in that directory. You see "Loaded .claude/rules/api-conventions.md" in your terminal, but not the rule content.',
    link: '/en/memory#path-specific-rules'
  }, {
    t: 0.38,
    kind: 'claude',
    label: 'Read middleware.ts',
    tokens: 1800,
    color: '#8A8880',
    vis: 'brief',
    desc: 'Tracing the auth flow deeper.',
    link: null
  }, {
    t: 0.41,
    kind: 'claude',
    label: 'Read auth.test.ts',
    tokens: 1600,
    color: '#8A8880',
    vis: 'brief',
    desc: 'Checking existing tests for expected behavior.',
    link: null
  }, {
    t: 0.44,
    kind: 'auto',
    label: 'Rule: testing.md',
    tokens: 290,
    color: '#4A9B8E',
    vis: 'brief',
    desc: 'Another path-scoped rule, this one matching `*.test.ts` files. Triggered when Claude read auth.test.ts. Shown as a one-line "Loaded" notice.',
    link: '/en/memory#path-specific-rules'
  }, {
    t: 0.47,
    kind: 'claude',
    label: 'grep "refreshToken"',
    tokens: 600,
    color: '#A09E96',
    vis: 'brief',
    desc: 'Search results across the codebase. You see the command ran, not the full output.',
    link: null
  }, {}, {
    t: 0.53,
    kind: 'claude',
    label: "Claude's analysis",
    tokens: 800,
    color: '#D97757',
    vis: 'full',
    desc: 'Explains the bug: token invalidated too early in the rotation. This text appears in your terminal.',
    link: null
  }, {
    t: 0.57,
    kind: 'claude',
    label: 'Edit auth.ts',
    tokens: 400,
    color: '#D97757',
    vis: 'full',
    desc: 'Fixes the token rotation order. The diff appears in your terminal.',
    link: null
  }, {
    t: 0.59,
    kind: 'hook',
    label: 'Hook: prettier',
    tokens: 120,
    color: '#B8860B',
    vis: 'hidden',
    desc: 'A PostToolUse hook in `settings.json` runs prettier after every file edit and reports back via `hookSpecificOutput.additionalContext`. That field enters Claude\'s context. Plain stdout on exit 0 does not. It is written to the debug log only.',
    tip: 'Output JSON with `additionalContext` to send info to Claude. For PostToolUse hooks, exit code 2 surfaces stderr as an error but cannot block since the tool already ran. Keep output concise since it enters context without truncation.',
    link: '/en/hooks-guide'
  }, {
    t: 0.62,
    kind: 'claude',
    label: 'Edit auth.test.ts',
    tokens: 600,
    color: '#D97757',
    vis: 'full',
    desc: 'Adds a regression test for the fix. The diff appears in your terminal.',
    link: null
  }, {
    t: 0.64,
    kind: 'hook',
    label: 'Hook: prettier',
    tokens: 100,
    color: '#B8860B',
    vis: 'hidden',
    desc: 'The same hook fires again for the test file. Every matching tool event triggers it.',
    link: '/en/hooks-guide'
  }, {
    t: 0.67,
    kind: 'claude',
    label: 'npm test output',
    tokens: 1200,
    color: '#A09E96',
    vis: 'brief',
    desc: 'Runs the test suite. You see "Running npm test..." and the pass count, not the full 1,200 tokens of output.',
    link: null
  }, {
    t: 0.70,
    kind: 'claude',
    label: 'Summary',
    tokens: 400,
    color: '#D97757',
    vis: 'full',
    desc: '"Fixed token rotation. Added regression test. All tests pass."',
    link: null
  }, {}, {
    t: 0.72,
    kind: 'user',
    label: 'Your follow-up',
    tokens: 40,
    color: '#558A42',
    vis: 'full',
    desc: '"Use a subagent to research session timeout handling, then fix it"',
    tip: 'Follow-ups add to the same context. Delegating research to a subagent keeps large file reads out of your main window.',
    link: null
  }, {
    t: 0.79,
    kind: 'claude',
    label: 'Spawn research subagent',
    tokens: 80,
    color: '#D97757',
    vis: 'brief',
    desc: "Claude delegates the research to a subagent with a fresh, separate context window. It loads CLAUDE.md and the same MCP and skill setup, but starts without your conversation history or the main session's auto memory.",
    link: '/en/sub-agents'
  }, {
    t: 0.795,
    kind: 'sub',
    label: 'System prompt',
    tokens: 0,
    subTokens: 900,
    color: '#6B6964',
    vis: 'hidden',
    desc: "The subagent gets its own system prompt, shorter than the main session's. For the general-purpose agent, it's a brief prompt plus environment details. The main session's auto memory is not included. If a custom agent has memory: in its frontmatter, it loads its own separate MEMORY.md here instead.",
    link: '/en/sub-agents#enable-persistent-memory'
  }, {
    t: 0.80,
    kind: 'sub',
    label: 'Project CLAUDE.md (own copy)',
    tokens: 0,
    subTokens: 1800,
    color: '#6A9BCC',
    vis: 'hidden',
    desc: "The subagent loads CLAUDE.md too. Same file, same content, but it counts against the subagent's context, not yours. The built-in Explore and Plan agents skip this for a smaller context.",
    link: '/en/sub-agents'
  }, {
    t: 0.805,
    kind: 'sub',
    label: 'MCP tools + skills',
    tokens: 0,
    subTokens: 970,
    color: '#9B7BC4',
    vis: 'hidden',
    desc: "The subagent has access to the same MCP servers and skills. It gets most of the parent's tools, minus several that don't apply in a nested context, including plan-mode controls, background-task tools, and by default the Agent tool itself to prevent recursion.",
    link: '/en/sub-agents'
  }, {
    t: 0.81,
    kind: 'sub',
    label: 'Task prompt from main',
    tokens: 0,
    subTokens: 120,
    color: '#558A42',
    vis: 'hidden',
    desc: "Instead of a user prompt, the subagent receives the task Claude wrote for it: 'Research session timeout handling in this codebase.'",
    link: '/en/sub-agents'
  }, {
    t: 0.82,
    kind: 'sub',
    label: 'Read session.ts',
    tokens: 0,
    subTokens: 2200,
    color: '#8A8880',
    vis: 'hidden',
    desc: "Now the subagent does its work. This file read fills the subagent's context, not yours.",
    link: '/en/sub-agents'
  }, {
    t: 0.825,
    kind: 'sub',
    label: 'Read timeouts.ts',
    tokens: 0,
    subTokens: 800,
    color: '#8A8880',
    vis: 'hidden',
    desc: "Another file read in the subagent's separate context.",
    link: '/en/sub-agents'
  }, {
    t: 0.83,
    kind: 'sub',
    label: 'Read config/*.ts',
    tokens: 0,
    subTokens: 3100,
    color: '#8A8880',
    vis: 'hidden',
    desc: "The subagent can read as many files as it needs. None of this touches your main context.",
    link: '/en/sub-agents'
  }, {

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