# Claude Code Context window

> How system prompts, CLAUDE.md, rules, file reads, tool output, hooks, and subagents use context, and when to use /clear, /compact, and /context.

URL: https://docs.passion8.cc/en/docs/claude-code/context
Language: en
Publisher: Passion8

Claude Code sends the required context to the model on each turn. The context window includes more than chat history: system prompts, project rules, file contents, tool output, hook feedback, subagent results, and your latest message.

## Where context comes from

| Source | When it enters context | Recommendation |
| --- | --- | --- |
| System prompt | Session startup | Managed by Claude Code; can change after upgrades |
| Environment information | Session startup | Working directory, platform, shell, Git state |
| `~/.claude/CLAUDE.md` | Session startup | Keep personal long-term preferences concise |
| Project `CLAUDE.md` | Session startup | Short, accurate team conventions, architecture, and commands |
| `.claude/rules` | When matching files are accessed | Use `paths:` for on-demand loading |
| Skill descriptions | Startup or invocation | Keep descriptions short; load bodies as needed |
| MCP tools | Startup or on-demand search | Tool search can load names and relevant schemas as needed |
| File reads | After Read/Grep/LSP/Bash | Large files consume more space |
| Hook output | When a hook returns additionalContext | Return only information Claude needs |
| Subagent results | When a subagent finishes | Main session receives the final summary; intermediate reads stay separate |




The most common waste is not a long prompt, but unrelated files, lengthy logs, and large tables read during one task.




## What /context shows

`/context` visualizes usage and identifies heavy sections.

| Observation | Action |
| --- | --- |
| File reads dominate | Narrow the task; locate before reading |
| Tool output dominates | Shorten output, or save it and read selected parts |
| Memory/rules are large | Split into path-scoped rules or skills |
| History is long | `/compact` at a natural break; `/clear` for a new task |
| Too many MCP tools | Use supported tool search, split servers, or disable unused tools |

`/context all` shows more detail when diagnosing a rapidly filling window.

## /clear, /compact, and /rewind

| Command | Action | Cache effect | Use |
| --- | --- | --- | --- |
| `/clear [name]` | Clear the conversation; old session remains resumable | New session reloads project context | An unrelated task |
| `/compact [instructions]` | Replace history with a summary | Summary can read old cache; next turn builds a shorter history cache | Continue a long task |
| `/rewind` | Return to an earlier checkpoint | Usually returns to a previously cached prefix | Undo a wrong direction |
| `/context` | Inspect usage only | No cache change | Decide whether to reorganize context |

`/compact` is not a universal cost-saving button: it adds a summarization request. Use it at natural breaks, not constantly. To undo a wrong direction, see [Checkpointing](https://docs.passion8.cc/en/docs/claude-code/checkpointing).

## What survives compaction

| Content | Result |
| --- | --- |
| System prompt and tools | Retained |
| Project CLAUDE.md and auto memory | Reloaded |
| Previously triggered path rules | Depend on the summary and subsequent file access |
| Important conclusions from invoked skills | Depend on summary quality |
| Full text of uninvoked skills | Not automatically retained |
| Intermediate subagent work | Not part of main context; final results may be summarized |
| Actual filesystem changes | Remain on disk independently of context |

If information must be available later, write it to a file, issue, or plan, or explicitly ask `/compact` to retain it.

## Practical context control





### Keep permanent rules short


Put stable conventions in `CLAUDE.md`, not long API references, logs, or TODO lists. Keep long material in skills or project documentation and read it when needed.




### Locate before reading


Use `rg`, the file tree, or symbol search to narrow the scope before reading key files. Avoid reading all of `src` first in a large repository.




### Delegate exploration


Use [subagents](https://docs.passion8.cc/en/docs/claude-code/subagents) to isolate large research reads. The main session receives conclusions instead of every file's contents.




### Save long output first


Write lengthy test logs or analysis reports to a file, then summarize or search the relevant sections.







## Relationship to caching

Context capacity and prompt caching are different but related:

| Action | Context effect | Cache effect |
| --- | --- | --- |
| Ordinary follow-up | Longer history | Existing prefix usually reusable |
| Read a large file | Noticeable increase | Appended content; old prefix can hit |
| `/compact` | Shorter history | Conversation layer becomes a summary |
| `/clear` | Clears current history | New session creates new cache entries |
| Change model or effort | Content may stay similar | Cache identity can change and invalidate reuse |

See [Prompt caching](https://docs.passion8.cc/en/docs/claude-code/prompt-caching) for TTL and hit checks, and [Subagents](https://docs.passion8.cc/en/docs/claude-code/subagents) for context boundaries.

Large repositories also need careful startup directories, layered instructions, Read deny rules, and sparse worktrees. See [Large codebases and monorepos](https://docs.passion8.cc/en/docs/claude-code/large-codebases).

## Official references

- [Explore the context window](https://code.claude.com/docs/en/context-window.md)
- [How Claude Code uses prompt caching](https://code.claude.com/docs/en/prompt-caching.md)
- [Manage sessions](https://code.claude.com/docs/en/sessions.md)