The context window

Claude's context window holds your conversation history, file contents, command outputs, CLAUDE.md, auto memory, loaded skills, and system instructions. As you work, it fills up.

When context fills up, Claude Code manages it automatically: it clears older tool outputs first, then summarizes the conversation if needed. Your requests and key code snippets are preserved, but detailed instructions from early in the conversation may be lost — put persistent rules in CLAUDE.md rather than relying on chat history.

terminal

# see what is using space
/context

# compact now, steering what to keep
/compact focus on the API changes

# check per-server MCP context cost
/mcp

To control what survives compaction, add a "Compact Instructions" section to CLAUDE.md or run /compact with a focus. MCP tool definitions are deferred by default — only tool names consume context until Claude uses a specific tool.

Thrashing

If a single file or tool output is so large that context refills immediately after each summary, auto-compaction stops after a few attempts and shows an error instead of looping.

Hands-on in your own terminal

This lesson ran in a simulated terminal. Work through it hands-on in your own terminal instead — the commands are safe to run in any repository.

• See what is filling the window — /context
• Summarise the conversation — /compact
• Check what MCP tool names cost you — /mcp

Commands used in this lesson: /context, /compact, /mcp

Check your understanding

Question 1What does Claude Code clear first when context fills?

AYour CLAUDE.mdBOlder tool outputsCThe system promptDAuto memory

Question 2Where should persistent rules live?

AEarly in the conversationBIn CLAUDE.mdCIn a pinned messageDIn shell history

Further reading

• Explore the context window
• Costs