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What happens when the Claude context window fills up?

Anonymous10/8/2026· Views 21

When I have a long conversation with Claude, sometimes it feels like it forgot what we talked about earlier. I can't tell if that's because the context window is full or if I'm just imagining it. If the context overflows, does the oldest content get cut off, or is it automatically summarized? Also, is there a way to check how full it is right now? I'm curious about when the right time is to switch to a new chat too.

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첫째StaffLv.64d ago

Hello, this is the admin team. To cut to the chase, when the context fills up, it doesn't necessarily just quietly cut off the oldest content. The behavior depends on the environment you're using. 1. claude.ai (web and app): According to official guidance, when code execution is turned on, the conversation gets summarized as it nears the context limit, and Claude continues from there by summarizing earlier messages. The full conversation history is still kept, and it's stated that earlier content can still be referenced. However, if this feature is turned off, or if the first message is very large, you may hit the length limit. 2. Claude Code: When the context fills up, auto-compact kicks in. You can check current usage by category with the /context command, compact it manually with /compact, or clear it with /clear. If you go over the limit, you'll get a "Prompt is too long" error. 3. API: There's a server-side compaction (summarization) feature, but it's currently labeled as beta and requires separate setup. In Claude Code, you can check how full it is with /context. I haven't confirmed whether the claude.ai web app shows a remaining-capacity indicator, so I'm not sure about that. The feeling that it has forgotten earlier content may come from details being lost during summarization, or from the influence of early instructions weakening as the conversation gets longer. It's hard to pin down the cause with certainty. Good times to start a new conversation: 1. When the topic has completely changed 2. When Claude keeps missing earlier decisions even after summarization has already happened 3. When usage drops quickly in a long conversation (long conversations are said to use up more of your usage) When moving to a new chat, have Claude summarize the decisions made so far and paste that summary into the first message of the new chat. I've written up a detailed guide on keeping context in long conversations here: https://usingclaude.com/ko/guides/features/claude-long-conversation-context