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HomeDocsClaude Developer Platform 7 Oct 2026
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context-windows changedbuild-with-claude/context-windows

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3333 
3434## Context window sizes by model
3535 
36Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5.5, Claude Sonnet 5, Claude Sonnet 4.6, and [Claude Mythos Preview](https://anthropic.com/glasswing) have a 1M-token context window. A single request to any of them can generate up to 128k output tokens (`max_tokens`). Other Claude models, including Claude Sonnet 4.5, have a 200k-token context window.
36Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5.5, Claude Sonnet 5, Claude Sonnet 4.6, and [Claude Mythos Preview](https://anthropic.com/glasswing) have a 1M-token context window. A single request to any of them can generate up to 128k output tokens (`max_tokens`). Other Claude models, including Claude Sonnet 4.5 (deprecated), have a 200k-token context window.
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3838For every model with a 1M-token context window, 1M is the default: you don't need a beta header, and long-context requests are billed at [standard pricing](https://platform.claude.com/docs/en/about-claude/pricing#long-context-pricing).
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9292 * The API uses cryptographic signatures to verify thinking block authenticity. If you modify a thinking block, the API returns an error.
9393 
9494<Note>
95 Most current Claude models support [interleaved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#interleaved-thinking), which lets Claude think between tool calls, including after it receives tool results. It is automatic on models with adaptive thinking; Claude Opus 4.5, Claude Sonnet 4.5, and earlier Claude 4 models require the `interleaved-thinking-2025-05-14` beta header, and Claude Haiku 4.5 does not support it.
95 Most current Claude models support [interleaved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#interleaved-thinking), which lets Claude think between tool calls, including after it receives tool results. It is automatic on models with adaptive thinking; Claude Opus 4.5, Claude Sonnet 4.5 (deprecated), and earlier Claude 4 models require the `interleaved-thinking-2025-05-14` beta header, and Claude Haiku 4.5 does not support it.
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9797 For more information about using tools with thinking, see [Thinking with tool use](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-with-tool-use).
9898</Note>
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101101 
102102## Context awareness
103103 
104Claude Sonnet 5, Claude Sonnet 4.6, Claude Sonnet 4.5, and Claude Haiku 4.5 have **context awareness:** these models track their remaining context window (their "token budget") throughout a conversation. This lets the model manage long-running tasks against the space that remains rather than guess how many tokens are left. Context awareness is automatic: there is nothing for you to enable, and you never send the tags shown in this section yourself. The API injects them.
104Claude Sonnet 5, Claude Sonnet 4.6, Claude Sonnet 4.5 (deprecated), and Claude Haiku 4.5 have **context awareness:** these models track their remaining context window (their "token budget") throughout a conversation. This lets the model manage long-running tasks against the space that remains rather than guess how many tokens are left. Context awareness is automatic: there is nothing for you to enable, and you never send the tags shown in this section yourself. The API injects them.
105105 
106106### How it works
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111111<budget:token_budget>200000</budget:token_budget>
112112```
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114The budget matches the context window available to your request: 1M tokens for Claude Sonnet 5 and Claude Sonnet 4.6, and 200k tokens for Claude Sonnet 4.5 and Claude Haiku 4.5. The examples in this section show a model with a 200k-token context window.
114The budget matches the context window available to your request: 1M tokens for Claude Sonnet 5 and Claude Sonnet 4.6, and 200k tokens for Claude Sonnet 4.5 (deprecated) and Claude Haiku 4.5. The examples in this section show a model with a 200k-token context window.
115115 
116116After each tool call, the API gives Claude an update on its remaining capacity:
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