Context Window

[ryukenshin546-a11y]

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Why does the Claude Opus4.6 token context window only have 128K input and 64K output, when the model can handle up to 1M?

[ryukenshin546-a11y]

[aryankumar06]

Claude Opus 4.6 “supporting 1M context” is basically the model’s maximum architectural capability, but in the actual product Anthropic sets practical limits for speed, cost, and reliability. At 1M tokens the compute and latency blow up massively, and even if the model can technically read that much, its effective recall and consistency can become less stable. That’s why they cap it at 128K input and 64K output to keep performance predictable, inference fast enough, and pricing manageable.

[anuragchauhan06]

In cursor you have 200k token context window and you're absolutely right.

[SeaDude]

I believe the 128k limit is imposed by GitHub Copilot's agent harness. Just like Claude Code, Cline, Cursor, etc., under the hood, GitHub Copilot is a "coding assistant agent" with a "harness"; a combination of programmatic and non-deterministic loops that handle input, context, and output.

If you want to work with 4.6's 1M context window right now, you'll have to select an agentic tool whose harness can handle 1M OR create your own custom harness.

[MuhammedSinanHQ]

Claude Opus 4.6 can support up to ~1M tokens at the model level.

Copilot Chat does not expose the model’s maximum context.

Copilot applies its own caps for:

• Cost control
• Latency
• Reliability
• Tool orchestration
• Multi-tenant fairness

So you see:

128K input / 64K output
even though the underlying model supports more.

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Important distinction

Model capability ≠ Product limit.

Vendors frequently gate large contexts behind:

• Enterprise tiers
• Private preview
• Custom contracts

Copilot currently uses a constrained configuration of Claude models.

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Why Copilot does this

• Long-context inference is expensive
• Tool calls + embeddings scale with context
• UI responsiveness degrades past ~100K tokens

GitHub optimizes for interactive developer workflows, not massive document ingestion.

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If you truly need >128K

Use Claude directly through Anthropic’s API or a provider that exposes 1M context.

Copilot is not the right surface today.

[nickchomey]

Claude Opus and Sonnet 4.6 both just changed 1M context window from beta to GA. Copilot should make use of it as well

https://claude.com/blog/1m-context-ga