Support 1M context window for Claude Opus 4.7 (parity with Claude Code)

[dezgit2025]

Describe the feature or problem you'd like to solve

Claude Code offers Opus 4.7 with 1M context as its default (recommended) model. The /model selector in Claude Code explicitly shows:

Opus 4.7 with 1M context · Most capable for complex work

GitHub Copilot CLI (v1.0.31) now has Opus 4.7 available (thanks for adding it in 1.0.29!), but it does not appear to support the 1M token context window. The context window seems capped at a smaller size (likely 192K–200K).

This creates a feature gap between two tools using the same underlying model — Claude Code gets the full 1M context while Copilot CLI does not.

Proposed solution

Add claude-opus-4.7-1m (or enable 1M context for the existing Opus 4.7 option) in the /model selector, similar to how claude-opus-4.6-1m was previously available (ref: #1395).

Ideally the 1M variant would be the default for Opus 4.7, matching Claude Code's behavior.

Example prompts or workflows

Large codebase refactoring, multi-file analysis, and long-session agentic workflows benefit significantly from the 1M context window. Without it, Opus 4.7 on Copilot CLI hits compaction much earlier than the same model on Claude Code.

Additional context

• Claude Code model selector screenshot showing "Opus 4.7 with 1M context" as default
• Copilot CLI v1.0.31 — Opus 4.7 is available but without 1M context
• Related: Support 1M token context with Opus 4.6 #1395 (Opus 4.6 1M — was added then removed from /model GUI)
• Related: ACP session.create rejects claude-opus-4.7 despite interactive path supporting it #2782 (Opus 4.7 ACP allowlist mismatch)