feat(zen): enable 1M context window for Opus 4.6 and Sonnet 4.5

[ruslan-kurchenko]

Summary

Extends the Anthropic 1M context window beta to Claude Opus 4.6 and Claude Sonnet 4.5 models. Previously only Sonnet 4 variants could leverage the extended context — this unlocks it for the newest high-capability models across both direct API and Bedrock paths.

Problem & Solution

Problem: The context-1m-2025-08-07 beta header was only sent for models matching claude-sonnet-*, so Opus 4.6 and Sonnet 4.5 users hit the default context limit despite the models supporting 1M tokens.

Solution: Replace the inline prefix check with a dedicated supports1MContext() predicate that matches all eligible model families, applied consistently to both API paths.

Key Changes

• Introduce supports1MContext() — a single source of truth for which models get the 1M context beta
• Apply the beta flag consistently across direct Anthropic API and AWS Bedrock request paths
• Cover Opus 4.6, Sonnet 4.5, and Sonnet 4 model families (with snapshot date suffixes)

System Design

Beta Header Injection Flow

sequenceDiagram
    participant Client
    participant Handler as handler.ts
    participant Select as selectProvider()
    participant Helper as anthropicHelper
    participant API as Anthropic API / Bedrock

    Client->>Handler: POST /zen (model: claude-opus-4-6)
    Handler->>Select: selectProvider(model)
    Select-->>Handler: providerInfo (anthropicHelper)

    rect rgb(40, 40, 40)
        note right of Handler: Request modification
        Handler->>Helper: modifyHeaders(headers, body, apiKey)
        Helper->>Helper: supports1MContext(body.model)?
        alt Model supported
            Helper-->>Handler: set anthropic-beta: context-1m-2025-08-07
        else Model not supported
            Helper-->>Handler: no beta header
        end
        Handler->>Helper: modifyBody(body)
        note right of Helper: Bedrock: injects anthropic_beta field
    end

    Handler->>API: fetch(url, { headers, body })
    API-->>Client: Response (up to 1M context)

Loading

The change affects the decision point inside modifyHeaders and modifyBody — replacing a hardcoded claude-sonnet- prefix check with supports1MContext() that covers three model prefixes.

Testing Strategy

The supports1MContext() helper is tested against all model families: positive cases for Opus 4.6, Sonnet 4.5, and Sonnet 4 (with and without snapshot suffixes), negative cases for Opus 4.5, Opus 4.1, Haiku, unknown models, and empty strings.

Reviewer Notes

• The prefix list ["claude-opus-4-6", "claude-sonnet-4-5", "claude-sonnet-4-"] intentionally uses a trailing hyphen on claude-sonnet-4- to match Sonnet 4 snapshots (e.g., claude-sonnet-4-20250514) without matching Sonnet 4.5 twice — Sonnet 4.5 is caught by its own explicit prefix first
• Bedrock path previously used isSonnet (a loose includes("sonnet") check) — now uses the same supports1MContext() for consistency

[github-actions]

Thanks for your contribution!

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[ruslan-kurchenko]

@codex review this

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Match canonical Sonnet 4 model for 1M context

The new prefix list excludes the documented claude-sonnet-4 model ID because it only matches claude-sonnet-4-... (with a trailing hyphen). When users request claude-sonnet-4, supports1MContext() returns false, so anthropic-beta: context-1m-2025-08-07 is no longer sent on either the direct Anthropic path or the Bedrock anthropic_beta path, which regresses that model back to the default context limit.

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[ruslan-kurchenko]

Resolved

[ruslan-kurchenko]

Closing in favor of anomalyco#12342 which supersedes this PR.

This PR only patched the Zen console proxy layer, while anomalyco#12342 provides a complete solution including core runtime beta headers, adaptive thinking for Opus 4.6, compaction overflow fix, and AI SDK v5→v6 upgrade.