feat(provider/anthropic): add support for claude-opus-4-8

[felixarntz]

Background

Anthropic released claude-opus-4-8, available on the first-party API as well as on Bedrock, Vertex, and the AI Gateway. Opus 4.8 is the first Claude model to support mid-conversation system messages.

Summary

• Add claude-opus-4-8 (and Bedrock / Vertex / Gateway variants) to provider model ID unions.
• Wire claude-opus-4-8 into getModelCapabilities with the same profile as claude-opus-4-7 (128k output, adaptive thinking, xhigh effort, structured output, sampling parameters rejected).
• Drop the UnsupportedFunctionalityError for "Multiple system messages that are separated by user/assistant messages", since the API will reject unsupported messages anyway. A second system block is now emitted inline in messages as { role: 'system', content: [{ type: 'text', text }] }, and the converter adds the mid-conversation-system-2026-04-07 beta header automatically. The first system block still flows into the top-level system field — so existing prompts hit a byte-identical request.
• Refresh docs and a handful of examples (anthropic xhigh-effort / task-budget, bedrock anthropic-reasoning) to use the new model, and add a new mid-conversation-system example.

Manual Verification

Run the relevant Opus 4.8 examples modified, including the new ones for mid-conversation system message.

Checklist

• All commits are signed (PRs with unsigned commits cannot be merged)
• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

[github-actions]

🚀 Published in:

Package	Version
ai	7.0.0-canary.157 github npm
@ai-sdk/amazon-bedrock	5.0.0-canary.73 github npm
@ai-sdk/angular	3.0.0-canary.157 github npm
@ai-sdk/anthropic	4.0.0-canary.61 github npm
@ai-sdk/anthropic-aws	1.0.0-canary.2 github npm
@ai-sdk/gateway	4.0.0-canary.93 github npm
@ai-sdk/google-vertex	5.0.0-canary.97 github npm
@ai-sdk/langchain	3.0.0-canary.157 github npm
@ai-sdk/llamaindex	3.0.0-canary.157 github npm
@ai-sdk/otel	1.0.0-canary.103 github npm
@ai-sdk/react	4.0.0-canary.159 github npm
@ai-sdk/rsc	3.0.0-canary.158 github npm
@ai-sdk/svelte	5.0.0-canary.157 github npm
@ai-sdk/vue	4.0.0-canary.157 github npm
@ai-sdk/workflow	1.0.0-canary.74 github npm