ai-anthropic: modelOptions.mcp_servers is forwarded without the mcp-client beta header

TanStack AI version

@tanstack/ai 0.56.0, @tanstack/ai-anthropic 0.18.8 (current main, bffdd186)

Framework/Library version

Any (server-side, framework-independent). @anthropic-ai/sdk 0.112.4.

Describe the bug and the steps to reproduce it

modelOptions.mcp_servers is forwarded to the Messages API body, but the required beta header is never sent. This is the same shape as #1074 (context_management forwarded without context-management-2025-06-27), which #1080 fixed.

• mcp_servers is in the validKeys allowlist in packages/ai-anthropic/src/adapters/text.ts, so it reaches the request body.
• computeAnthropicBetas in the same file only looks at thinking, context_management, and the code-execution / skills tools. It never adds mcp-client-2025-11-20.
• The SDK does not inject it either: client.beta.messages.create sends exactly the betas it is given (resources/beta/messages/messages.js, 'anthropic-beta': betas?.toString()).
• The existing test forwards container, mcp_servers, service_tier, ... to the beta client asserts the body but not payload.betas, so nothing catches it.

Anthropic's MCP connector docs require the header: https://platform.claude.com/docs/en/agents-and-tools/mcp-connector (anthropic-beta: mcp-client-2025-11-20).

Steps to reproduce (no API key needed):

1. chat({ adapter: anthropicText('claude-sonnet-4-5'), messages, modelOptions: { mcp_servers: [{ type: 'url', name: 'x', url: 'https://mcp.example.com' }] } })
2. Inspect the betaMessagesCreate payload (or the outgoing request): mcp_servers is present, betas is undefined.

Your Minimal, Reproducible Example - (Sandbox Highly Recommended)

Reproduces in the package's own test suite — add expect(payload.betas).toContain('mcp-client-2025-11-20') to the mcp_servers test in packages/ai-anthropic/tests/anthropic-adapter.test.ts and it fails on main.

Screenshots or Videos (Optional)

No response

Do you intend to try to help solve this bug with your own PR?

Yes — PR incoming: add the beta in computeAnthropicBetas when mcp_servers is a non-empty array, assert payload.betas in the test, patch changeset.

Terms & Code of Conduct

• I agree to follow this project's Code of Conduct
• I understand that if my bug cannot be reliable reproduced in a debuggable environment, it will probably not be fixed and this issue may even be closed.