[Bug] Anthropic provider sends invalid API parameters (thinking.type: adaptive, output_config.effort) when variant is set #17876

Description

When using the native Anthropic provider with claude-opus-4-6 and a "max" variant (configured via oh-my-opencode plugin), OpenCode's bundled @ai-sdk/anthropic provider generates invalid request parameters, causing the Anthropic API to return HTTP 400 invalid_request_error.

The request body sent to https://api.anthropic.com/v1/messages includes:

{
  "model": "claude-opus-4-6",
  "max_tokens": 32000,
  "thinking": {"type": "adaptive"},
  "output_config": {"effort": "max"}
}

• "thinking": {"type": "adaptive"} — not a documented Anthropic API parameter (should be {"type": "enabled", "budget_tokens": N})
• "output_config": {"effort": "max"} — not part of the Anthropic Messages API spec

The API response:

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "Error"
  }
}

This was working on 2026-03-16 and broke on 2026-03-17 with no changes to OpenCode version, config, or plugins. This suggests either a server-side Anthropic API validation change, or an issue in how OpenCode's @ai-sdk/anthropic bundle maps variant options to Anthropic API parameters.

Plugins

oh-my-opencode (sets "variant": "max" on agent configs), [email protected]

OpenCode Version

1.2.27

Steps to Reproduce

1. Configure an agent with "model": "anthropic/claude-opus-4-6" and "variant": "max" (via oh-my-opencode or directly)
2. Start a session and send any message
3. OpenCode attempts to call https://api.anthropic.com/v1/messages with the invalid parameters
4. API returns 400 invalid_request_error

Operating System

macOS (Darwin 25.3.0)

Additional Context

From the logs:

service=provider providerID=anthropic pkg=@ai-sdk/anthropic using bundled provider

ERROR service=session.processor error=Error stack="AI_APICallError: Error\n    at <anonymous> (/$bunfs/root/src/index.js:101905:30)"

Key observation: Custom provider models defined in opencode.json with explicit variant mappings (e.g., Antigravity proxy models using thinkingConfig.thinkingBudget) work correctly. The issue is specifically with the built-in Anthropic provider interpreting the variant and producing non-standard API parameters.

Workaround: Removing the "variant": "max" from the model config allows claude-opus-4-6 to work without extended thinking.