act() fails with Anthropic models: response wrapped in $PARAMETER_NAME key

[caug-rodriguez]

Description

When using an Anthropic model (e.g., anthropic/claude-sonnet-4-5) as the default model for act(), the response is wrapped in a $PARAMETER_NAME key, causing Zod validation to fail with AI_NoObjectGeneratedError.

Expected behavior

act() should return the response at the top level:

{"elementId": "11-811", "description": "Create Invoice link button", "method": "click", "arguments": [], "twoStep": false}

Actual behavior

Claude wraps the response in a $PARAMETER_NAME key:

{"$PARAMETER_NAME": {"elementId": "11-811", "description": "Create Invoice link button", "method": "click", "arguments": [], "twoStep": false}}

This causes the Zod schema validation to fail because all required fields (elementId, description, method, arguments, twoStep) are undefined at the top level.

Reproduction

const stagehand = new Stagehand({
  env: "BROWSERBASE",
  model: "anthropic/claude-sonnet-4-5",
  modelClientOptions: { apiKey: process.env.ANTHROPIC_API_KEY },
});
await stagehand.init();
// Navigate to any page
await stagehand.act("Click the submit button", { page });
// → throws AI_NoObjectGeneratedError

Environment

• Stagehand version: 3.2.1
• AI SDK version: 5.x
• Anthropic model: claude-sonnet-4-5-20250929
• The issue does NOT occur with OpenAI models (gpt-4o, gpt-4.1-mini)

Root cause

Claude's tool_use response wraps the output in the tool's parameter name. The AI SDK or Stagehand's response parsing doesn't unwrap this wrapper when using Anthropic models.

From the raw response:

{
  "type": "tool_use",
  "name": "json",
  "input": {
    "$PARAMETER_NAME": {
      "elementId": "11-811",
      ...
    }
  }
}