Act with model option fails

[gal-checksum]

Reopenning #1263 as it was closed wrongly.

See repro below. Act with instructions and custom model fails as well

/**
 * Bug: stagehand.act() fails when passing model override parameter
 *
 * Expected: act() should work with model override, like observe() does
 * Actual: act() throws "An unexpected error occurred"
 */

const { Stagehand } = require("@browserbasehq/stagehand");

async function reproduceActBug() {
  const stagehand = new Stagehand({
    env: "BROWSERBASE",
    apiKey: process.env.BROWSERBASE_API_KEY,
    projectId: process.env.BROWSERBASE_PROJECT_ID,
    verbose: 1,
    model: {
      modelName: "google/gemini-2.5-pro",
      apiKey: process.env.GOOGLE_API_KEY,
    },
  });

  await stagehand.init();
  const page = stagehand.context.pages()[0];
  await page.goto("https://example.com");

  // ✅ This works - observe() with model override
  console.log("Testing observe() with model override...");
  const observations = await stagehand.observe("Find the first link", {
    model: { modelName: "google/gemini-2.5-pro" },
  });
  console.log("✅ observe() SUCCESS -", observations.length, "elements found");

  // ❌ This fails - act() with model override
  console.log("\nTesting act() with model override...");
  try {
    await stagehand.act("Click on the first link", {
      model: { modelName: "anthropic/claude-sonnet-4-5" },
    });
    console.log("✅ act() SUCCESS");
  } catch (error) {
    console.log("❌ act() FAILED:", error.message);
  }

  await stagehand.close();
}

reproduceActBug().catch(console.error);