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Magnitude can be customized by passing in options when you start a browser agent: 
await startBrowserAgent({
    // Starting URL for agent
    url: "https://google.com", 
    // Show thoughts and actions
    narrate: true,
    // LLM configuration
    llm: {
        provider: 'anthropic',
        options: {
            model: 'claude-sonnet-4-20250514',
            apiKey: process.env.ANTHROPIC_API_KEY
        }
    },
    // Any system instructions specific to your agent or website
    prompt: 'Prefer mouse to keyboard when filling out form fields'
});
Only some LLMs are compatible with Magnitude - see compatible LLMs for details.
For information on configuring the test runner instead see Configure Test Runner

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Browser Options

Various browser options can also be passed, such as browser launch options or context options: 
const agent = await startBrowserAgent({
    url: "https://google.com", 
    browser: {
        // Configured launched browser:
        launchOptions: {
            // chromium launch options, for example enabling CDP
            args: ["--remote-debugging-port=9222"]
        },
        contextOptions: {
            // see https://playwright.dev/docs/api/class-browser#browser-new-context
            // for comprehensive list of options
            viewport: {
                width: 1280,
                height: 720
            }
        }
    }
});
See Playwright’s docs on Launch Options and Browser Context for more details on what can be configured. You can also connect via CDP to an open CDP-enabled browser: 
const agent = await startBrowserAgent({
    url: "https://google.com", 
    browser: {
        cdp: "http://localhost:9222"
    }
});