App Builder passes local.envVars, which @cursor/sdk ignores

Description

When App Builder creates the session agent it tries to inject BROWSER=none into the local agent:

https://github.com/cursor/cookbook/blob/main/sdk/app-builder/src/lib/app-builder/server.ts#L1002-L1011

session.agent = await Agent.create({
  apiKey: session.apiKey,
  model: { id: process.env.CURSOR_MODEL ?? "composer-2" },
  local: {
    cwd: session.projectPath,
    envVars: {
      BROWSER: "none",
    },
  },
})

envVars is a CloudAgentOptions field. LocalAgentOptions has cwd, dirs, settingSources, sandboxOptions, customTools, store, and enableAgentRetries — not envVars. The extra property is dropped at the type boundary and never reaches the local executor.

The preview host process already sets BROWSER: "none" when it starts Vite (server.ts around the runCommand spawn). That does not cover shells the agent starts later (npm run dev, vite, a browser opener). Those child processes still see a normal BROWSER and can pop a system browser on top of the in-app iframe.

Expected

Local session agents run with BROWSER=none (or an equivalent local-supported setting) so preview stays inside the App Builder iframe.

Suggested fix

Pick one:

• Pass the var through a supported local channel if the SDK adds one.
• Set BROWSER=none in the workspace .env / generated project scripts the agent actually runs.
• Stop passing a no-op local.envVars so the example does not teach an invalid option.

Start in sdk/app-builder/src/lib/app-builder/server.ts around lines 1002–1011 and compare the create call with the SDK's LocalAgentOptions. Trace how the preview host starts Vite and how later agent-launched commands receive environment variables. Done means choosing and validating a supported way to keep local agent previews from opening a system browser, or removing the invalid option if no such channel exists.