This error means the MCP server tried to start a Browserbase session and had nothing to connect to. If you want to run fully local without API keys, pass a local CDP URL so the MCP drives your own browser instead of a Browserbase session - that ability was added in PR 43 of the repo. Otherwise set your Browserbase API key in the MCP env config and let it create a cloud session. Do not mix the two: a local Stagehand path plus no API key is the combination that produces the proxy error.

Context: Issue browserbase/mcp-server-browserbase#34 (closed, 7 comments): running the Browserbase MCP server fully local on MacOS without API keys failed with Failed to initialize Stagehand - Cannot create proxy with a non-object as target or handler. The reporter was pointing the MCP at a local Stagehand build with no Browserbase session behind it.