vercel/errors/now-static-build-failed-to-detect-a-server.md

@vercel/static-build Failed to detect a server running

Why This Warning Occurred

When running vercel dev, the @vercel/static-build builder proxies relevant HTTP requests to the server that is created by the dev script in the package.json file.

In order for vercel dev to know which port the server is running on, the builder is provided a $PORT environment variable that the server must bind to. The error "Failed to detect a server running on port" is printed if the builder fails to detect a server listening on that specific port within five minutes.

Possible Ways to Fix It

Please ensure that your dev script binds the spawned development server on the provided $PORT that the builder expects the server to bind to.

For example, if you are using Gatsby, your dev script must use the -p (port) option to bind to the $PORT specified from the builder:

In Windows environments, reference the PORT environment variable with %PORT%

{
  ...
  "scripts": {
    ...
    "dev": "gatsby develop -p $PORT"
  }
}

Consult your static builder program's --help or documentation to figure out what the command line flag to bind to a specific port is (in many cases, it is one of: -p / -P / --port).