Server startup and connection failures

Stdio process exits during initialization

Run the configured command directly from the configured working directory. Check that its stdout contains only MCP protocol traffic; write diagnostics to stderr. Confirm every argument is a separate StdioServer.args value.

Resolve server and manifest paths from Path(__file__).resolve(), because the subprocess may run from another working directory. -m package.module is also stable.

HTTP connection fails

Start the endpoint separately and verify the exact Streamable HTTP URL. A URL to the service root may not be the MCP route. Add endpoint credentials through HTTPServer.headers.

Nonlocal HTTP trust error

Classify a public or trusted private endpoint explicitly. A direct HTTPServer(name=..., url=...) defaults to TrustLevel.UNTRUSTED, so kit.direct(...) raises EndpointTrustError ("untrusted MCP endpoint resolved to a private or local address") for a loopback or private URL. Pass trust=TrustLevel.TRUSTED_PRIVATE, as in Test a Streamable HTTP server.

Omitting trust is accepted only for servers selected through servers=[{"type": "http", ...}] in the m3 marker or through m3 test --server http --url URL without --trust, and only when the URL host is localhost or a loopback IP address. The connection is then treated as TRUSTED_PRIVATE and must resolve exclusively to loopback. Supplying trust in either form skips this inference.

A nonlocal HTTP agent server defaults to untrusted, which cannot be exposed to an agent. Use TrustLevel.PUBLIC or TRUSTED_PRIVATE. A direct public endpoint may keep the default.