The mu-plugin header describes the split as 'the adapter authorises the transport, the callback authorises the operation'. The second half held, every ability carries infinito_mcp_may_read. The first did not: the adapter answered the initialize handshake to anyone, so the server and its protocol version were readable without a credential.
Measured in run 34653754175, the swarm WordPress#0: test-mcp-guest.js posts an anonymous initialize and expects at least 400, and the error context reads 'Expected: >= 400, Received: 200'. Its neighbour addons/mcp-adapter.spec.js posts an anonymous tools/list and passed, so the tool call was already refused and only the handshake was not.
The filter hangs on rest_pre_dispatch and refuses every route under the mcp namespace while the site cannot attribute the request. It leaves an already short-circuited response alone rather than overwriting it, scopes itself by route prefix so the rest of the REST API stays as open as it was, and reaches is_user_logged_in, which triggers determine_current_user and with it wp_validate_application_password, so a client presenting its application password passes. A WP_Error renders no jsonrpc key, which is the second thing the spec asserts.
The new unit test reads the guarded prefix out of the PHP and the endpoint out of the spec and requires the endpoint to sit under the prefix. Without it an adapter that moves namespace would leave the guard protecting nothing while both negative specs keep passing.
A positive spec was tried and reverted: handing the MCP credential to the Playwright env is forbidden by tests/lint/ansible/services/test_mcp_secret_not_in_playwright.py, because the trace travels in the CI artifact. No role probes its MCP endpoint with the credential at deploy time either, so the authenticated direction stays unproven.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
kevinveenbirkenbachandClaude Opus 5