Troubleshooting mcpreflight
Most mcpreflight failures trace to a short list of causes: the endpoint sits behind auth (expected, reported as UNKNOWN), an inputSchema comes back empty after zod transforms, required properties are missing from the declared schema, a response exceeds client size limits, or the server closes the stream right after initialize. In the October 2026 registry scan, 37 servers returned a tools/list that fails JSON Schema validation outright and 23 returned an invalid initialize response. This page maps each symptom to its fix. Numbers: State of MCP Servers.
See the full troubleshooting guide for NO_402, NO_PAYMENT_DOCUMENT, NO_ACCEPTS, REQUIRED_PROPERTY_UNDEFINED, and UNKNOWN.