Unit tests can pass while your webhook receives a different input.

In a self-initiated n8n 2.46.6 demo, the direct function rejected JSON null with OBJECT_REQUIRED. The loopback HTTP route converted null to {} before the Code node, so it rejected the request with UNSUPPORTED_SCHEMA instead.

Both paths rejected the input, but the transport changed what the mapping code received.

I tested the same 49 synthetic fixtures at three layers: direct JavaScript, native n8n execution, and loopback HTTP. The documented HTTP expectation accounts for that null-body conversion. Two additional transport checks passed too.

The lesson: test the request path as well as the function, and document intentional differences instead of hiding them behind one green total.

AI-assisted demonstration, not client work. The webhook has no authentication or signature verification and must stay isolated.

Case study, test counts and limits: https://contra.com/p/sXees3lU-n8n-webhook-mapping-guard