n8n 'No testing function found for this credential' Fix

What actually changed You built a custom n8n node with its own credential type. The node works in a workflow. You open the credential in the n8n UI, click Test, and instead of a green checkmark you get: No testing function found for this credential. You double-check the node — credentialTest is defined in methods, testedBy is set on the credential declaration, everything compiles. n8n just refuses to see it. This is one of the most-reported custom-node issues in n8n's history — the original report is Stack Overflow q/75109822 and the underlying bug is tracked in n8n-io/n8n#8188, with users still reproducing it on 1.58+ and 1.94 well after the original fix landed. The fix The root cause is not a missing function. It is in LoadNodesAndCredentials.ts: n8n generates nodesToTestWith in dist/known/credentials.json but only reads supportedNodes when linking a credential to its test function. For custom and community nodes the two keys never match, so the li