Troubleshooting
HMRC authentication failure on CT600
Five usual causes — wrong credentials, CT not enrolled, expired session, mismatched UTR, or environment mix-up.
Reconnect the client’s HMRC authorisation from Settings → HMRC. Confirm you are on the test environment when testing and live only with live credentials.
Enrol the company for Corporation Tax Online via Government Gateway if it has never filed electronically. Activation codes can take several days to arrive.
Never paste test client IDs into a live Hydra deployment — the platform blocks that bleed by design.