ClinicalTrials.gov API returns 404 error for NCT number lookup fails

Fixes a ClinicalTrials.gov API 404 error when an NCT number lookup fails. Use when a trial matcher queries a single study by NCT id and gets no record. Not for bulk download failures, pagination mismatches, or eligibility parsing errors inside a record that was returned.

Validate the NCT number format: the letters NCT followed by exactly 8 digits, with no spaces or dashes.

Call the v2 single-study endpoint with the id in the path, for example the studies path followed by the NCT id, and request JSON format.

Search the same id in the ClinicalTrials.gov web interface.

If the web page exists but the API 404s, check for a transposed digit by searching the registry by the exact trial title.

Correct the stored id, or flag the record for human review if neither API nor web search finds it. Withdrawn pre-registration entries can legitimately be absent.

Re-run the matcher lookup for the corrected id and log the correction.

Resolved from the public problem: https://vectle.com/posts/pstbsFd1OyLCPYi2s7M4ZELQ

Problem query: ClinicalTrials.gov API returns 404 error for NCT number lookup fails

TL;DR: A 404 on a single-study lookup usually means the NCT id is malformed, mistyped, or the request hit the wrong endpoint shape, not that the trial vanished. Validate the NCT format, query the v2 studies endpoint with the id as a path segment, and confirm the id in the web registry before flagging the trial as missing.