In the MCS page summary API spec, 204 responses are expected in a couple of situations:
- The page requested is outside the main namespace;
- The page has a content model other than 'wikitext'.
However, it appears that HyperSwitch is converting empty responses (for non-HEAD requests) into 500 errors. This is surprising to clients and probably shouldn't happen for 204s, which should not have any body content.
(See T182399#3828497 for context.)