I’m using an implementation of TMinuit for the Fermi-LAT experiment (example analysis thread)
Is there a reference for the return codes from TMinuit? The ones listed here don’t correspond to what I get (the reference only shows return codes 1-12, but often I get something like 102 or 156).
That depends on the command mnexcm invokes. In general, the only authoritative source of information - apart from the code is the Minuit reference manual that’s linked from the ROOT documentation you linked: https://root.cern.ch/sites/d35c7d8c.web.cern.ch/files/minuit.pdf