I have 700,000 latitude/longitude pairs I need to convert to US Census tracts. Is there a free API that offers this in batches? So far the only option I have found is from the FCC and does not state a rate limit but has the form of a 1-1 call to return.
Have you tried the Census geocoder? https://www.census.gov/geo/maps-data/data/geocoder.html
The limit is 1,000 per batch, but that should be more doable for your project.