Using your examples from the API docs I can't create a new discussion, or add an existing comment to an existing discussion. I can do everything else that I've tried.

When I try to add a new discussion I get this error:

[["body","can't be blank"]]

and when I try to add a new comment to an existing discussion I get this error:

[["body","can't be blank"],["user_email","is missing"]]

I am providing the body in both cases, and in both cases I'm providing an author_email and I've tried changing that to user_email for adding a comment, and get the same error. Below (minus my API keys) are the curl attempts I've provided:

# creating new
curl -H "Accept: application/vnd.tender-v1+json" -H "X-Tender-Auth: ...snip..." -d "{'author_email':'[email blocked]','title':'Test, please ignore','body':'Yet another test','skip_spam':true}" "https://api.tenderapp.com/amco/categories/23402/discussions"

# adding to existing
curl -H "Accept: application/vnd.tender-v1+json" -H "X-Tender-Auth: ...snip..." -d "{'author_email':'[email blocked]','body':'Yet another test','skip_spam':true}" "https://api.tenderapp.com/amco/discussions/4654033/comments"
curl -H "Accept: application/vnd.tender-v1+json" -H "X-Tender-Auth: ...snip..." -d "{'user_email':'[email blocked]','body':'Yet another test','skip_spam':true}" "https://api.tenderapp.com/amco/discussions/4654033/comments"

I also tried switching to username/password auth, because that's what your example used. Same result. Thwarts my every effort.