How to get the modify index of a key in consul after setting it?

Question

When I set a key in consul, e.g.:

curl -w "\n" -v --request PUT --data value http://127.0.0.1:8500/v1/kv/my-key?cas=1234

How can I get the index for the value I just set? I was surprised to see the response doesn't contain the header X-Consul-Index (why not?).

If I issue another request, e.g.:

curl -w "\n" -v http://127.0.0.1:8500/v1/kv/my-key

I will get the index for the current value but there are two problems with that:

1. It's a race condition. The key could have been changed in the meantime and I'll get a different index.
2. It's wasteful, the response should have that info.

What's the correct way of setting a key/value and knowing the index?