I've re-deployed the app from ToT once more, but still see no Backend option here: https://test-results.appspot.com/admin/settings/tsmon.

The gclient solution was fresh and luci-go is at 0e72694a59aac5340e5b5a571bb3f7d3ddce67a3 in the DEPS file.

Perhaps this is because this is a mixed python-go app?

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:-/ 

/admin/settings/tsmon seems to be served by the correct module: https://screenshot.googleplex.com/oMcVcP6FxTy.png

It's the one you've deployed recently: https://screenshot.googleplex.com/JHRS9Jx1cHg.png

So it looks like a stale code gets deployed somehow.

What's your GOPATH env var in a console you use to run deployment script? My current theory is that you have some stale copy of luci-go in GOPATH. Do you use eval `go/env.py` to setup GOROOT and GOPATH (https://chromium.googlesource.com/infra/infra/+/master/go/README.md#Bootstrap)?

Here's my GOPATH:
$ echo $GOPATH
/Users/vadimsh/infra-git/infra/go/.vendor:/Users/vadimsh/infra-git/infra/go

(/Users/vadimsh/infra-git/ is the gclient solution root)

Please also check that infra/go/src/github.com/luci/luci-go is indeed checked out at  0e72694a59aac5340e5b5a571bb3f7d3ddce67a. Maybe gclient didn't update it.

(I could probably try to deploy new version myself, but your next update will overwrite it back with stale version of luci-go deps, so we need to figure out why it happens on your machine).

Turned out I had a local branch in my luci-go checkout and gclient-sync doesn't change branches. New version was deployed and admin page now had Backend parameter. Vadim said he'll do the rest of the configuration changes.