msdemos VS Finch.io

You're not confined to the usual suggestions below (play, http4s). There's a ton of options. (I wrote test cases using a bunch of different frameworks a few years ago at https://github.com/hohonuuli/msdemos). Having written services using a variety of frameworks in production, I would strongly suggest using one that auto-generates API docs (openapi, swagger) for you. That will save you a huge amount of time later on. For heavier services, like the one at https://fathomnet.org/, I tend to the Java side (Quarkus is my current top choice, but Micronaut and Helidon are both great). For everything else I use Scala. My go-to right now is tapir using a vertx backend. See https://tapir.softwaremill.com/

I've put together simple examples (in Scala 3), that all do exactly the same thing, using various frameworks and benchmarked them. The source code is at https://github.com/hohonuuli/msdemos. For the record, I usually use Scalatra for Scala projects, Helidon or Micronaut for Java projects. If you're new and looking for something super simple, try out Cask or SparkJava.

I've written sample apps in Scala 3 that all do the same thing at https://github.com/hohonuuli/msdemos using akka-http (some issues at the moment), cask, finatra (which doesn't work with Scala 3), helidon, http4s, javalin, scalatra, sparkjava, vertx, and zio-http. I wrote those as an exercise in understanding particular frameworks and benchmarking them. (I write a lot of microservices). You can peruse the code there to get a feel for what it takes to write a service in a particular framework

Finagle, possibly with Finch on top.