I'm finding this question incomplete without proper usage example of how things should be set up.

Since op did not mention target language I'm feeling like to share Kotlin setup that works in my case:

build.gradle.kts

implementation("org.springframework.boot:spring-boot-starter-webflux")
implementation("com.playtika.reactivefeign:feign-reactor-core:3.2.1")
implementation("com.playtika.reactivefeign:feign-reactor-spring-configuration:3.2.1")
implementation("com.playtika.reactivefeign:feign-reactor-webclient:3.2.1")

Config.kt

@Configuration
@EnableWebFlux
@EnableReactiveFeignClients
class Config {
}

MyFeignClient.kt

@Component
@ReactiveFeignClient(
        url = "\${package.service.my-service-url}",
        name = "client"
)
interface MyFeignClient {
    @GetMapping(value = ["/my/url?my_param={my_value}"], consumes = ["application/json"])
    fun getValues(
            @PathVariable(name = "my_value") myValue: String?,
        ): Mono<MyEntity?>?
}

Then here goes code in some service class:

val myClient: MyFeignClient = WebReactiveFeign.builder<MyFeignClient>()
        .contract(ReactiveContract(SpringMvcContract()))
        .target(MyFeignClient::class.java, "http://example.com")
// feel free to add .block() to get unpacked value or just chain your logic further
val response = myClient.getValues(param)

Feign includes simple Reactive streams support that allows for the use of Reactive return types. This library wraps the method execution in a Reactive wrapper. It is not "reactive all the way down", the method execution is still blocking, but it can be integrated into an existing Reactive execution chain.

To use the provided library, include it as a dependency:

<dependency>
   <groupId>io.github.openfeign</groupId>
   <artifactId>feign-reactive-wrappers</artifactId>
   <version>10.4.0</version>
</dependency>

And use the appropriate builder for the reactive streams implementation you want, Reactor and RxJava2+ are supported.

public class ExampleReactor {
  public static void main(String args[]) {
    GitHubReactor gitHub = ReactorFeign.builder()      
      .target(GitHubReactor.class, "https://api.github.com");

    List<Contributor> contributors = gitHub.contributors("OpenFeign", "feign")
      .collect(Collectors.toList())
      .block();
  }
}

You can use this library to get you started. Our roadmap includes full reactive support in the near future.