Spring AI @McpTool annotated suspending functions are ignored in ASYNC server mode

Bug description

Similar to https://github.com/spring-projects/spring-ai/issues/4392. Only @McpTool functions that return Mono, etc, are used in ASYNC mode. This should also include Kotlin suspend functions (as well as ideally Flow returning functions).

I've categorized it as a bug, but if Kotlin isn't officially supported (i.e. https://github.com/spring-projects/spring-ai/issues/3718) it may fit better as a feature request.

Environment Java 24, Kotlin 2.2.10, Spring AI 1.1.0-M2

Steps to reproduce Have a @McpTool method that returns Mono that works with an async server. Change it to not return Mono but be suspend.

Expected behavior The function is still detected.

Minimal Complete Reproducible example

@SpringBootApplication
class Application {
    companion object {
        @JvmStatic
        fun main(args: Array<String>) {
            runApplication<Application>(*args)
        }
    }

    @McpTool
    fun testTool(): Mono<String> {
        return Mono.just("test")
    }

    @McpTool
    suspend fun testTool2(): String {
        return "test"
    }
}

testTool is registered, testTool2 is not. Config:

spring:
  ai:
    mcp:
      server:
        type: ASYNC
        annotation-scanner:
          enabled: true
        capabilities:
          tool: true
          resource: true
          prompt: true
          completion: true
        stdio: false

Comment From: jmirc

I had the same issue.

🔍 What's happening in the code?

The line you're pointing to is part of a stream pipeline that processes a list of tool objects to extract method specifications annotated with @McpTool. Here's a simplified breakdown:

List<AsyncToolSpecification> toolSpecs = this.toolObjects.stream()
            .map(toolObject -> Stream.of(this.doGetClassMethods(toolObject))
                .filter(method -> method.isAnnotationPresent(McpTool.class))
                .filter(McpProviderUtils.isReactiveReturnType)
                                 ....