diff --git a/sonar-kotlin-api/src/main/java/org/sonarsource/kotlin/api/checks/ApiExtensions.kt b/sonar-kotlin-api/src/main/java/org/sonarsource/kotlin/api/checks/ApiExtensions.kt
index 69ab86129..d881497d8 100644
--- a/sonar-kotlin-api/src/main/java/org/sonarsource/kotlin/api/checks/ApiExtensions.kt
+++ b/sonar-kotlin-api/src/main/java/org/sonarsource/kotlin/api/checks/ApiExtensions.kt
@@ -403,12 +403,26 @@ private fun KtTypeReference.determineType(bindingContext: BindingContext) =
 fun KtTypeReference.determineTypeAsString(bindingContext: BindingContext, printTypeArguments: Boolean = false) =
     determineType(bindingContext)?.getKotlinTypeFqName(printTypeArguments)
 
+@Deprecated("use kotlin-analysis-api instead", ReplaceWith("this.returnTypeAsString()"))
 fun KtNamedFunction.returnTypeAsString(bindingContext: BindingContext, printTypeArguments: Boolean = false) =
     returnType(bindingContext)?.getKotlinTypeFqName(printTypeArguments)
 
+@Deprecated("use kotlin-analysis-api", ReplaceWith("this.returnType"))
 fun KtNamedFunction.returnType(bindingContext: BindingContext) =
     createTypeBindingForReturnType(bindingContext)?.type
 
+/**
+ * In the following example for the platform type `java.lang.String!`
+ * ```
+ * fun flexibleAkaPlatformType() = java.lang.String.valueOf(1)
+ * ```
+ * this function returns its lower bound `"java.lang.String"`.
+ * @see asFqNameString
+ */
+fun KtNamedFunction.returnTypeAsString(): String? = withKaSession {
+    returnType.asFqNameString()
+}
+
 fun KtLambdaArgument.isSuspending(
     bindingContext: BindingContext,
 ) = (parent as? KtCallExpression)
diff --git a/sonar-kotlin-checks/src/main/java/org/sonarsource/kotlin/checks/FlowChannelReturningFunsNotSuspendingCheck.kt b/sonar-kotlin-checks/src/main/java/org/sonarsource/kotlin/checks/FlowChannelReturningFunsNotSuspendingCheck.kt
index 6ecfaa1be..0ba76d486 100644
--- a/sonar-kotlin-checks/src/main/java/org/sonarsource/kotlin/checks/FlowChannelReturningFunsNotSuspendingCheck.kt
+++ b/sonar-kotlin-checks/src/main/java/org/sonarsource/kotlin/checks/FlowChannelReturningFunsNotSuspendingCheck.kt
@@ -30,13 +30,12 @@ import org.sonarsource.kotlin.api.frontend.KotlinFileContext
 private val FORBIDDEN_RETURN_TYPES = listOf(COROUTINES_FLOW, COROUTINES_CHANNEL)
 private const val MESSAGE = """Functions returning "Flow" or "Channel" should not be suspending"""
 
-@org.sonarsource.kotlin.api.frontend.K1only
 @Rule(key = "S6309")
 class FlowChannelReturningFunsNotSuspendingCheck : AbstractCheck() {
 
     override fun visitNamedFunction(function: KtNamedFunction, kotlinFileContext: KotlinFileContext) {
         function.suspendModifier()?.let { suspendModifier ->
-            if (function.returnTypeAsString(kotlinFileContext.bindingContext) in FORBIDDEN_RETURN_TYPES) {
+            if (function.returnTypeAsString() in FORBIDDEN_RETURN_TYPES) {
                 val secondaries = function.typeReference
                     ?.let { listOf(SecondaryLocation(kotlinFileContext.textRange(it))) }
                     ?: emptyList()