diff --git a/build.sbt b/build.sbt
index c05b69832a..a50ecea99e 100644
--- a/build.sbt
+++ b/build.sbt
@@ -15,7 +15,7 @@ import scala.concurrent.duration.DurationInt
 import scala.sys.process.Process
 
 val scala2_12 = "2.12.21"
-val scala2_13 = "2.13.18"
+val scala2_13 = "3.9.0"
 val scala3 = "3.3.8"
 val scala3_7 = "3.7.4"
 
diff --git a/core/src/main/scala-2/sttp/tapir/internal/CaseClassUtil.scala b/core/src/main/scala-2/sttp/tapir/internal/CaseClassUtil.scala
index d6bf2547aa..e11cd6d60d 100644
--- a/core/src/main/scala-2/sttp/tapir/internal/CaseClassUtil.scala
+++ b/core/src/main/scala-2/sttp/tapir/internal/CaseClassUtil.scala
@@ -65,7 +65,7 @@ private[tapir] class CaseClassUtil[C <: blackbox.Context, T: C#WeakTypeTag](val
           case List(Select(_, name @ TermName(_))) if name.decodedName.toString.startsWith("<init>$default") =>
             None
           case List(Literal(Constant(str: String))) =>
-            // before Scala 2.13.18, the above test for a default value worked; now we need this additional condition
+            // before Scala 3.9.0, the above test for a default value worked; now we need this additional condition
             if (str == "") None else Some(str)
           case _ => throw new IllegalStateException(s"Cannot extract annotation argument from: ${c.universe.showRaw(a.tree)}")
         }
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/caching/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/caching/build.sbt
index 8cab20d4cf..2ca87d1198 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/caching/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/caching/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiGenerateEndpointTypes := true
   )
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/minimal/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/minimal/build.sbt
index 8cab20d4cf..2ca87d1198 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/minimal/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/minimal/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiGenerateEndpointTypes := true
   )
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/multi_file/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/multi_file/build.sbt
index 96753a4a03..e1b330bf8c 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/multi_file/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/multi_file/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiGenerateEndpointTypes := true
   )
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/nifi_test/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/nifi_test/build.sbt
index c62781ac8e..8cf7217a25 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/nifi_test/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/nifi_test/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiStreamingImplementation := "pekko",
     openapiGenerateEndpointTypes := true,
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip-zio/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip-zio/build.sbt
index 02d3ecea1f..487b813e5e 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip-zio/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip-zio/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiJsonSerdeLib := "zio",
     openapiAddDisambiguationCodes := true
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip/build.sbt
index 3bb9db4bf7..afe264a8d7 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiStreamingImplementation := "pekko",
     openapiGenerateEndpointTypes := true,
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip_jsoniter/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip_jsoniter/build.sbt
index f6ecda90bb..65632fb3e9 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip_jsoniter/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/oneOf-json-roundtrip_jsoniter/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiJsonSerdeLib := "jsoniter",
     openapiStreamingImplementation := "pekko",
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/option-overrides/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/option-overrides/build.sbt
index a31312fc23..5e0e9f6d6c 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/option-overrides/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/option-overrides/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiPackage := "com.example.generated.apis",
     openapiObject := "MyExampleEndpoints",
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/petstore/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/petstore/build.sbt
index 4a2ebc9e2a..9c78ab921b 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/petstore/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/petstore/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiStreamingImplementation := "pekko",
     openapiGenerateEndpointTypes := true,
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/schema_inheritance/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/schema_inheritance/build.sbt
index 03164540ac..25f9e0fa29 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/schema_inheritance/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/schema_inheritance/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiGenerateEndpointTypes := true,
     openapiSwaggerFile := baseDirectory.value / "specs" / "core.yaml",
diff --git a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/seperate_files_for_models/build.sbt b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/seperate_files_for_models/build.sbt
index 6226e77c02..de7ad3d038 100644
--- a/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/seperate_files_for_models/build.sbt
+++ b/openapi-codegen/sbt-plugin/src/sbt-test/sbt-openapi-codegen/seperate_files_for_models/build.sbt
@@ -1,7 +1,7 @@
 lazy val root = (project in file("."))
   .enablePlugins(OpenapiCodegenPlugin)
   .settings(
-    scalaVersion := "2.13.18",
+    scalaVersion := "3.9.0",
     version := "0.1",
     openapiSeperateFilesForModels := true,
     openapiAdditionalPackages := List(