diff --git a/build.gradle.kts b/build.gradle.kts
index aa96ba569379..5b765a73b6e8 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -195,6 +195,19 @@ subprojects {
     }
   }
 
+  // Configure all projects' source sets (jvm and kmp)
+  plugins.matching { it.javaClass.name.startsWith("org.jetbrains.kotlin") }.configureEach {
+    kotlinExtension.sourceSets {
+      // Exclude all test source sets (containing Test)
+      matching { !it.name.contains("Test", ignoreCase = true) }.all {
+        languageSettings {
+          apiVersion = "2.0"
+          languageVersion = "2.0"
+        }
+      }
+    }
+  }
+
   val platform = System.getProperty("okhttp.platform", "jdk9")
   val testJavaVersion = System.getProperty("test.java.version", "21").toInt()
 
diff --git a/okhttp/api/android/okhttp.api b/okhttp/api/android/okhttp.api
index 1f0b9839ac73..80bd7c8a110a 100644
--- a/okhttp/api/android/okhttp.api
+++ b/okhttp/api/android/okhttp.api
@@ -1241,7 +1241,7 @@ public abstract interface class okhttp3/TrailersSource {
 	public static final field Companion Lokhttp3/TrailersSource$Companion;
 	public static final field EMPTY Lokhttp3/TrailersSource;
 	public abstract fun get ()Lokhttp3/Headers;
-	public fun peek ()Lokhttp3/Headers;
+	public abstract fun peek ()Lokhttp3/Headers;
 }
 
 public final class okhttp3/TrailersSource$Companion {
diff --git a/okhttp/api/jvm/okhttp.api b/okhttp/api/jvm/okhttp.api
index ca4df1afdcfa..ff226222cb99 100644
--- a/okhttp/api/jvm/okhttp.api
+++ b/okhttp/api/jvm/okhttp.api
@@ -1240,7 +1240,7 @@ public abstract interface class okhttp3/TrailersSource {
 	public static final field Companion Lokhttp3/TrailersSource$Companion;
 	public static final field EMPTY Lokhttp3/TrailersSource;
 	public abstract fun get ()Lokhttp3/Headers;
-	public fun peek ()Lokhttp3/Headers;
+	public abstract fun peek ()Lokhttp3/Headers;
 }
 
 public final class okhttp3/TrailersSource$Companion {