diff --git a/build.gradle.kts b/build.gradle.kts
index 9514641ebabb..2eefaa3344c4 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -196,6 +196,17 @@ subprojects {
     }
   }
 
+  tasks.withType<KotlinCompile>().configureEach {
+    if (name.contains("test")) return@configureEach // Skip test source sets.
+  
+    compilerOptions {
+      // Pin language level to 2.0 to ensure compatibility with older Gradle versions and other libraries that depend on okhttp.
+      // https://docs.gradle.org/current/userguide/compatibility.html#kotlin
+      languageVersion = org.jetbrains.kotlin.gradle.dsl.KotlinVersion.KOTLIN_2_0
+      apiVersion = org.jetbrains.kotlin.gradle.dsl.KotlinVersion.KOTLIN_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 91980d116b2f..aa234fe098dd 100644
--- a/okhttp/api/android/okhttp.api
+++ b/okhttp/api/android/okhttp.api
@@ -1253,7 +1253,7 @@ 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$Companion {
diff --git a/okhttp/api/jvm/okhttp.api b/okhttp/api/jvm/okhttp.api
index 493b859afb9a..c6666babd7c0 100644
--- a/okhttp/api/jvm/okhttp.api
+++ b/okhttp/api/jvm/okhttp.api
@@ -1252,7 +1252,7 @@ public abstract interface class okhttp3/TrailersSource {
 	public static final field Companion Lokhttp3/TrailersSource$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 {