diff --git a/extensions/google-java/build.gradle.kts b/extensions/google-java/build.gradle.kts
index 1219bcb8..fa9e1bab 100644
--- a/extensions/google-java/build.gradle.kts
+++ b/extensions/google-java/build.gradle.kts
@@ -8,6 +8,10 @@ plugins {
     id("com.vanniktech.maven.publish.base")
 }
 
+kotlin {
+    compilerOptions.allWarningsAsErrors.set(true)
+}
+
 sourceSets {
     main {
         java {
diff --git a/extensions/google-javalite/build.gradle.kts b/extensions/google-javalite/build.gradle.kts
index 3695ac37..1347b4b6 100644
--- a/extensions/google-javalite/build.gradle.kts
+++ b/extensions/google-javalite/build.gradle.kts
@@ -8,6 +8,10 @@ plugins {
     id("com.vanniktech.maven.publish.base")
 }
 
+kotlin {
+    compilerOptions.allWarningsAsErrors.set(true)
+}
+
 dependencies {
     testImplementation(libs.assertj)
     testImplementation(libs.junit)
diff --git a/library/build.gradle.kts b/library/build.gradle.kts
index 17adb3be..20c2680c 100644
--- a/library/build.gradle.kts
+++ b/library/build.gradle.kts
@@ -8,6 +8,10 @@ plugins {
     alias(libs.plugins.ksp)
 }
 
+kotlin {
+    compilerOptions.allWarningsAsErrors.set(true)
+}
+
 dependencies {
     testImplementation(libs.assertj)
     testImplementation(libs.junit)
diff --git a/okhttp/build.gradle.kts b/okhttp/build.gradle.kts
index a365e6f5..4b96f1fc 100644
--- a/okhttp/build.gradle.kts
+++ b/okhttp/build.gradle.kts
@@ -7,6 +7,10 @@ plugins {
     id("com.vanniktech.maven.publish.base")
 }
 
+kotlin {
+    compilerOptions.allWarningsAsErrors.set(true)
+}
+
 dependencies {
     api(libs.okhttp.core)
     implementation(libs.kotlin.coroutines.core)
diff --git a/protoc-gen-connect-kotlin/build.gradle.kts b/protoc-gen-connect-kotlin/build.gradle.kts
index 41cc9269..a6398322 100644
--- a/protoc-gen-connect-kotlin/build.gradle.kts
+++ b/protoc-gen-connect-kotlin/build.gradle.kts
@@ -14,6 +14,10 @@ application {
     mainClass.set("com.connectrpc.protocgen.connect.Main")
 }
 
+kotlin {
+    compilerOptions.allWarningsAsErrors.set(true)
+}
+
 tasks {
     jar {
         manifest {
diff --git a/protoc-gen-connect-kotlin/src/main/kotlin/com/connectrpc/protocgen/connect/internal/ProtoHelpers.kt b/protoc-gen-connect-kotlin/src/main/kotlin/com/connectrpc/protocgen/connect/internal/ProtoHelpers.kt
index 16168fae..b1ae1504 100644
--- a/protoc-gen-connect-kotlin/src/main/kotlin/com/connectrpc/protocgen/connect/internal/ProtoHelpers.kt
+++ b/protoc-gen-connect-kotlin/src/main/kotlin/com/connectrpc/protocgen/connect/internal/ProtoHelpers.kt
@@ -76,14 +76,14 @@ internal fun parseGeneratorParameter(
  * `java_multiple_files` options specified in the .proto file.
  */
 internal fun getProtocJavaFileName(descriptor: Descriptors.Descriptor): String {
-    var descriptor = descriptor
     val fullName: String
     if (descriptor.file.options.javaMultipleFiles) {
         var containingType: Descriptors.Descriptor
-        while (descriptor.containingType.also { containingType = it } != null) {
-            descriptor = containingType
+        var currentDescriptor = descriptor
+        while (currentDescriptor.containingType.also { containingType = it } != null) {
+            currentDescriptor = containingType
         }
-        fullName = getClassName(descriptor)
+        fullName = getClassName(currentDescriptor)
     } else {
         fullName = getClassNameForFile(descriptor.file)
     }
@@ -214,14 +214,14 @@ internal fun getFileJavaPackage(file: FileDescriptor): String {
  * `java_multiple_files` options specified in the .proto file.
  */
 fun getJavaFileName(descriptor: Descriptors.Descriptor): String {
-    var descriptor: Descriptors.Descriptor = descriptor
     val fullName: String
     if (descriptor.file.options.javaMultipleFiles) {
+        var currentDescriptor = descriptor
         var containingType: Descriptors.Descriptor?
-        while (descriptor.containingType.also { containingType = it } != null) {
-            descriptor = containingType!!
+        while (currentDescriptor.containingType.also { containingType = it } != null) {
+            currentDescriptor = containingType!!
         }
-        fullName = getClassName(descriptor)
+        fullName = getClassName(currentDescriptor)
     } else {
         fullName = getClassNameForFile(descriptor.file)
     }
@@ -317,44 +317,44 @@ private fun getFieldName(field: Descriptors.FieldDescriptor): String {
  * letter should be upper-case or lower-case.
  *
  * @param input         string to be converted
- * @param capNextLetter `true` if the first letter should be turned to
+ * @param capFirstLetter `true` if the first letter should be turned to
  * upper-case.
  * @return the camel-cased string
  */
 private fun underscoresToCamelCaseImpl(
     input: String,
-    capNextLetter: Boolean,
-): String {
-    var capNextLetter = capNextLetter
-    val result = StringBuilder(input.length)
-    var i = 0
-    val l = input.length
-    while (i < l) {
-        val c = input[i]
-        capNextLetter = if ('a' <= c && c <= 'z') {
-            if (capNextLetter) {
-                result.append((c.code + ('A'.code - 'a'.code)).toChar())
-            } else {
-                result.append(c)
+    capFirstLetter: Boolean,
+): String = buildString(input.length) {
+    var capNextLetter = capFirstLetter
+    for ((i, c) in input.withIndex()) {
+        capNextLetter = when (c) {
+            in 'a'..'z' -> {
+                if (capNextLetter) {
+                    append(c.uppercaseChar())
+                } else {
+                    append(c)
+                }
+                false
             }
-            false
-        } else if ('A' <= c && c <= 'Z') {
-            if (i == 0 && !capNextLetter) {
-                // Force first letter to lower-case unless explicitly told
-                // to capitalize it.
-                result.append((c.code + ('a'.code - 'A'.code)).toChar())
-            } else {
-                // Capital letters after the first are left as-is.
-                result.append(c)
+
+            in 'A'..'Z' -> {
+                if (i == 0 && !capNextLetter) {
+                    // Force first letter to lower-case unless explicitly told
+                    // to capitalize it.
+                    append(c.lowercaseChar())
+                } else {
+                    // Capital letters after the first are left as-is.
+                    append(c)
+                }
+                false
             }
-            false
-        } else if ('0' <= c && c <= '9') {
-            result.append(c)
-            true
-        } else {
-            true
+
+            in '0'..'9' -> {
+                append(c)
+                true
+            }
+
+            else -> true
         }
-        i++
     }
-    return result.toString()
 }
diff --git a/protoc-gen-connect-kotlin/src/test/kotlin/PluginGenerationTest.kt b/protoc-gen-connect-kotlin/src/test/kotlin/PluginGenerationTest.kt
index 832dcd89..f1699b17 100644
--- a/protoc-gen-connect-kotlin/src/test/kotlin/PluginGenerationTest.kt
+++ b/protoc-gen-connect-kotlin/src/test/kotlin/PluginGenerationTest.kt
@@ -111,6 +111,6 @@ class PluginGenerationTest {
 
     @Test
     fun evilCommentsCompiles() {
-        val client = EvilCommentsServiceClient(mock { })
+        EvilCommentsServiceClient(mock { })
     }
 }