diff --git a/.idea/inspectionProfiles/Project_Default.xml b/.idea/inspectionProfiles/Project_Default.xml
index 9ece71d5..aeb663b6 100644
--- a/.idea/inspectionProfiles/Project_Default.xml
+++ b/.idea/inspectionProfiles/Project_Default.xml
@@ -94,6 +94,7 @@
       <option name="processLiterals" value="true" />
       <option name="processComments" value="true" />
     </inspection_tool>
+    <inspection_tool class="StringToUpperWithoutLocale" enabled="true" level="WARNING" enabled_by_default="true" editorAttributes="WARNING_ATTRIBUTES" />
     <inspection_tool class="dd497f47-d38f-3fab-9ed7-eabe699620c8" enabled="true" level="ERROR" enabled_by_default="true" editorAttributes="ERRORS_ATTRIBUTES" />
   </profile>
 </component>
\ No newline at end of file
diff --git a/build-logic/src/main/kotlin/pklJSpecify.gradle.kts b/build-logic/src/main/kotlin/pklJSpecify.gradle.kts
index e50ca0e0..a41115ce 100644
--- a/build-logic/src/main/kotlin/pklJSpecify.gradle.kts
+++ b/build-logic/src/main/kotlin/pklJSpecify.gradle.kts
@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+import net.ltgt.gradle.errorprone.CheckSeverity
 import net.ltgt.gradle.errorprone.errorprone
 import net.ltgt.gradle.nullaway.nullaway
 import org.gradle.accessors.dm.LibrariesForLibs
@@ -36,6 +37,7 @@ nullaway { onlyNullMarked = true }
 
 tasks.withType<JavaCompile>().configureEach {
   options.errorprone.disableAllChecks = true
+  options.errorprone.check("StringCaseLocaleUsage", CheckSeverity.ERROR)
   options.errorprone.nullaway {
     error()
     onlyNullMarked = true
diff --git a/pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java b/pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java
index 96affffa..ff28d098 100644
--- a/pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java
+++ b/pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java
@@ -27,6 +27,7 @@ import java.net.URISyntaxException;
 import java.util.ArrayList;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Objects;
 import java.util.Set;
 import java.util.function.BiFunction;
@@ -2541,7 +2542,7 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
       int modifier = visitModifier(ctx);
       if ((modifier & validModifiers) == 0) {
         throw exceptionBuilder()
-            .evalError(errorMessage, ctx.getValue().name().toLowerCase())
+            .evalError(errorMessage, ctx.getValue().name().toLowerCase(Locale.ROOT))
             .withSourceSection(createSourceSection(ctx))
             .build();
       }
diff --git a/pkl-core/src/main/java/org/pkl/core/evaluatorSettings/PklEvaluatorSettings.java b/pkl-core/src/main/java/org/pkl/core/evaluatorSettings/PklEvaluatorSettings.java
index 8d94344d..255f9284 100644
--- a/pkl-core/src/main/java/org/pkl/core/evaluatorSettings/PklEvaluatorSettings.java
+++ b/pkl-core/src/main/java/org/pkl/core/evaluatorSettings/PklEvaluatorSettings.java
@@ -22,6 +22,7 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Objects;
@@ -115,7 +116,7 @@ public record PklEvaluatorSettings(
         (Map<String, String>) pSettings.get("env"),
         allowedModules,
         allowedResources,
-        color == null ? null : Color.valueOf(color.toUpperCase()),
+        color == null ? null : Color.valueOf(color.toUpperCase(Locale.ROOT)),
         (Boolean) pSettings.get("noCache"),
         moduleCacheDir,
         modulePath,
@@ -124,7 +125,7 @@ public record PklEvaluatorSettings(
         Http.parse((Value) pSettings.get("http")),
         externalModuleReaders,
         externalResourceReaders,
-        traceMode == null ? null : TraceMode.valueOf(traceMode.toUpperCase()));
+        traceMode == null ? null : TraceMode.valueOf(traceMode.toUpperCase(Locale.ROOT)));
   }
 
   public record Http(
diff --git a/pkl-core/src/main/java/org/pkl/core/runtime/CommandSpecParser.java b/pkl-core/src/main/java/org/pkl/core/runtime/CommandSpecParser.java
index a1ae4ea9..400a4e1a 100644
--- a/pkl-core/src/main/java/org/pkl/core/runtime/CommandSpecParser.java
+++ b/pkl-core/src/main/java/org/pkl/core/runtime/CommandSpecParser.java
@@ -27,6 +27,7 @@ import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.function.BiFunction;
@@ -737,7 +738,7 @@ public final class CommandSpecParser {
         if (each == null)
           each =
               (rawValue, workingDirUri) -> {
-                var value = rawValue.toLowerCase();
+                var value = rawValue.toLowerCase(Locale.ROOT);
                 if (TRUE_VALUES.contains(value)) {
                   return true;
                 } else if (FALSE_VALUES.contains(value)) {
diff --git a/pkl-core/src/main/java/org/pkl/core/util/IoUtils.java b/pkl-core/src/main/java/org/pkl/core/util/IoUtils.java
index 63811ccf..9ef9a44f 100644
--- a/pkl-core/src/main/java/org/pkl/core/util/IoUtils.java
+++ b/pkl-core/src/main/java/org/pkl/core/util/IoUtils.java
@@ -889,7 +889,7 @@ public final class IoUtils {
   }
 
   private static boolean isReservedHeaderName(String headerName) {
-    var normalizedHeader = headerName.toLowerCase();
+    var normalizedHeader = headerName.toLowerCase(Locale.ROOT);
     for (var reservedHeader : reservedHeaderNames) {
       if (normalizedHeader.equals(reservedHeader)) {
         return true;
@@ -899,7 +899,7 @@ public final class IoUtils {
   }
 
   private static boolean hasReservedHeaderPrefix(String headerName) {
-    var normalizedHeader = headerName.toLowerCase();
+    var normalizedHeader = headerName.toLowerCase(Locale.ROOT);
     for (var prefix : reservedHeaderPrefixes) {
       if (normalizedHeader.startsWith(prefix)) {
         return true;
diff --git a/pkl-gradle/src/externalReader/java/org/pkl/gradle/test/extreader/Main.java b/pkl-gradle/src/externalReader/java/org/pkl/gradle/test/extreader/Main.java
index aba2a5ea..9dad4447 100644
--- a/pkl-gradle/src/externalReader/java/org/pkl/gradle/test/extreader/Main.java
+++ b/pkl-gradle/src/externalReader/java/org/pkl/gradle/test/extreader/Main.java
@@ -17,6 +17,7 @@ package org.pkl.gradle.test.extreader;
 
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
+import java.util.Locale;
 import org.msgpack.core.MessagePack;
 import org.msgpack.value.Value;
 import org.msgpack.value.ValueFactory;
@@ -77,7 +78,7 @@ public class Main {
 
           var colonIndex = uri.indexOf(':');
           var schemeSpecific = colonIndex >= 0 ? uri.substring(colonIndex + 1) : uri;
-          var contents = schemeSpecific.toUpperCase().getBytes(StandardCharsets.UTF_8);
+          var contents = schemeSpecific.toUpperCase(Locale.ROOT).getBytes(StandardCharsets.UTF_8);
 
           packer.packArrayHeader(2);
           packer.packInt(READ_RESOURCE_RESPONSE);
diff --git a/pkl-gradle/src/main/java/org/pkl/gradle/PklPlugin.java b/pkl-gradle/src/main/java/org/pkl/gradle/PklPlugin.java
index fcc76faf..a65cacbb 100644
--- a/pkl-gradle/src/main/java/org/pkl/gradle/PklPlugin.java
+++ b/pkl-gradle/src/main/java/org/pkl/gradle/PklPlugin.java
@@ -18,6 +18,7 @@ package org.pkl.gradle;
 import java.io.File;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Optional;
 import java.util.Set;
 import java.util.function.Function;
@@ -453,7 +454,7 @@ public class PklPlugin implements Plugin<Project> {
                         + spec.getName()
                         + "'. Either apply a JVM plugin (e.g. 'java') or set the sourceSet property explicitly.");
               }
-              if (sourceSet.getName().toLowerCase().contains("test")) {
+              if (sourceSet.getName().toLowerCase(Locale.ROOT).contains("test")) {
                 module.getTestSources().from(append(module.getTestSources().getFiles(), outputDir));
               } else {
                 module.setSourceDirs(append(module.getSourceDirs(), outputDir));
diff --git a/pkl-gradle/src/main/java/org/pkl/gradle/utils/PluginUtils.java b/pkl-gradle/src/main/java/org/pkl/gradle/utils/PluginUtils.java
index f9cb1914..43a4a75b 100644
--- a/pkl-gradle/src/main/java/org/pkl/gradle/utils/PluginUtils.java
+++ b/pkl-gradle/src/main/java/org/pkl/gradle/utils/PluginUtils.java
@@ -25,6 +25,7 @@ import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 import java.util.function.Function;
 import org.gradle.api.InvalidUserDataException;
 import org.gradle.api.file.FileSystemLocation;
@@ -183,7 +184,7 @@ public final class PluginUtils {
       return TestReporter.getDefault();
     }
     try {
-      return TestReporter.valueOf(inputStr.toUpperCase());
+      return TestReporter.valueOf(inputStr.toUpperCase(Locale.ROOT));
     } catch (IllegalArgumentException e) {
       var sb = new StringBuilder("Invalid test reporter: '");
       sb.append(inputStr).append("'. ");
@@ -197,7 +198,7 @@ public final class PluginUtils {
         } else {
           sb.append(", ");
         }
-        sb.append('\'').append(value.toString().toLowerCase()).append('\'');
+        sb.append('\'').append(value.toString().toLowerCase(Locale.ROOT)).append('\'');
       }
       sb.append(".");
       throw new InvalidUserDataException(sb.toString());
diff --git a/pkl-parser/src/main/java/org/pkl/parser/Token.java b/pkl-parser/src/main/java/org/pkl/parser/Token.java
index 32bbbf05..4a013447 100644
--- a/pkl-parser/src/main/java/org/pkl/parser/Token.java
+++ b/pkl-parser/src/main/java/org/pkl/parser/Token.java
@@ -15,6 +15,8 @@
  */
 package org.pkl.parser;
 
+import java.util.Locale;
+
 public enum Token {
   ABSTRACT,
   AMENDS,
@@ -233,6 +235,6 @@ public enum Token {
     if (this == UNDERSCORE) {
       return "_";
     }
-    return name().toLowerCase();
+    return name().toLowerCase(Locale.ROOT);
   }
 }