diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 9976217..30f057b 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -24,16 +24,16 @@ jobs:
     runs-on: macos-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up JDK
-        uses: actions/setup-java@v4
+        uses: actions/setup-java@v5
         with:
           distribution: oracle
           java-version: '26'
 
       - name: Set up Gradle
-        uses: gradle/actions/setup-gradle@v4
+        uses: gradle/actions/setup-gradle@v5
 
       - name: Build
         run: ./gradlew clean build
@@ -51,16 +51,16 @@ jobs:
       ORG_GRADLE_PROJECT_signingInMemoryKeyPassword: ${{ secrets.SIGNING_IN_MEMORY_KEY_PASSWORD }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up JDK
-        uses: actions/setup-java@v4
+        uses: actions/setup-java@v5
         with:
           distribution: oracle
           java-version: '26'
 
       - name: Set up Gradle
-        uses: gradle/actions/setup-gradle@v4
+        uses: gradle/actions/setup-gradle@v5
 
       - name: Check release version
         run: |
diff --git a/README.md b/README.md
index 042dbb5..d62ca1b 100644
--- a/README.md
+++ b/README.md
@@ -45,7 +45,7 @@ Apply the plugin after the Kotlin JVM plugin in Kotlin modules:
 ```kotlin
 plugins {
     kotlin("jvm")
-    id("no.beint.thim") version "0.5.0"
+    id("no.beint.thim") version "0.5.1"
 }
 ```
 
@@ -54,7 +54,7 @@ Java modules need only the Java and Thim plugins:
 ```kotlin
 plugins {
     java
-    id("no.beint.thim") version "0.5.0"
+    id("no.beint.thim") version "0.5.1"
 }
 ```
 
diff --git a/build.gradle.kts b/build.gradle.kts
index 50d911e..af4b682 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -15,7 +15,7 @@ plugins {
 
 allprojects {
     group = "no.beint.thim"
-    version = "0.5.0"
+    version = "0.5.1"
 }
 
 subprojects {
diff --git a/compiler/src/main/kotlin/no/beint/thim/compiler/MessageCatalog.kt b/compiler/src/main/kotlin/no/beint/thim/compiler/MessageCatalog.kt
index 05d0075..f778161 100644
--- a/compiler/src/main/kotlin/no/beint/thim/compiler/MessageCatalog.kt
+++ b/compiler/src/main/kotlin/no/beint/thim/compiler/MessageCatalog.kt
@@ -110,23 +110,27 @@ internal class MessageCatalog private constructor(
             }
             if (!Files.exists(directory)) return MessageCatalog(emptyMap(), canonicalDefault, canonicalSupported)
             require(Files.isDirectory(directory)) { "Message catalog directory does not exist: $directory" }
-            val shortExtensions = Files.walk(directory).use { paths ->
-                paths.filter { Files.isRegularFile(it) && it.extension == "yml" }
-                    .map(Path::toString)
+            val catalogFiles = Files.walk(directory).use { paths ->
+                paths.filter {
+                    Files.isRegularFile(it) && it.extension.lowercase(Locale.ROOT) in setOf("yaml", "yml")
+                }
                     .sorted()
                     .toList()
             }
-            require(shortExtensions.isEmpty()) {
-                "Message catalogs must use the .yaml extension, found $shortExtensions"
+            val invalidExtensions = catalogFiles.filter { it.extension != "yaml" }
+            require(invalidExtensions.isEmpty()) {
+                "Message catalogs must use the .yaml extension, found $invalidExtensions"
             }
-
-            val discovered = Files.list(directory).use { paths ->
-                paths.filter { Files.isDirectory(it) }
-                    .filter { localeDirectory -> containsYaml(localeDirectory) }
-                    .map { it.name }
-                    .sorted()
-                    .toList()
+            val yamlFiles = catalogFiles.filter { it.extension == "yaml" }
+            val misplaced = yamlFiles.filter { directory.relativize(it).nameCount < 2 }
+            require(misplaced.isEmpty()) {
+                "Message catalogs must be stored inside a locale directory, found $misplaced"
             }
+
+            val discovered = yamlFiles
+                .map { directory.relativize(it).getName(0).toString() }
+                .distinct()
+                .sorted()
             val unexpected = discovered - canonicalSupported.toSet()
             require(unexpected.isEmpty()) { "Message catalog has unsupported locale directories $unexpected" }
 
@@ -404,10 +408,6 @@ internal class MessageCatalog private constructor(
             return canonical
         }
 
-        private fun containsYaml(directory: Path): Boolean = Files.walk(directory).use { paths ->
-            paths.anyMatch { Files.isRegularFile(it) && it.extension == "yaml" }
-        }
-
         private fun suggestion(value: String, candidates: Collection<String>): String {
             val nearest = candidates.minByOrNull { distance(value, it) } ?: return ""
             return if (distance(value, nearest) <= 2) "; did you mean '$nearest'?" else ""
diff --git a/compiler/src/main/kotlin/no/beint/thim/compiler/RendererGenerator.kt b/compiler/src/main/kotlin/no/beint/thim/compiler/RendererGenerator.kt
index b047427..ff5b6cb 100644
--- a/compiler/src/main/kotlin/no/beint/thim/compiler/RendererGenerator.kt
+++ b/compiler/src/main/kotlin/no/beint/thim/compiler/RendererGenerator.kt
@@ -17,11 +17,12 @@ internal data class CompiledTemplate(
 
 internal class StaticContent {
     private val output = ByteArrayOutputStream()
+    private val ranges = hashMapOf<String, IntRange>()
 
-    fun append(value: String): IntRange {
+    fun append(value: String): IntRange = ranges.getOrPut(value) {
         val start = output.size()
         output.writeBytes(value.toByteArray(StandardCharsets.UTF_8))
-        return start until output.size()
+        start until output.size()
     }
 
     fun bytes(): ByteArray = output.toByteArray()
@@ -289,7 +290,7 @@ internal class RendererGenerator(
             }
             renderErrors(errorsAttribute, element, scope, code)
         } else if (fieldExpansion?.content != null) {
-            code.statement("output.text(${fieldExpansion?.content});")
+            code.statement("output.text(${fieldExpansion.content});")
         } else if (text == null && safeHtml == null) {
             element.children.forEach { renderNodeCollecting(it, scope, code, context) }
         } else if (safeHtml != null) {
diff --git a/compiler/src/test/kotlin/no/beint/thim/compiler/MessageCatalogTest.kt b/compiler/src/test/kotlin/no/beint/thim/compiler/MessageCatalogTest.kt
index 22c74a6..12b71fa 100644
--- a/compiler/src/test/kotlin/no/beint/thim/compiler/MessageCatalogTest.kt
+++ b/compiler/src/test/kotlin/no/beint/thim/compiler/MessageCatalogTest.kt
@@ -139,6 +139,22 @@ class MessageCatalogTest {
         }
     }
 
+    @Test
+    fun `rejects case variant extensions and catalogs outside locale directories`() {
+        write("en/home.yaml", "title: Hello")
+        write("en/extra.YAML", "subtitle: Welcome")
+
+        assertProblem("must use the .yaml extension") {
+            MessageCatalog.load(directory, "en", listOf("en"))
+        }
+
+        Files.delete(directory.resolve("en/extra.YAML"))
+        write("root.yaml", "title: Ignored")
+        assertProblem("must be stored inside a locale directory") {
+            MessageCatalog.load(directory, "en", listOf("en"))
+        }
+    }
+
     @Test
     fun `requires the same keys and argument contract in every locale`() {
         write("en/home.yaml", "title: Hello {name}\nsubtitle: Welcome")
diff --git a/compiler/src/test/kotlin/no/beint/thim/compiler/StaticContentTest.kt b/compiler/src/test/kotlin/no/beint/thim/compiler/StaticContentTest.kt
new file mode 100644
index 0000000..8a20a67
--- /dev/null
+++ b/compiler/src/test/kotlin/no/beint/thim/compiler/StaticContentTest.kt
@@ -0,0 +1,22 @@
+package no.beint.thim.compiler
+
+import java.nio.charset.StandardCharsets
+import kotlin.test.Test
+import kotlin.test.assertContentEquals
+import kotlin.test.assertEquals
+
+class StaticContentTest {
+    @Test
+    fun `reuses the byte range for identical static content`() {
+        val content = StaticContent()
+
+        val first = content.append("Save")
+        val second = content.append("Cancel")
+        val repeated = content.append("Save")
+
+        assertEquals(first, repeated)
+        assertEquals(0 until 4, first)
+        assertEquals(4 until 10, second)
+        assertContentEquals("SaveCancel".toByteArray(StandardCharsets.UTF_8), content.bytes())
+    }
+}
diff --git a/gradle-plugin/src/main/java/no/beint/thim/gradle/ThimPlugin.java b/gradle-plugin/src/main/java/no/beint/thim/gradle/ThimPlugin.java
index 493db0a..e6abe8a 100644
--- a/gradle-plugin/src/main/java/no/beint/thim/gradle/ThimPlugin.java
+++ b/gradle-plugin/src/main/java/no/beint/thim/gradle/ThimPlugin.java
@@ -87,7 +87,7 @@ private void configureKotlinProject(Project project, ThimExtension extension) {
             task.getInputs().files(extension.getTemplates().map(directory -> htmlFiles(project, directory.getAsFile())))
                     .withPropertyName("thimTemplates")
                     .withPathSensitivity(PathSensitivity.RELATIVE);
-            task.getInputs().files(extension.getMessages().map(directory -> yamlFiles(project, directory.getAsFile())))
+            task.getInputs().files(extension.getMessages().map(directory -> catalogFiles(project, directory.getAsFile())))
                     .withPropertyName("thimMessages")
                     .withPathSensitivity(PathSensitivity.RELATIVE);
         });
@@ -207,8 +207,8 @@ private FileTree htmlFiles(Project project, java.io.File directory) {
         return project.fileTree(directory, files -> files.include("**/*.html"));
     }
 
-    private FileTree yamlFiles(Project project, java.io.File directory) {
-        return project.fileTree(directory, files -> files.include("**/*.yaml"));
+    private FileTree catalogFiles(Project project, java.io.File directory) {
+        return project.fileTree(directory, files -> files.include("**/*.yaml", "**/*.yml", "**/*.YAML", "**/*.YML"));
     }
 
     private String generatedPackage(Project project) {