diff --git a/ktor-client/ktor-client-core/jvm/src/io/ktor/client/plugins/cache/storage/FileCacheStorage.kt b/ktor-client/ktor-client-core/jvm/src/io/ktor/client/plugins/cache/storage/FileCacheStorage.kt
index db3c00d2234..be2d7b03901 100644
--- a/ktor-client/ktor-client-core/jvm/src/io/ktor/client/plugins/cache/storage/FileCacheStorage.kt
+++ b/ktor-client/ktor-client-core/jvm/src/io/ktor/client/plugins/cache/storage/FileCacheStorage.kt
@@ -205,6 +205,15 @@ private class FileCacheStorage(
         channel.writeFully(cache.body)
     }
 
+    /**
+     * Deserialize a single CachedResponseData from the provided ByteReadChannel.
+     *
+     * Reads the cached-entry fields in the stored binary format: request URL, HTTP status and version,
+     * headers, request/response/expiration timestamps, vary keys (converted to lowercase), and the body bytes.
+     *
+     * @param channel Source channel positioned at the start of a serialized cache entry.
+     * @return The reconstructed CachedResponseData instance.
+     */
     private suspend fun readCache(channel: ByteReadChannel): CachedResponseData {
         val url = channel.readUTF8Line()!!
         val status = HttpStatusCode(channel.readInt(), channel.readUTF8Line()!!)
@@ -222,7 +231,7 @@ private class FileCacheStorage(
         val varyKeysCount = channel.readInt()
         val varyKeys = buildMap {
             for (j in 0 until varyKeysCount) {
-                val key = channel.readUTF8Line()!!
+                val key = channel.readUTF8Line()!!.lowercase()
                 val value = channel.readUTF8Line()!!
                 put(key, value)
             }
@@ -242,4 +251,4 @@ private class FileCacheStorage(
             body = body
         )
     }
-}
+}
\ No newline at end of file
diff --git a/ktor-test-server/src/main/kotlin/test/server/tests/Cache.kt b/ktor-test-server/src/main/kotlin/test/server/tests/Cache.kt
index 2b426fae7d9..5c629548ea9 100644
--- a/ktor-test-server/src/main/kotlin/test/server/tests/Cache.kt
+++ b/ktor-test-server/src/main/kotlin/test/server/tests/Cache.kt
@@ -17,6 +17,14 @@ import java.util.concurrent.atomic.AtomicInteger
 
 internal val counter = AtomicInteger(0)
 
+/**
+ * Registers test routes under `/cache` that exercise caching and conditional header behaviors.
+ *
+ * Installs the `CachingHeaders` and `ConditionalHeaders` plugins and declares endpoints used in tests:
+ * handlers for various `Cache-Control` directives, `Expires`, `ETag`, `Last-Modified`, and `Vary`
+ * header scenarios, including endpoints that return 304 Not Modified responses and case-sensitive
+ * vary-header behaviors.
+ */
 internal fun Application.cacheTestServer() {
     routing {
         route("/cache") {
@@ -149,6 +157,11 @@ internal fun Application.cacheTestServer() {
                     else -> error("Should not be invoked")
                 }
             }
+
+            get("/vary-header-not-modified") {
+                call.response.header(HttpHeaders.Vary, HttpHeaders.AcceptLanguage)
+                call.respond(HttpStatusCode.NotModified)
+            }
         }
     }
-}
+}
\ No newline at end of file