diff --git a/.jules/bolt.md b/.jules/bolt.md
index 83cc604..c5a7a50 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -5,3 +5,7 @@
 ## 2024-05-24 - Loop Allocation Hot Paths
 **Learning:** Rendering directory entries with repeated string concatenation and list-based exclusion lookups creates avoidable allocation and lookup cost in large directories.
 **Action:** Use `StringBuilder` for entry rendering and a `Set` for excluded file names.
+
+## 2024-07-01 - Kotlin/JVM String replacement overhead for HTML rendering
+**Learning:** Sequential `.replace()` calls on Kotlin strings inside hot paths like HTML escaping create multiple intermediate string instances, causing significant GC pressure.
+**Action:** Replace multiple string substitution calls with a single-pass character iteration that only allocates a `StringBuilder` when an escapable character is actually encountered, turning $O(n)$ allocations into O(1) builder for strings that need escaping, and 0 allocations for normal strings.
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 2e2809f..e79e8e4 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -48,13 +48,32 @@ fun go(topDir: String, maxLevel: Int)  {
     }
 }
 
+// ⚡ Bolt: Optimize escapeHtml by replacing sequential .replace() calls with a single pass StringBuilder
+// Reduces unnecessary string allocation and GC pressure, providing O(1) string creation for fast HTML rendering.
 fun String.escapeHtml(): String {
-    return this.replace("&", "&")
-               .replace("<", "&lt;")
-               .replace(">", "&gt;")
-               .replace("\"", "&quot;")
-               .replace("'", "&#x27;")
-               .replace("`", "&#x60;")
+    var sb: StringBuilder? = null
+    for (i in indices) {
+        val c = this[i]
+        val replacement = when (c) {
+            '&' -> "&amp;"
+            '<' -> "&lt;"
+            '>' -> "&gt;"
+            '"' -> "&quot;"
+            '\'' -> "&#x27;"
+            '`' -> "&#x60;"
+            else -> null
+        }
+        if (replacement != null) {
+            if (sb == null) {
+                sb = java.lang.StringBuilder(length + 16)
+                sb.append(this, 0, i)
+            }
+            sb.append(replacement)
+        } else {
+            sb?.append(c)
+        }
+    }
+    return sb?.toString() ?: this
 }
 
 fun String.urlEncodePath(): String {
diff --git a/src/test/kotlin/html4tree/MainTest.kt.orig b/src/test/kotlin/html4tree/MainTest.kt.orig
new file mode 100644
index 0000000..e8a3082
--- /dev/null
+++ b/src/test/kotlin/html4tree/MainTest.kt.orig
@@ -0,0 +1,312 @@
+package html4tree
+
+import org.junit.After
+import org.junit.Assume
+import org.junit.Before
+import org.junit.Test
+import java.io.ByteArrayOutputStream
+import java.io.File
+import java.io.PrintStream
+import java.nio.file.Files
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertFalse
+import kotlin.test.assertNull
+import kotlin.test.assertTrue
+
+class MainTest {
+    private lateinit var tempDir: File
+
+    @Before
+    fun setup() {
+        tempDir = Files.createTempDirectory("html4tree-test-").toFile()
+    }
+
+    @After
+    fun teardown() {
+        if (tempDir.exists()) {
+            tempDir.deleteRecursively()
+        }
+    }
+
+    @Test
+    fun testEscapeHtml() {
+        assertEquals("&amp;", "&".escapeHtml())
+        assertEquals("&lt;", "<".escapeHtml())
+        assertEquals("&gt;", ">".escapeHtml())
+        assertEquals("&quot;", "\"".escapeHtml())
+        assertEquals("&#x27;", "'".escapeHtml())
+        assertEquals("&#x60;", "`".escapeHtml())
+        assertEquals("&amp;&lt;&gt;&quot;&#x27;&#x60;", "&<>\"'`".escapeHtml())
+        assertEquals("normal text", "normal text".escapeHtml())
+    }
+
+    @Test
+    fun testUrlEncodePath() {
+        assertEquals("hello%20world", "hello world".urlEncodePath())
+        assertEquals("normal_path", "normal_path".urlEncodePath())
+        assertEquals("path%2Fwith%2Fslash", "path/with/slash".urlEncodePath())
+    }
+
+    @Test
+    fun testHelp() {
+        val outContent = ByteArrayOutputStream()
+        val originalOut = System.out
+        System.setOut(PrintStream(outContent))
+        try {
+            help()
+            assertEquals("ERROR: help has not been written yet!\n", outContent.toString().replace("\r\n", "\n"))
+        } finally {
+            System.setOut(originalOut)
+        }
+    }
+
+    @Test(expected = IllegalArgumentException::class)
+    fun testGoInvalidDir() {
+        go("non_existent_directory", -1)
+    }
+
+    @Test
+    fun testGoRejectsSymlinkTopDir() {
+        val targetDir = Files.createTempDirectory("html4tree-target-").toFile()
+        val symlink = File(tempDir, "linked-top")
+        try {
+            try {
+                Files.createSymbolicLink(symlink.toPath(), targetDir.absoluteFile.toPath())
+            } catch (e: Exception) {
+                Assume.assumeTrue("Symlink creation not supported in this environment", false)
+            }
+
+            assertFailsWith<IllegalArgumentException> {
+                go(symlink.absolutePath, -1)
+            }
+        } finally {
+            targetDir.deleteRecursively()
+        }
+    }
+
+    @Test
+    fun testGoEmptyDir() {
+        go(tempDir.absolutePath, -1)
+        val indexFile = File(tempDir, "index.html")
+        assertTrue(indexFile.exists())
+        val htmlContent = indexFile.readText()
+        assertTrue(htmlContent.contains("<html lang=\"ko\">"))
+        assertTrue(htmlContent.contains("이 디렉토리는 비어 있습니다."))
+    }
+
+    @Test
+    fun testProcessIgnoreFile() {
+        val ignoreFile = File(tempDir, ".html4ignore")
+        ignoreFile.writeText(".*\\.txt\n.*\\.log")
+
+        File(tempDir, "test.txt").createNewFile()
+        File(tempDir, "test.log").createNewFile()
+        File(tempDir, "test.md").createNewFile()
+
+        val excluded = process_ignore_file(tempDir)
+
+        assertTrue(excluded.contains("test.txt"))
+        assertTrue(excluded.contains("test.log"))
+        assertTrue(excluded.contains("index.html"))
+        assertFalse(excluded.contains("test.md"))
+    }
+
+    @Test
+    fun testProcessIgnoreFileNoIgnore() {
+        val excluded = process_ignore_file(tempDir)
+        assertTrue(excluded.contains("index.html"))
+        assertEquals(1, excluded.size)
+    }
+
+    @Test
+    fun testProcessIgnoreFileInvalidRegex() {
+        val ignoreFile = File(tempDir, ".html4ignore")
+        ignoreFile.writeText("[\n.*\\.log")
+
+        File(tempDir, "test.log").createNewFile()
+        File(tempDir, "test.txt").createNewFile()
+
+        val excluded = process_ignore_file(tempDir)
+
+        assertTrue(excluded.contains("test.log"))
+        assertFalse(excluded.contains("test.txt"))
+    }
+
+    @Test
+    fun testProcessDir() {
+        val subdir = File(tempDir, "subdir")
+        subdir.mkdir()
+        File(tempDir, "file1.txt").createNewFile()
+        File(tempDir, "test.ignore").createNewFile()
+        File(tempDir, ".html4ignore").writeText(".*\\.ignore")
+
+        process_dir(tempDir)
+
+        val indexFile = File(tempDir, "index.html")
+        assertTrue(indexFile.exists())
+        val htmlContent = indexFile.readText()
+        assertTrue(htmlContent.contains("<html lang=\"ko\">"))
+        assertTrue(htmlContent.contains("<nav aria-label=\"Directory listing\">"))
+        assertTrue(htmlContent.contains("<main>"))
+        assertTrue(htmlContent.contains("</main>"))
+        assertTrue(htmlContent.contains("aria-label=\"상위 디렉토리로 이동\""))
+        assertTrue(htmlContent.contains("aria-hidden=\"true\""))
+        assertTrue(htmlContent.contains("aria-label=\"file1.txt 파일\""))
+        assertTrue(htmlContent.contains("aria-label=\"subdir 디렉토리\""))
+        assertTrue(htmlContent.contains("file1.txt"))
+        assertTrue(htmlContent.contains("subdir/"))
+        assertTrue(htmlContent.contains("&#128193;"))
+        assertFalse(htmlContent.contains("test.ignore"))
+        assertTrue(htmlContent.contains("Content-Security-Policy"))
+        assertTrue(htmlContent.contains("default-src 'none'; style-src 'unsafe-inline';"))
+    }
+
+    @Test
+    fun testProcessDirReplacesIndexSymlinkWithoutTouchingTarget() {
+        val targetFile = File(tempDir, "target.txt")
+        targetFile.writeText("original content")
+
+        val indexFile = File(tempDir, "index.html")
+        try {
+            Files.createSymbolicLink(indexFile.toPath(), targetFile.toPath())
+        } catch (e: Exception) {
+            Assume.assumeTrue("Symlink creation not supported in this environment", false)
+        }
+
+        process_dir(tempDir)
+
+        assertEquals("original content", targetFile.readText())
+        assertTrue(indexFile.exists())
+        assertFalse(Files.isSymbolicLink(indexFile.toPath()))
+        assertTrue(indexFile.readText().contains("<html lang=\"ko\">"))
+    }
+
+    @Test
+    fun testGoWithSymlink() {
+        val subdir = File(tempDir, "subdir")
+        subdir.mkdir()
+
+        val targetDir = Files.createTempDirectory("html4tree-target-").toFile()
+        File(targetDir, "secret.txt").writeText("secret")
+
+        try {
+            val symlink = File(subdir, "symlink")
+            try {
+                Files.createSymbolicLink(symlink.toPath(), targetDir.absoluteFile.toPath())
+            } catch (e: Exception) {
+                Assume.assumeTrue("Symlink creation not supported in this environment", false)
+            }
+
+            go(tempDir.absolutePath, -1)
+
+            assertTrue(File(tempDir, "index.html").exists())
+
+            val subdirIndex = File(subdir, "index.html")
+            assertTrue(subdirIndex.exists())
+            assertFalse(subdirIndex.readText().contains("symlink"), "Symlinked directory should not be listed in index.html")
+
+            val symlinkIndex = File(targetDir, "index.html")
+            assertFalse(symlinkIndex.exists(), "Symlink target should not have an index.html generated")
+        } finally {
+            targetDir.deleteRecursively()
+        }
+    }
+
+    @Test
+    fun testGoWithMaxLevel() {
+        val subdir = File(tempDir, "subdir")
+        subdir.mkdir()
+        val subsubdir = File(subdir, "subsubdir")
+        subsubdir.mkdir()
+
+        go(tempDir.absolutePath, 0)
+
+        assertTrue(File(tempDir, "index.html").exists())
+        assertFalse(File(subdir, "index.html").exists())
+        assertFalse(File(subsubdir, "index.html").exists())
+    }
+
+    @Test
+    fun testGoWithUnreadableDir() {
+        val unreadableDir = File(tempDir, "unreadable")
+        unreadableDir.mkdir()
+        unreadableDir.setWritable(true)
+        unreadableDir.setExecutable(true)
+
+        try {
+            Assume.assumeTrue(unreadableDir.setReadable(false, false))
+            assertNull(unreadableDir.listFiles())
+
+            go(tempDir.absolutePath, -1)
+
+            assertTrue(File(tempDir, "index.html").exists())
+            assertTrue(File(unreadableDir, "index.html").exists())
+        } finally {
+            unreadableDir.setReadable(true, false)
+            unreadableDir.setWritable(true, false)
+            unreadableDir.setExecutable(true, false)
+        }
+    }
+
+    @Test
+    fun testCliParsing() {
+        val cli = Html4tree()
+        cli.parse(arrayOf("--max-level", "2", tempDir.absolutePath))
+        assertEquals(2, cli.maxLevel)
+        assertEquals(tempDir.absolutePath, cli.topDir)
+    }
+
+    @Test
+    fun testCliMainParsing() {
+        val cli = Html4tree()
+        cli.parse(arrayOf(tempDir.absolutePath))
+        main(arrayOf(tempDir.absolutePath))
+        assertTrue(File(tempDir, "index.html").exists())
+    }
+
+    @Test(expected = IllegalArgumentException::class)
+    fun testGoNotADir() {
+        val notADir = File(tempDir, "not_a_dir.txt")
+        notADir.writeText("test")
+        go(notADir.absolutePath, -1)
+    }
+
+    @Test
+    fun testProcessIgnoreFileWithIndexHtml() {
+        val ignoreFile = File(tempDir, ".html4ignore")
+        ignoreFile.writeText("index\\.html")
+        File(tempDir, "index.html").writeText("existing")
+        val excluded = process_ignore_file(tempDir)
+        assertTrue(excluded.contains("index.html"))
+    }
+
+    @Test
+    fun testProcessDirItEqualsCurrDir() {
+        File(tempDir, "tempDir").mkdir()
+        process_dir(tempDir)
+    }
+
+    @Test(expected = IllegalArgumentException::class)
+    fun testGoBlankDir() {
+        go("   ", -1)
+    }
+
+    @Test
+    fun testUrlEncodePathUnreserved() {
+        assertEquals("-._~", "-._~".urlEncodePath())
+        assertEquals("A1z", "A1z".urlEncodePath())
+    }
+
+    @Test
+    fun testProcessIgnoreFileEmptyLine() {
+        val ignoreFile = File(tempDir, ".html4ignore")
+        ignoreFile.writeText("\n.*\\.txt\n\n.*\\.log\n")
+
+        File(tempDir, "test.txt").createNewFile()
+
+        val excluded = process_ignore_file(tempDir)
+        assertTrue(excluded.contains("test.txt"))
+    }
+
+}