diff --git a/src/test/java/com/dostalgia/ConfigBuilderTest.java b/src/test/java/com/dostalgia/ConfigBuilderTest.java
new file mode 100644
index 0000000..5d0ab0e
--- /dev/null
+++ b/src/test/java/com/dostalgia/ConfigBuilderTest.java
@@ -0,0 +1,80 @@
+package com.dostalgia;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+
+class ConfigBuilderTest {
+
+    private final ConfigBuilder builder = new ConfigBuilder();
+
+    @Test
+    void buildDosboxConf_noCdImages() {
+        String conf = builder.buildDosboxConf("FALLOUT.EXE", List.of());
+        assertTrue(conf.contains("memsize=64"), "Should include memsize");
+        assertTrue(conf.contains("mount c ."), "Should mount C drive");
+        assertTrue(conf.contains("FALLOUT.EXE"), "Should reference executable");
+        assertFalse(conf.contains("imgmount"), "No CD images should produce no imgmount");
+    }
+
+    @Test
+    void buildDosboxConf_withCdImages() {
+        String conf = builder.buildDosboxConf("DOTT.EXE", List.of("game.cue", "game.bin"));
+        assertTrue(conf.contains("imgmount D \"game.bin\" -t cdrom -fs iso"), "Should mount .bin with iso flag");
+        assertFalse(conf.contains("game.cue"), "Should skip .cue when .bin exists");
+    }
+
+    @Test
+    void buildDosboxConf_subdirectory() {
+        String conf = builder.buildDosboxConf("FALLOUT/FALLOUT.EXE", List.of());
+        assertTrue(conf.contains("cd FALLOUT"), "Should cd into subdirectory");
+        assertTrue(conf.contains("path=c:\\"), "Should set path");
+    }
+
+    @Test
+    void buildCdOnlyDosboxConf() {
+        String conf = builder.buildCdOnlyDosboxConf(List.of("game.iso"));
+        assertTrue(conf.contains("runs from the CD-ROM"), "CD-only message");
+        assertTrue(conf.contains("imgmount D"), "Should mount CD");
+    }
+
+    @Test
+    void buildJsdosJson() {
+        byte[] json = builder.buildJsdosJson("GAME.EXE", List.of());
+        String s = new String(json);
+        assertTrue(s.contains("\"script\""), "Should have script key");
+        assertTrue(s.contains("\"autolock\""), "Should have autolock");
+        assertTrue(s.contains("GAME.EXE"), "Should contain executable name");
+    }
+
+    @Test
+    void splitExePath_rootLevel() {
+        var parts = ConfigBuilder.splitExePath("GAME.EXE");
+        assertNull(parts.dir());
+        assertEquals("GAME.EXE", parts.exe());
+    }
+
+    @Test
+    void splitExePath_subdirectory() {
+        var parts = ConfigBuilder.splitExePath("FALLOUT1/FALLOUT.EXE");
+        assertEquals("FALLOUT1", parts.dir());
+        assertEquals("FALLOUT.EXE", parts.exe());
+    }
+
+    @Test
+    void splitExePath_nullOrEmpty() {
+        assertNull(ConfigBuilder.splitExePath(null).dir());
+        assertEquals("", ConfigBuilder.splitExePath("").exe());
+    }
+
+    @Test
+    void escapeJson() {
+        assertEquals("hello", ConfigBuilder.escapeJson("hello"));
+        assertEquals("\\\\n", ConfigBuilder.escapeJson("\\n"));
+        assertEquals("he said \\\"hi\\\"", ConfigBuilder.escapeJson("he said \"hi\""));
+    }
+}
diff --git a/src/test/java/com/dostalgia/PlatformDetectorTest.java b/src/test/java/com/dostalgia/PlatformDetectorTest.java
new file mode 100644
index 0000000..b89af2f
--- /dev/null
+++ b/src/test/java/com/dostalgia/PlatformDetectorTest.java
@@ -0,0 +1,90 @@
+package com.dostalgia;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+class PlatformDetectorTest {
+
+    private final PlatformDetector detector = new PlatformDetector();
+
+    @Test
+    void detect_comFile_returnsDos(@TempDir Path dir) throws Exception {
+        Path f = dir.resolve("test.com");
+        Files.write(f, new byte[]{0x4D, 0x5A}); // MZ header
+        assertEquals("dos", detector.detect(f));
+    }
+
+    @Test
+    void detect_batFile_returnsDos(@TempDir Path dir) throws Exception {
+        Path f = dir.resolve("test.bat");
+        Files.writeString(f, "@echo off");
+        assertEquals("dos", detector.detect(f));
+    }
+
+    @Test
+    void detect_plainMZ_returnsDos(@TempDir Path dir) throws Exception {
+        byte[] buf = new byte[0x80];
+        buf[0] = 0x4D; // M
+        buf[1] = 0x5A; // Z
+        // e_lfanew points to garbage — should be treated as plain DOS
+        buf[0x3C] = 0x00;
+        buf[0x3D] = 0x00;
+        buf[0x3E] = 0x00;
+        buf[0x3F] = 0x00;
+
+        Path f = dir.resolve("test.exe");
+        Files.write(f, buf);
+        assertEquals("dos", detector.detect(f));
+    }
+
+    @Test
+    void detect_peHeader_returnsWindows(@TempDir Path dir) throws Exception {
+        byte[] buf = new byte[0x100];
+        buf[0] = 0x4D; // M
+        buf[1] = 0x5A; // Z
+        // e_lfanew = 0x80
+        buf[0x3C] = (byte) 0x80;
+        buf[0x3D] = 0x00;
+        buf[0x3E] = 0x00;
+        buf[0x3F] = 0x00;
+        // PE signature at offset 0x80
+        buf[0x80] = 0x50; // P
+        buf[0x81] = 0x45; // E
+
+        Path f = dir.resolve("test.exe");
+        Files.write(f, buf);
+        assertEquals("windows", detector.detect(f));
+    }
+
+    @Test
+    void detect_neHeader_returnsWindows(@TempDir Path dir) throws Exception {
+        byte[] buf = new byte[0x100];
+        buf[0] = 0x4D;
+        buf[1] = 0x5A;
+        buf[0x3C] = (byte) 0x80;
+        buf[0x80] = 0x4E; // N
+        buf[0x81] = 0x45; // E
+
+        Path f = dir.resolve("test.exe");
+        Files.write(f, buf);
+        assertEquals("windows", detector.detect(f));
+    }
+
+    @Test
+    void detect_emptyFile_returnsNull(@TempDir Path dir) throws Exception {
+        Path f = dir.resolve("empty.exe");
+        Files.write(f, new byte[0]);
+        assertNull(detector.detect(f));
+    }
+
+    @Test
+    void detect_noMZ_returnsNull(@TempDir Path dir) throws Exception {
+        Path f = dir.resolve("bad.exe");
+        Files.write(f, new byte[]{0x00, 0x00});
+        assertNull(detector.detect(f));
+    }
+}