diff --git a/shared/build.gradle.kts b/shared/build.gradle.kts
index 64e3fb5..9d7c0ea 100644
--- a/shared/build.gradle.kts
+++ b/shared/build.gradle.kts
@@ -33,6 +33,12 @@ kotlin {
     sourceSets {
         commonMain.dependencies {
         }
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+        }
+        androidUnitTest.dependencies {
+            implementation(kotlin("test-junit"))
+        }
     }
 }
 
diff --git a/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadErrorTest.kt b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadErrorTest.kt
new file mode 100644
index 0000000..255422d
--- /dev/null
+++ b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadErrorTest.kt
@@ -0,0 +1,88 @@
+package dev.onexeor.kdownloader
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+import kotlin.test.assertTrue
+
+class DownloadErrorTest {
+
+    @Test
+    fun networkErrorHasMessage() {
+        val error = DownloadError.Network("Connection timeout")
+        assertEquals("Connection timeout", error.message)
+        assertNull(error.cause)
+    }
+
+    @Test
+    fun networkErrorCanHaveCause() {
+        val cause = RuntimeException("Underlying error")
+        val error = DownloadError.Network("Connection failed", cause)
+        assertEquals("Connection failed", error.message)
+        assertEquals(cause, error.cause)
+    }
+
+    @Test
+    fun httpErrorHasStatusCode() {
+        val error = DownloadError.Http(404, "Not Found")
+        assertEquals(404, error.statusCode)
+        assertEquals("Not Found", error.message)
+    }
+
+    @Test
+    fun storageErrorHasMessage() {
+        val error = DownloadError.Storage("Disk full")
+        assertEquals("Disk full", error.message)
+    }
+
+    @Test
+    fun invalidUrlErrorHasMessage() {
+        val error = DownloadError.InvalidUrl("Malformed URL: abc")
+        assertEquals("Malformed URL: abc", error.message)
+    }
+
+    @Test
+    fun cancelledErrorHasDefaultMessage() {
+        val error = DownloadError.Cancelled
+        assertEquals("Download cancelled", error.message)
+    }
+
+    @Test
+    fun unknownErrorHasMessage() {
+        val error = DownloadError.Unknown("Something went wrong")
+        assertEquals("Something went wrong", error.message)
+    }
+
+    @Test
+    fun errorsCanBeUsedInWhenExpression() {
+        val errors: List<DownloadError> = listOf(
+            DownloadError.Network("test"),
+            DownloadError.Http(500, "Server error"),
+            DownloadError.Storage("test"),
+            DownloadError.InvalidUrl("test"),
+            DownloadError.Cancelled,
+            DownloadError.Unknown("test")
+        )
+
+        errors.forEach { error ->
+            val result = when (error) {
+                is DownloadError.Network -> "network"
+                is DownloadError.Http -> "http-${error.statusCode}"
+                is DownloadError.Storage -> "storage"
+                is DownloadError.InvalidUrl -> "invalid-url"
+                is DownloadError.Cancelled -> "cancelled"
+                is DownloadError.Unknown -> "unknown"
+            }
+            assertTrue(result.isNotEmpty())
+        }
+    }
+
+    @Test
+    fun httpErrorCoversCommonStatusCodes() {
+        val codes = listOf(400, 401, 403, 404, 500, 502, 503)
+        codes.forEach { code ->
+            val error = DownloadError.Http(code, "Error $code")
+            assertEquals(code, error.statusCode)
+        }
+    }
+}
diff --git a/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadProgressTest.kt b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadProgressTest.kt
new file mode 100644
index 0000000..da245f1
--- /dev/null
+++ b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadProgressTest.kt
@@ -0,0 +1,79 @@
+package dev.onexeor.kdownloader
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class DownloadProgressTest {
+
+    @Test
+    fun zeroProgressHasCorrectDefaults() {
+        val progress = DownloadProgress.ZERO
+        assertEquals(0, progress.bytesDownloaded)
+        assertEquals(-1, progress.totalBytes)
+        assertEquals(-1, progress.percentage)
+    }
+
+    @Test
+    fun isSizeKnownReturnsTrueWhenTotalBytesPositive() {
+        val progress = DownloadProgress(
+            bytesDownloaded = 100,
+            totalBytes = 1000,
+            percentage = 10
+        )
+        assertTrue(progress.isSizeKnown)
+    }
+
+    @Test
+    fun isSizeKnownReturnsFalseWhenTotalBytesNegative() {
+        val progress = DownloadProgress(
+            bytesDownloaded = 100,
+            totalBytes = -1,
+            percentage = -1
+        )
+        assertFalse(progress.isSizeKnown)
+    }
+
+    @Test
+    fun isSizeKnownReturnsFalseWhenTotalBytesZero() {
+        val progress = DownloadProgress(
+            bytesDownloaded = 0,
+            totalBytes = 0,
+            percentage = 0
+        )
+        assertFalse(progress.isSizeKnown)
+    }
+
+    @Test
+    fun hasStartedReturnsTrueWhenBytesDownloaded() {
+        val progress = DownloadProgress(
+            bytesDownloaded = 1,
+            totalBytes = 100,
+            percentage = 1
+        )
+        assertTrue(progress.hasStarted)
+    }
+
+    @Test
+    fun hasStartedReturnsFalseWhenNoBytesDownloaded() {
+        val progress = DownloadProgress(
+            bytesDownloaded = 0,
+            totalBytes = 100,
+            percentage = 0
+        )
+        assertFalse(progress.hasStarted)
+    }
+
+    @Test
+    fun progressValuesAreCorrect() {
+        val progress = DownloadProgress(
+            bytesDownloaded = 500,
+            totalBytes = 1000,
+            percentage = 50
+        )
+        assertEquals(500, progress.bytesDownloaded)
+        assertEquals(1000, progress.totalBytes)
+        assertEquals(50, progress.percentage)
+    }
+}
diff --git a/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadRequestBuilderTest.kt b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadRequestBuilderTest.kt
new file mode 100644
index 0000000..2677a55
--- /dev/null
+++ b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadRequestBuilderTest.kt
@@ -0,0 +1,100 @@
+package dev.onexeor.kdownloader
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+import kotlin.test.assertTrue
+
+class DownloadRequestBuilderTest {
+
+    @Test
+    fun builderSetsUrl() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").build()
+        assertEquals("https://example.com/file.zip", request.url)
+    }
+
+    @Test
+    fun builderSetsFileName() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").apply {
+            fileName = "custom.zip"
+        }.build()
+        assertEquals("custom.zip", request.fileName)
+    }
+
+    @Test
+    fun builderSetsDirectory() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").apply {
+            directory = "/downloads"
+        }.build()
+        assertEquals("/downloads", request.directory)
+    }
+
+    @Test
+    fun builderDefaultsToNoOverwrite() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").build()
+        assertEquals(false, request.overwrite)
+    }
+
+    @Test
+    fun overwriteIfExistsSetsOverwriteTrue() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").apply {
+            overwriteIfExists()
+        }.build()
+        assertTrue(request.overwrite)
+    }
+
+    @Test
+    fun wifiOnlySetsNetworkType() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").apply {
+            wifiOnly()
+        }.build()
+        assertEquals(NetworkType.WIFI_ONLY, request.networkType)
+    }
+
+    @Test
+    fun headersDslAddsHeaders() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").apply {
+            headers {
+                "X-Custom" to "value1"
+                "Accept" to "application/json"
+            }
+        }.build()
+        assertEquals("value1", request.headers["X-Custom"])
+        assertEquals("application/json", request.headers["Accept"])
+    }
+
+    @Test
+    fun authBearerSetsAuth() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").apply {
+            auth {
+                bearer("my-token")
+            }
+        }.build()
+        val auth = request.auth
+        assertTrue(auth is Auth.Bearer)
+        assertEquals("my-token", auth.token)
+    }
+
+    @Test
+    fun authBasicSetsAuth() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").apply {
+            auth {
+                basic("user", "pass")
+            }
+        }.build()
+        val auth = request.auth
+        assertTrue(auth is Auth.Basic)
+        assertEquals("user", auth.username)
+        assertEquals("pass", auth.password)
+    }
+
+    @Test
+    fun defaultsAreNull() {
+        val request = DownloadRequestBuilder("https://example.com/file.zip").build()
+        assertNull(request.fileName)
+        assertNull(request.directory)
+        assertNull(request.auth)
+        assertNull(request.networkType)
+        assertTrue(request.headers.isEmpty())
+    }
+}
diff --git a/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadStateTest.kt b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadStateTest.kt
new file mode 100644
index 0000000..e37c24d
--- /dev/null
+++ b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/DownloadStateTest.kt
@@ -0,0 +1,70 @@
+package dev.onexeor.kdownloader
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class DownloadStateTest {
+
+    @Test
+    fun pendingStateIsCorrectType() {
+        val state: DownloadState = DownloadState.Pending
+        assertTrue(state is DownloadState.Pending)
+    }
+
+    @Test
+    fun downloadingStateContainsProgress() {
+        val progress = DownloadProgress(50, 100, 50)
+        val state = DownloadState.Downloading(progress)
+        assertEquals(progress, state.progress)
+    }
+
+    @Test
+    fun pausedStateContainsReason() {
+        val state = DownloadState.Paused("Waiting for WiFi")
+        assertEquals("Waiting for WiFi", state.reason)
+    }
+
+    @Test
+    fun completedStateContainsFilePath() {
+        val state = DownloadState.Completed("/path/to/file.zip")
+        assertEquals("/path/to/file.zip", state.filePath)
+    }
+
+    @Test
+    fun failedStateContainsError() {
+        val error = DownloadError.Network("Connection failed")
+        val state = DownloadState.Failed(error)
+        assertEquals(error, state.error)
+    }
+
+    @Test
+    fun cancelledStateIsCorrectType() {
+        val state: DownloadState = DownloadState.Cancelled
+        assertTrue(state is DownloadState.Cancelled)
+    }
+
+    @Test
+    fun statesCanBeUsedInWhenExpression() {
+        val states = listOf(
+            DownloadState.Pending,
+            DownloadState.Downloading(DownloadProgress.ZERO),
+            DownloadState.Paused("test"),
+            DownloadState.Completed("/path"),
+            DownloadState.Failed(DownloadError.Cancelled),
+            DownloadState.Cancelled
+        )
+
+        states.forEach { state ->
+            val result = when (state) {
+                is DownloadState.Pending -> "pending"
+                is DownloadState.Downloading -> "downloading"
+                is DownloadState.Paused -> "paused"
+                is DownloadState.Completed -> "completed"
+                is DownloadState.Failed -> "failed"
+                is DownloadState.Cancelled -> "cancelled"
+            }
+            assertTrue(result.isNotEmpty())
+        }
+    }
+}
diff --git a/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/KDownloaderConfigTest.kt b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/KDownloaderConfigTest.kt
new file mode 100644
index 0000000..96944a1
--- /dev/null
+++ b/shared/src/commonTest/kotlin/dev/onexeor/kdownloader/KDownloaderConfigTest.kt
@@ -0,0 +1,42 @@
+package dev.onexeor.kdownloader
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+
+class KDownloaderConfigTest {
+
+    @Test
+    fun defaultConfigHasNullDirectory() {
+        val config = KDownloaderConfig()
+        assertNull(config.defaultDirectory)
+    }
+
+    @Test
+    fun defaultConfigHasAnyNetworkType() {
+        val config = KDownloaderConfig()
+        assertEquals(NetworkType.ANY, config.defaultNetworkType)
+    }
+
+    @Test
+    fun configCanSetDirectory() {
+        val config = KDownloaderConfig(defaultDirectory = "Downloads/MyApp")
+        assertEquals("Downloads/MyApp", config.defaultDirectory)
+    }
+
+    @Test
+    fun configCanSetNetworkType() {
+        val config = KDownloaderConfig(defaultNetworkType = NetworkType.WIFI_ONLY)
+        assertEquals(NetworkType.WIFI_ONLY, config.defaultNetworkType)
+    }
+
+    @Test
+    fun configCanSetBothValues() {
+        val config = KDownloaderConfig(
+            defaultDirectory = "custom/path",
+            defaultNetworkType = NetworkType.WIFI_ONLY
+        )
+        assertEquals("custom/path", config.defaultDirectory)
+        assertEquals(NetworkType.WIFI_ONLY, config.defaultNetworkType)
+    }
+}