diff --git a/src/SentenceStudio.Shared/AssemblyInfo.cs b/src/SentenceStudio.Shared/AssemblyInfo.cs
new file mode 100644
index 00000000..49394e0f
--- /dev/null
+++ b/src/SentenceStudio.Shared/AssemblyInfo.cs
@@ -0,0 +1,3 @@
+using System.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("SentenceStudio.UnitTests")]
diff --git a/tests/SentenceStudio.UnitTests/Services/ClassifyContentAsyncTests.cs b/tests/SentenceStudio.UnitTests/Services/ClassifyContentAsyncTests.cs
new file mode 100644
index 00000000..bb88b92c
--- /dev/null
+++ b/tests/SentenceStudio.UnitTests/Services/ClassifyContentAsyncTests.cs
@@ -0,0 +1,173 @@
+using FluentAssertions;
+using Microsoft.Data.Sqlite;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+using Moq;
+using SentenceStudio.Abstractions;
+using SentenceStudio.Data;
+using SentenceStudio.Shared.Models;
+using SentenceStudio.Services;
+using Xunit;
+
+namespace SentenceStudio.UnitTests.Services;
+
+/// <summary>
+/// Tests for ContentImportService.ClassifyContentAsync — covering all 3 branches:
+/// null AI response, AI exception, and the 4-arm type-switch (vocabulary/phrases/sentences/transcript + unknown).
+/// </summary>
+public class ClassifyContentAsyncTests : IDisposable
+{
+    private readonly SqliteConnection _connection;
+    private readonly ServiceProvider _serviceProvider;
+    private readonly Mock<IAiService> _mockAiService;
+
+    public ClassifyContentAsyncTests()
+    {
+        _connection = new SqliteConnection("DataSource=:memory:");
+        _connection.Open();
+
+        var mockPreferences = new Mock<IPreferencesService>();
+        mockPreferences.Setup(p => p.Get(It.IsAny<string>(), It.IsAny<string>()))
+            .Returns((string key, string defaultValue) => defaultValue);
+
+        var mockFileSystem = new Mock<IFileSystemService>();
+        _mockAiService = new Mock<IAiService>();
+
+        var services = new ServiceCollection();
+
+        services.AddDbContext<ApplicationDbContext>(options =>
+        {
+            options.UseSqlite(_connection);
+            options.ConfigureWarnings(w =>
+                w.Ignore(Microsoft.EntityFrameworkCore.Diagnostics.RelationalEventId.PendingModelChangesWarning));
+        });
+
+        services.AddLogging(builder => builder.AddDebug());
+        services.AddSingleton(mockPreferences.Object);
+        services.AddSingleton<ISyncService>(new NoOpSyncService());
+        services.AddSingleton(mockFileSystem.Object);
+        services.AddSingleton(_mockAiService.Object);
+        services.AddScoped<LearningResourceRepository>();
+        services.AddScoped<VocabularyProgressRepository>();
+        services.AddScoped<ContentImportService>();
+
+        _serviceProvider = services.BuildServiceProvider();
+
+        using var scope = _serviceProvider.CreateScope();
+        var dbContext = scope.ServiceProvider.GetRequiredService<ApplicationDbContext>();
+        dbContext.Database.EnsureCreated();
+    }
+
+    public void Dispose()
+    {
+        _serviceProvider.Dispose();
+        _connection.Close();
+        _connection.Dispose();
+    }
+
+    // -----------------------------------------------------------------
+    // Branch 1: null AI response → Vocabulary / 0.5f
+    // -----------------------------------------------------------------
+
+    [Fact]
+    public async Task ClassifyContent_NullAiResponse_DefaultsToVocabulary()
+    {
+        // Arrange
+        _mockAiService
+            .Setup(s => s.SendPrompt<ContentClassificationAiResponse>(It.IsAny<string>()))
+            .ReturnsAsync((ContentClassificationAiResponse?)null);
+
+        using var scope = _serviceProvider.CreateScope();
+        var service = scope.ServiceProvider.GetRequiredService<ContentImportService>();
+
+        // Act
+        var result = await service.ClassifyContentAsync("apple,사과\nbanana,바나나", null);
+
+        // Assert
+        result.ContentType.Should().Be(ContentType.Vocabulary);
+        result.Confidence.Should().BeApproximately(0.5f, 0.001f);
+    }
+
+    // -----------------------------------------------------------------
+    // Branch 2: AI throws → Vocabulary / 0.3f with error signals
+    // -----------------------------------------------------------------
+
+    [Fact]
+    public async Task ClassifyContent_AiException_DefaultsWithLowConfidence()
+    {
+        // Arrange
+        _mockAiService
+            .Setup(s => s.SendPrompt<ContentClassificationAiResponse>(It.IsAny<string>()))
+            .ThrowsAsync(new InvalidOperationException("AI unavailable"));
+
+        using var scope = _serviceProvider.CreateScope();
+        var service = scope.ServiceProvider.GetRequiredService<ContentImportService>();
+
+        // Act
+        var result = await service.ClassifyContentAsync("apple,사과\nbanana,바나나", null);
+
+        // Assert
+        result.ContentType.Should().Be(ContentType.Vocabulary);
+        result.Confidence.Should().BeApproximately(0.3f, 0.001f);
+        result.Signals.Should().Contain("error");
+        result.Signals.Should().Contain(nameof(InvalidOperationException));
+    }
+
+    // -----------------------------------------------------------------
+    // Branch 3: type-switch arms
+    // -----------------------------------------------------------------
+
+    [Theory]
+    [InlineData("vocabulary", ContentType.Vocabulary)]
+    [InlineData("phrases", ContentType.Phrases)]
+    [InlineData("sentences", ContentType.Sentences)]
+    [InlineData("transcript", ContentType.Transcript)]
+    public async Task ClassifyContent_KnownType_MapsCorrectly(string aiType, ContentType expectedType)
+    {
+        // Arrange
+        _mockAiService
+            .Setup(s => s.SendPrompt<ContentClassificationAiResponse>(It.IsAny<string>()))
+            .ReturnsAsync(new ContentClassificationAiResponse
+            {
+                Type = aiType,
+                Confidence = 0.9f,
+                Reasoning = $"Looks like {aiType}",
+                Signals = new List<string> { aiType }
+            });
+
+        using var scope = _serviceProvider.CreateScope();
+        var service = scope.ServiceProvider.GetRequiredService<ContentImportService>();
+
+        // Act
+        var result = await service.ClassifyContentAsync("some content sample", null);
+
+        // Assert
+        result.ContentType.Should().Be(expectedType);
+        result.Confidence.Should().BeApproximately(0.9f, 0.001f);
+    }
+
+    [Fact]
+    public async Task ClassifyContent_UnknownType_FallsBackToVocabulary()
+    {
+        // Arrange — AI returns a type not in the switch arms
+        _mockAiService
+            .Setup(s => s.SendPrompt<ContentClassificationAiResponse>(It.IsAny<string>()))
+            .ReturnsAsync(new ContentClassificationAiResponse
+            {
+                Type = "unknown_future_type",
+                Confidence = 0.6f,
+                Reasoning = "unrecognised",
+                Signals = new List<string>()
+            });
+
+        using var scope = _serviceProvider.CreateScope();
+        var service = scope.ServiceProvider.GetRequiredService<ContentImportService>();
+
+        // Act
+        var result = await service.ClassifyContentAsync("some content sample", null);
+
+        // Assert
+        result.ContentType.Should().Be(ContentType.Vocabulary);
+    }
+}