diff --git a/src/LinqTests/Bugs/Bug_4043_include_plus_contains.cs b/src/LinqTests/Bugs/Bug_4043_include_plus_contains.cs
new file mode 100644
index 0000000000..4f7bf18865
--- /dev/null
+++ b/src/LinqTests/Bugs/Bug_4043_include_plus_contains.cs
@@ -0,0 +1,174 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Linq.Expressions;
+using System.Threading.Tasks;
+using Marten.Linq;
+using Marten.Testing.Harness;
+
+namespace LinqTests.Bugs;
+
+public class Bug_4043_include_plus_contains : BugIntegrationContext
+{
+    /// <summary>
+    /// Contains() alone in compiled query - WORKS
+    /// </summary>
+    [Fact]
+    public async Task CompiledQuery_WithContainsOnly_Works()
+    {
+        var ownerId = Guid.NewGuid();
+        var document = new TestDocument
+        {
+            Id = Guid.NewGuid(),
+            Name = "Test",
+            Owners = [ownerId]
+        };
+
+        await using (var session = theStore!.LightweightSession())
+        {
+            session.Store(document);
+            await session.SaveChangesAsync();
+        }
+
+        await using (var session = theStore.QuerySession())
+        {
+            var query = new FindByOwnerQuery { OwnerId = ownerId };
+            var results = await session.QueryAsync(query);
+
+            Assert.Single(results);
+            Assert.Equal(document.Id, results.First().Id);
+        }
+    }
+
+    /// <summary>
+    /// Include() alone in compiled query - WORKS
+    /// </summary>
+    [Fact]
+    public async Task CompiledQuery_WithIncludeOnly_Works()
+    {
+        var relatedDoc = new RelatedDocument { Id = Guid.NewGuid(), Description = "Related" };
+        var document = new TestDocument
+        {
+            Id = Guid.NewGuid(),
+            Name = "Test",
+            Owners = [],
+            RelatedDocumentId = relatedDoc.Id
+        };
+
+        await using (var session = theStore!.LightweightSession())
+        {
+            session.Store(relatedDoc);
+            session.Store(document);
+            await session.SaveChangesAsync();
+        }
+
+        await using (var session = theStore.QuerySession())
+        {
+            var query = new FindByIdWithIncludeQuery { DocumentId = document.Id };
+            var results = await session.QueryAsync(query);
+
+            Assert.Single(results);
+            Assert.Single(query.RelatedDocuments);
+        }
+    }
+
+    /// <summary>
+    /// Include() + Contains() in compiled query - FAILS with code generation error!
+    ///
+    /// The generated C# code contains unescaped JSON:
+    ///   parameters[0].Value = "{"Owners":["guid"]}";  // INVALID
+    /// Should be:
+    ///   parameters[0].Value = "{\"Owners\":[\"guid\"]}";  // VALID
+    /// </summary>
+    [Fact]
+    public async Task CompiledQuery_WithIncludeAndContains_Fails()
+    {
+        var ownerId = Guid.NewGuid();
+        var relatedDoc = new RelatedDocument { Id = Guid.NewGuid(), Description = "Related" };
+        var document = new TestDocument
+        {
+            Id = Guid.NewGuid(),
+            Name = "Test",
+            Owners = [ownerId],
+            RelatedDocumentId = relatedDoc.Id
+        };
+
+        await using (var session = theStore!.LightweightSession())
+        {
+            session.Store(relatedDoc);
+            session.Store(document);
+            await session.SaveChangesAsync();
+        }
+
+        await using (var session = theStore.QuerySession())
+        {
+            var query = new FindByOwnerWithIncludeQuery { OwnerId = ownerId };
+
+            // This throws due to invalid code generation
+            var exception = await Assert.ThrowsAnyAsync<Exception>(async () =>
+            {
+                await session.QueryAsync(query);
+            });
+
+            // Should contain CS1002 or similar code generation error
+            Assert.True(
+                exception.Message.Contains("CS1002") ||
+                exception.Message.Contains("CS0103") ||
+                exception.Message.Contains("Compilation failures"),
+                $"Expected code generation error, got: {exception.Message}");
+        }
+    }
+}
+
+
+public class TestDocument
+{
+    public Guid Id { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public HashSet<Guid> Owners { get; set; } = [];
+    public Guid RelatedDocumentId { get; set; }
+}
+
+public class RelatedDocument
+{
+    public Guid Id { get; set; }
+    public string Description { get; set; } = string.Empty;
+}
+
+
+public class FindByOwnerQuery : ICompiledListQuery<TestDocument>
+{
+    public Guid OwnerId { get; set; }
+
+    public Expression<Func<IMartenQueryable<TestDocument>, IEnumerable<TestDocument>>> QueryIs()
+    {
+        return q => q.Where(x => x.Owners.Contains(OwnerId));
+    }
+}
+
+public class FindByIdWithIncludeQuery : ICompiledListQuery<TestDocument>
+{
+    public Guid DocumentId { get; set; }
+    public Dictionary<Guid, RelatedDocument> RelatedDocuments { get; } = [];
+
+    public Expression<Func<IMartenQueryable<TestDocument>, IEnumerable<TestDocument>>> QueryIs()
+    {
+        return q => q
+            .Include(x => x.RelatedDocumentId, RelatedDocuments)
+            .Where(x => x.Id == DocumentId);
+    }
+}
+
+public class FindByOwnerWithIncludeQuery : ICompiledListQuery<TestDocument>
+{
+    public Guid OwnerId { get; set; }
+    public Dictionary<Guid, RelatedDocument> RelatedDocuments { get; } = [];
+
+    public Expression<Func<IMartenQueryable<TestDocument>, IEnumerable<TestDocument>>> QueryIs()
+    {
+        return q => q
+            .Include(x => x.RelatedDocumentId, RelatedDocuments)
+            .Where(x => x.Owners.Contains(OwnerId));
+    }
+}
+