Cosmos DB: DateTime serialization format causing errors in ordering and filtering

using Microsoft.EntityFrameworkCore;

var endpoint = Environment.GetEnvironmentVariable("COSMOS_ENDPOINT")
    ?? throw new InvalidOperationException("COSMOS_ENDPOINT is not set.");

var key = Environment.GetEnvironmentVariable("COSMOS_KEY")
    ?? throw new InvalidOperationException("COSMOS_KEY is not set.");

var databaseName =
    Environment.GetEnvironmentVariable("COSMOS_DATABASE")
    ?? "EfCoreDateTimeRepro";

var options = new DbContextOptionsBuilder<TestDbContext>()
    .UseCosmos(endpoint, key, databaseName)
    .EnableSensitiveDataLogging()
    .LogTo(Console.WriteLine)
    .Options;

await using var db = new TestDbContext(options);

await db.Database.EnsureCreatedAsync();

if (await db.Items.CountAsync() == 0)
{
    db.Items.AddRange(
        new TestItem
        {
            Id = "exact-second",
            Timestamp = new DateTime(
                2026, 1, 1, 0, 0, 0,
                DateTimeKind.Utc)
        },
        new TestItem
        {
            Id = "fractional-second",
            Timestamp = new DateTime(
                2026, 1, 1, 0, 0, 0, 123,
                DateTimeKind.Utc)
        });

    await db.SaveChangesAsync();
}

// The result of the inserts are these docs. Note that the timestamps have a different
// number of decimal digits.

// {
//     "id": "exact-second",
//     "$type": "TestItem",
//     "Timestamp": "2026-01-01T00:00:00Z",
//     "_rid": "fedHAL39aMkBAAAAAAAAAA==",
//     "_self": "dbs/fedHAA==/colls/fedHAL39aMk=/docs/fedHAL39aMkBAAAAAAAAAA==/",
//     "_etag": "\"02009396-0000-3000-0000-6aba61850000\"",
//     "_attachments": "attachments/",
//     "_ts": 1790599557
// }
// {
//     "id": "fractional-second",
//     "$type": "TestItem",
//     "Timestamp": "2026-01-01T00:00:00.123Z",
//     "_rid": "fedHAL39aMkCAAAAAAAAAA==",
//     "_self": "dbs/fedHAA==/colls/fedHAL39aMk=/docs/fedHAL39aMkCAAAAAAAAAA==/",
//     "_etag": "\"02009496-0000-3000-0000-6aba61850000\"",
//     "_attachments": "attachments/",
//     "_ts": 1790599557
// }


// Problem 1: Incorrect ordering.

var all = await db.Items.OrderBy(x => x.Timestamp).ToListAsync();

Console.WriteLine(string.Join(",", all.Select(x => x.Id)));

// `all` is ['fractional-second', `exact-second`]
// The order is wrong because "2026-01-01T00:00:00Z" > "2026-01-01T00:00:00.123Z".


// Problem 2: Incorrect filtering

var boundary = new DateTime(
    2026, 1, 1, 0, 0, 0,
    DateTimeKind.Utc);

var results = await db.Items
    .Where(x => x.Timestamp > boundary)
    .OrderBy(x => x.Timestamp)
    .ToListAsync();

Console.WriteLine(results.Count);

// Results is empty but it should contain "fractional-second".
// It doesn't because boundary is serialized to "2026-01-01T00:00:00Z"
// and "2026-01-01T00:00:00Z" > "2026-01-01T00:00:00.123Z".

// info: 2026/09/28 14:51:26.494 CosmosEventId.ExecutedReadNext[30102] (Microsoft.EntityFrameworkCore.Database.Command)
//       Executed ReadNext (349.2587 ms, 2.79 RU) ActivityId='a336e1d3-fe82-4e90-b053-7166421065f9', Container='Items', Partition='None', Parameters=[@boundary='2026-01-01T00:00:00Z']
//       SELECT VALUE c
//       FROM root c
//       WHERE (c["Timestamp"] > @boundary)
//       ORDER BY c["Timestamp"]


public sealed class TestDbContext(DbContextOptions<TestDbContext> options)
    : DbContext(options)
{
    public DbSet<TestItem> Items => Set<TestItem>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<TestItem>(entity =>
        {
            entity.ToContainer("Items");
            entity.HasKey(x => x.Id);
            entity.HasPartitionKey(x => x.Id);
        });
    }
}

public sealed class TestItem
{
    public string Id { get; set; } = null!;

    public DateTime Timestamp { get; set; }
}