diff --git a/OpenProject.Api.Test/Tests/WorkPackageScheduleJsonTests.cs b/OpenProject.Api.Test/Tests/WorkPackageScheduleJsonTests.cs
new file mode 100644
index 0000000..2f7dba9
--- /dev/null
+++ b/OpenProject.Api.Test/Tests/WorkPackageScheduleJsonTests.cs
@@ -0,0 +1,62 @@
+using OpenProject.Api.Data.Models;
+using OpenProject.Api.Data.Models.Update;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace OpenProject.Api.Test.Tests;
+
+/// <summary>
+/// Work-package schedule fields are grouped in <see cref="WorkPackageSchedule"/> but stay flat in OpenProject's JSON.
+/// </summary>
+public class WorkPackageScheduleJsonTests
+{
+	private static readonly JsonSerializerOptions Options = OpenProjectClient.CreateJsonOptions();
+
+	[Fact]
+	public void WorkPackage_ReadsFlatScheduleFieldsIntoSchedule()
+	{
+		var workPackage = JsonSerializer.Deserialize<WorkPackage>(
+			"""
+			{"_type":"WorkPackage","_links":{},"id":7,"subject":"Ship it","percentageDone":10,"derivedPercentageDone":20,
+			"scheduleManually":true,"startDate":"2026-11-02T00:00:00","dueDate":"2026-11-06T00:00:00",
+			"date":null,"estimatedTime":"PT8H","derivedEstimatedTime":"PT16H"}
+			""",
+			Options)!;
+
+		workPackage.Id.Should().Be(7);
+		workPackage.Subject.Should().Be("Ship it");
+		workPackage.Schedule.ScheduleManually.Should().BeTrue();
+		workPackage.Schedule.StartDate.Should().Be(new DateTime(2026, 11, 2));
+		workPackage.Schedule.DueDate.Should().Be(new DateTime(2026, 11, 6));
+		workPackage.Schedule.Date.Should().BeNull();
+		workPackage.Schedule.EstimatedTime.Should().Be("PT8H");
+		workPackage.Schedule.DerivedEstimatedTime.Should().Be("PT16H");
+	}
+
+	[Fact]
+	public void WorkPackageUpdate_WritesScheduleFieldsFlatAndLeavesNullsOut()
+	{
+		var update = new WorkPackageUpdate
+		{
+			LockVersion = 3,
+			Schedule = new WorkPackageSchedule { StartDate = new DateTime(2026, 11, 2), EstimatedTime = "PT4H" }
+		};
+
+		var json = JsonNode.Parse(JsonSerializer.Serialize(update, Options))!.AsObject();
+
+		json.Select(property => property.Key).Should().BeEquivalentTo("lockVersion", "startDate", "estimatedTime");
+		json["lockVersion"]!.GetValue<int>().Should().Be(3);
+		json["startDate"]!.GetValue<string>().Should().Be("2026-11-02T00:00:00");
+		json["estimatedTime"]!.GetValue<string>().Should().Be("PT4H");
+	}
+
+	[Fact]
+	public void WorkPackagesInACollection_AreReadThroughTheSameConverter()
+	{
+		var workPackages = JsonSerializer.Deserialize<List<WorkPackage>>(
+			"""[{"_type":"WorkPackage","_links":{},"id":1,"subject":"a","percentageDone":0,"derivedPercentageDone":0,"dueDate":"2026-12-24T00:00:00"}]""",
+			Options)!;
+
+		workPackages.Should().ContainSingle().Which.Schedule.DueDate.Should().Be(new DateTime(2026, 12, 24));
+	}
+}
diff --git a/OpenProject.Api/Data/FlatScheduleConverterFactory.cs b/OpenProject.Api/Data/FlatScheduleConverterFactory.cs
new file mode 100644
index 0000000..2d3914c
--- /dev/null
+++ b/OpenProject.Api/Data/FlatScheduleConverterFactory.cs
@@ -0,0 +1,63 @@
+using OpenProject.Api.Data.Models;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using Type = System.Type;
+
+namespace OpenProject.Api.Data;
+
+/// <summary>
+/// Reads and writes <see cref="IHasSchedule"/> models with their <see cref="WorkPackageSchedule"/> fields at the
+/// top level of the JSON object, where OpenProject puts them.
+/// </summary>
+internal sealed class FlatScheduleConverterFactory : JsonConverterFactory
+{
+	public override bool CanConvert(Type typeToConvert)
+		=> typeToConvert.IsClass && typeof(IHasSchedule).IsAssignableFrom(typeToConvert);
+
+	public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
+		=> (JsonConverter)Activator.CreateInstance(
+			typeof(FlatScheduleConverter<>).MakeGenericType(typeToConvert),
+			WithoutThisFactory(options))!;
+
+	// Without this, serializing the model inside the converter would re-enter the converter.
+	private static JsonSerializerOptions WithoutThisFactory(JsonSerializerOptions options)
+	{
+		var inner = new JsonSerializerOptions(options);
+		for (var i = inner.Converters.Count - 1; i >= 0; i--)
+		{
+			if (inner.Converters[i] is FlatScheduleConverterFactory)
+			{
+				inner.Converters.RemoveAt(i);
+			}
+		}
+
+		return inner;
+	}
+
+	private sealed class FlatScheduleConverter<T>(JsonSerializerOptions inner) : JsonConverter<T>
+		where T : class, IHasSchedule
+	{
+		public override T? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+		{
+			using var document = JsonDocument.ParseValue(ref reader);
+			var item = document.RootElement.Deserialize<T>(inner);
+			if (item is not null)
+			{
+				item.Schedule = document.RootElement.Deserialize<WorkPackageSchedule>(inner) ?? new();
+			}
+
+			return item;
+		}
+
+		public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
+		{
+			var node = JsonSerializer.SerializeToNode(value, inner)!.AsObject();
+			foreach (var (name, field) in JsonSerializer.SerializeToNode(value.Schedule, inner)!.AsObject())
+			{
+				node[name] = field?.DeepClone();
+			}
+
+			node.WriteTo(writer, inner);
+		}
+	}
+}
diff --git a/OpenProject.Api/Data/Models/IHasSchedule.cs b/OpenProject.Api/Data/Models/IHasSchedule.cs
new file mode 100644
index 0000000..f794b71
--- /dev/null
+++ b/OpenProject.Api/Data/Models/IHasSchedule.cs
@@ -0,0 +1,12 @@
+namespace OpenProject.Api.Data.Models;
+
+/// <summary>
+/// A model whose schedule fields are flat in OpenProject's JSON but grouped in <see cref="Schedule"/>.
+/// </summary>
+public interface IHasSchedule
+{
+	/// <summary>
+	/// The scheduling and estimate fields.
+	/// </summary>
+	WorkPackageSchedule Schedule { get; set; }
+}
diff --git a/OpenProject.Api/Data/Models/Update/WorkPackageUpdate.cs b/OpenProject.Api/Data/Models/Update/WorkPackageUpdate.cs
index 1f093cf..10befcd 100644
--- a/OpenProject.Api/Data/Models/Update/WorkPackageUpdate.cs
+++ b/OpenProject.Api/Data/Models/Update/WorkPackageUpdate.cs
@@ -1,11 +1,12 @@
-using System.Text.Json.Serialization;
+using OpenProject.Api.Data.Models;
+using System.Text.Json.Serialization;
 
 namespace OpenProject.Api.Data.Models.Update;
 
 /// <summary>
 /// Represents the fields that can be updated on an existing work package.
 /// </summary>
-public class WorkPackageUpdate
+public class WorkPackageUpdate : IHasSchedule
 {
 	/// <summary>
 	/// The value of lockVersion is used to implement optimistic locking.
@@ -26,38 +27,8 @@ public class WorkPackageUpdate
 	public Formattable? Description { get; set; }
 
 	/// <summary>
-	/// If false (default) schedule automatically.
+	/// Scheduling and estimate fields to change; fields left null are not sent.
 	/// </summary>
-	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-	public bool? ScheduleManually { get; set; }
-
-	/// <summary>
-	/// Scheduled beginning of a work package
-	/// </summary>
-	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-	public DateTime? StartDate { get; set; }
-
-	/// <summary>
-	/// Scheduled end of a work package
-	/// </summary>
-	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-	public DateTime? DueDate { get; set; }
-
-	/// <summary>
-	/// Date on which a milestone is achieved
-	/// </summary>
-	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-	public DateTime? Date { get; set; }
-
-	/// <summary>
-	/// Corresponds to work. Time a work package likely needs to be completed.	
-	/// </summary>
-	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-	public string? EstimatedTime { get; set; }
-
-	/// <summary>
-	/// Corresponds to total work. Time a work package likely needs to be completed including itself and its descendants.
-	/// </summary>
-	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-	public string? DerivedEstimatedTime { get; set; }
+	[JsonIgnore]
+	public WorkPackageSchedule Schedule { get; set; } = new();
 }
diff --git a/OpenProject.Api/Data/Models/WorkPackage.cs b/OpenProject.Api/Data/Models/WorkPackage.cs
index 7c79877..80f400d 100644
--- a/OpenProject.Api/Data/Models/WorkPackage.cs
+++ b/OpenProject.Api/Data/Models/WorkPackage.cs
@@ -6,7 +6,7 @@ namespace OpenProject.Api.Data.Models;
 /// A work package in OpenProject can basically be everything you need to keep track off within your projects. It can be e.g. a task, a feature, a bug, a risk, a milestone or a project phase.
 /// <para>See <a href='https://www.openproject.org/docs/getting-started/work-packages-introduction/'/></para>
 /// </summary>
-public class WorkPackage : IdentifiedItem<int>, IHasTimestamps
+public class WorkPackage : IdentifiedItem<int>, IHasTimestamps, IHasSchedule
 {
 	/// <summary>
 	/// The version of the item as used for optimistic locking
@@ -27,24 +27,10 @@ public class WorkPackage : IdentifiedItem<int>, IHasTimestamps
 	public Formattable? Description { get; set; }
 
 	/// <summary>
-	/// If false (default) schedule automatically.
+	/// Scheduling and estimate fields: start, due and milestone dates, manual scheduling and estimated time.
 	/// </summary>
-	public bool? ScheduleManually { get; set; }
-
-	/// <summary>
-	/// Scheduled beginning of a work package	
-	/// </summary>
-	public DateTime? StartDate { get; set; }
-
-	/// <summary>
-	/// Scheduled end of a work package
-	/// </summary>
-	public DateTime? DueDate { get; set; }
-
-	/// <summary>
-	/// Date on which a milestone is achieved
-	/// </summary>
-	public DateTime? Date { get; set; }
+	[JsonIgnore]
+	public WorkPackageSchedule Schedule { get; set; } = new();
 
 	/// <summary>
 	/// Similar to start date but is not set by a client but rather deduced by the work packages’ descendants. If manual scheduleManually is active, the two dates can deviate.
@@ -61,16 +47,6 @@ public class WorkPackage : IdentifiedItem<int>, IHasTimestamps
 	/// </summary>
 	public string? Duration { get; set; }
 
-	/// <summary>
-	/// Corresponds to work. Time a work package likely needs to be completed.
-	/// </summary>
-	public string? EstimatedTime { get; set; }
-
-	/// <summary>
-	/// Corresponds to total work. Time a work package likely needs to be completed including itself and its descendants.	
-	/// </summary>
-	public string? DerivedEstimatedTime { get; set; }
-
 	/// <summary>
 	/// Corresponds to remaining work. Remaining time a work package likely needs to be completed.
 	/// </summary>
diff --git a/OpenProject.Api/Data/Models/WorkPackageSchedule.cs b/OpenProject.Api/Data/Models/WorkPackageSchedule.cs
new file mode 100644
index 0000000..0331539
--- /dev/null
+++ b/OpenProject.Api/Data/Models/WorkPackageSchedule.cs
@@ -0,0 +1,46 @@
+using System.Text.Json.Serialization;
+
+namespace OpenProject.Api.Data.Models;
+
+/// <summary>
+/// The scheduling and estimate fields of a work package. They are top-level fields in OpenProject's JSON;
+/// <see cref="IHasSchedule"/> models group them here.
+/// </summary>
+public class WorkPackageSchedule
+{
+	/// <summary>
+	/// If false (default) schedule automatically.
+	/// </summary>
+	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+	public bool? ScheduleManually { get; set; }
+
+	/// <summary>
+	/// Scheduled beginning of a work package
+	/// </summary>
+	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+	public DateTime? StartDate { get; set; }
+
+	/// <summary>
+	/// Scheduled end of a work package
+	/// </summary>
+	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+	public DateTime? DueDate { get; set; }
+
+	/// <summary>
+	/// Date on which a milestone is achieved
+	/// </summary>
+	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+	public DateTime? Date { get; set; }
+
+	/// <summary>
+	/// Corresponds to work. Time a work package likely needs to be completed.
+	/// </summary>
+	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+	public string? EstimatedTime { get; set; }
+
+	/// <summary>
+	/// Corresponds to total work. Time a work package likely needs to be completed including itself and its descendants.
+	/// </summary>
+	[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+	public string? DerivedEstimatedTime { get; set; }
+}
diff --git a/OpenProject.Api/OpenProject.Api.csproj b/OpenProject.Api/OpenProject.Api.csproj
index 308e46f..5e4bc5a 100644
--- a/OpenProject.Api/OpenProject.Api.csproj
+++ b/OpenProject.Api/OpenProject.Api.csproj
@@ -38,4 +38,7 @@
     <None Include="Icon.png" Pack="true" PackagePath="" />
     <None Include="..\README.md" Pack="true" PackagePath="\" />
   </ItemGroup>
+  <ItemGroup>
+    <InternalsVisibleTo Include="OpenProject.Api.Test" />
+  </ItemGroup>
 </Project>
\ No newline at end of file
diff --git a/OpenProject.Api/OpenProjectClient.cs b/OpenProject.Api/OpenProjectClient.cs
index 252be26..ce89623 100644
--- a/OpenProject.Api/OpenProjectClient.cs
+++ b/OpenProject.Api/OpenProjectClient.cs
@@ -29,6 +29,7 @@ public OpenProjectClient(HttpClient client)
 		_httpClient = client;
 		_refitSettings = new RefitSettings
 		{
+			ContentSerializer = new SystemTextJsonContentSerializer(CreateJsonOptions()),
 			//UrlParameterFormatter = new OpenProjectUrlParameterFormatter(),
 		};
 
@@ -59,6 +60,16 @@ public OpenProjectClient(HttpClient client)
 		Types = RefitFor<ITypes>();
 	}
 
+	/// <summary>
+	/// Refit's default JSON options, plus the converter that keeps work-package schedule fields flat on the wire.
+	/// </summary>
+	internal static System.Text.Json.JsonSerializerOptions CreateJsonOptions()
+	{
+		var options = SystemTextJsonContentSerializer.GetDefaultJsonSerializerOptions();
+		options.Converters.Add(new FlatScheduleConverterFactory());
+		return options;
+	}
+
 	private T RefitFor<T>()
 		=> RestService.For<T>(_httpClient, _refitSettings);
 
diff --git a/version.json b/version.json
index 6f8af6b..4ecc882 100644
--- a/version.json
+++ b/version.json
@@ -1,6 +1,6 @@
 {
 	"$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/master/src/NerdBank.GitVersioning/version.schema.json",
-	"version": "3.0",
+	"version": "3.1",
 	"publicReleaseRefSpec": [
 		"^refs/heads/main$",
 		"^refs/heads/release/\\d+\\.\\d+$",