Add typed context tier support (#1503)

* Add typed context tier support

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Trim context tier docs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

dotnet/src/Client.cs

@@ -2276,7 +2276,7 @@ internal record CreateSessionRequest(
         string? ClientName,
         string? ReasoningEffort,
         ReasoningSummary? ReasoningSummary,
-        string? ContextTier,
+        ContextTier? ContextTier,
         IList<ToolDefinition>? Tools,
         SystemMessageConfig? SystemMessage,
         IList<string>? AvailableTools,
@@ -2362,7 +2362,7 @@ internal record ResumeSessionRequest(
         string? Model,
         string? ReasoningEffort,
         ReasoningSummary? ReasoningSummary,
-        string? ContextTier,
+        ContextTier? ContextTier,
         IList<ToolDefinition>? Tools,
         SystemMessageConfig? SystemMessage,
         IList<string>? AvailableTools,

dotnet/src/Types.cs

@@ -2390,6 +2390,65 @@ public sealed class CloudSessionOptions
     public CloudSessionRepository? Repository { get; set; }
 }
 
+/// <summary>
+/// Context window tier for models that support tiered context windows.
+/// </summary>
+[JsonConverter(typeof(ContextTier.Converter))]
+[DebuggerDisplay("{Value,nq}")]
+public readonly struct ContextTier : IEquatable<ContextTier>
+{
+    private readonly string? _value;
+
+    /// <summary>Initializes a new instance of the <see cref="ContextTier"/> struct.</summary>
+    /// <param name="value">The value to associate with this <see cref="ContextTier"/>.</param>
+    [JsonConstructor]
+    public ContextTier(string value)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(value);
+        _value = value;
+    }
+
+    /// <summary>Gets the value associated with this <see cref="ContextTier"/>.</summary>
+    public string Value => _value ?? string.Empty;
+
+    /// <summary>Default context tier with standard context window size.</summary>
+    public static ContextTier Default { get; } = new("default");
+
+    /// <summary>Extended context tier with a larger context window.</summary>
+    public static ContextTier LongContext { get; } = new("long_context");
+
+    /// <summary>Returns a value indicating whether two <see cref="ContextTier"/> instances are equivalent.</summary>
+    public static bool operator ==(ContextTier left, ContextTier right) => left.Equals(right);
+
+    /// <summary>Returns a value indicating whether two <see cref="ContextTier"/> instances are not equivalent.</summary>
+    public static bool operator !=(ContextTier left, ContextTier right) => !left.Equals(right);
+
+    /// <inheritdoc/>
+    public override bool Equals([NotNullWhen(true)] object? obj) => obj is ContextTier other && Equals(other);
+
+    /// <inheritdoc/>
+    public bool Equals(ContextTier other) => string.Equals(Value, other.Value, StringComparison.OrdinalIgnoreCase);
+
+    /// <inheritdoc/>
+    public override int GetHashCode() => StringComparer.OrdinalIgnoreCase.GetHashCode(Value);
+
+    /// <inheritdoc/>
+    public override string ToString() => Value;
+
+    /// <summary>Provides a <see cref="JsonConverter{ContextTier}"/> for serializing <see cref="ContextTier"/> instances.</summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class Converter : JsonConverter<ContextTier>
+    {
+        /// <inheritdoc/>
+        public override ContextTier Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
+            new(GeneratedStringEnumJson.ReadValue(ref reader, typeToConvert));
+
+        /// <inheritdoc/>
+        public override void Write(Utf8JsonWriter writer, ContextTier value, JsonSerializerOptions options) =>
+            GeneratedStringEnumJson.WriteValue(writer, value.Value, typeof(ContextTier));
+    }
+}
+
 /// <summary>
 /// Shared configuration properties for creating or resuming a Copilot session.
 /// Use <see cref="SessionConfig"/> when creating a new session, or
@@ -2497,9 +2556,10 @@ protected SessionConfigBase(SessionConfigBase? other)
 
     /// <summary>
     /// Context window tier for models that support it.
-    /// Valid values: "default", "long_context".
+    /// Use <see cref="ContextTier.Default"/> or <see cref="ContextTier.LongContext"/>
+    /// for the currently known tiers.
     /// </summary>
-    public string? ContextTier { get; set; }
+    public ContextTier? ContextTier { get; set; }
 
     /// <summary>Per-property overrides for model capabilities, deep-merged over runtime defaults.</summary>
     public ModelCapabilitiesOverride? ModelCapabilities { get; set; }

dotnet/test/Unit/CloneTests.cs

@@ -69,7 +69,7 @@ public void SessionConfig_Clone_CopiesAllProperties()
             Model = "gpt-4",
             ReasoningEffort = "high",
             ReasoningSummary = ReasoningSummary.Detailed,
-            ContextTier = "long_context",
+            ContextTier = ContextTier.LongContext,
             ConfigDirectory = "/config",
             AvailableTools = ["tool1", "tool2"],
             ExcludedTools = ["tool3"],
@@ -386,7 +386,7 @@ public void ResumeSessionConfig_Clone_CopiesContextTier()
     {
         var original = new ResumeSessionConfig
         {
-            ContextTier = "long_context",
+            ContextTier = ContextTier.LongContext,
         };
 
         var clone = original.Clone();

dotnet/test/Unit/SerializationTests.cs

@@ -196,6 +196,31 @@ public void SessionRequests_CanSerializeReasoningSummary_WithSdkOptions()
         Assert.Equal("none", resumeDocument.RootElement.GetProperty("reasoningSummary").GetString());
     }
 
+    [Fact]
+    public void SessionRequests_CanSerializeContextTier_WithSdkOptions()
+    {
+        var options = GetSerializerOptions();
+        var createRequestType = GetNestedType(typeof(CopilotClient), "CreateSessionRequest");
+        var createRequest = CreateInternalRequest(
+            createRequestType,
+            ("SessionId", "session-id"),
+            ("ContextTier", ContextTier.LongContext));
+
+        var createJson = JsonSerializer.Serialize(createRequest, createRequestType, options);
+        using var createDocument = JsonDocument.Parse(createJson);
+        Assert.Equal("long_context", createDocument.RootElement.GetProperty("contextTier").GetString());
+
+        var resumeRequestType = GetNestedType(typeof(CopilotClient), "ResumeSessionRequest");
+        var resumeRequest = CreateInternalRequest(
+            resumeRequestType,
+            ("SessionId", "session-id"),
+            ("ContextTier", ContextTier.Default));
+
+        var resumeJson = JsonSerializer.Serialize(resumeRequest, resumeRequestType, options);
+        using var resumeDocument = JsonDocument.Parse(resumeJson);
+        Assert.Equal("default", resumeDocument.RootElement.GetProperty("contextTier").GetString());
+    }
+
     [Fact]
     public void SessionRequests_CanSerializePluginDirectoriesAndLargeOutput_WithSdkOptions()
     {

go/client_test.go

@@ -436,7 +436,7 @@ func TestSessionRequests_ReasoningSummary(t *testing.T) {
 
 func TestSessionRequests_ContextTier(t *testing.T) {
 	t.Run("create includes contextTier in JSON when set", func(t *testing.T) {
-		req := createSessionRequest{ContextTier: "long_context"}
+		req := createSessionRequest{ContextTier: ContextTierLongContext}
 		data, err := json.Marshal(req)
 		if err != nil {
 			t.Fatalf("Failed to marshal: %v", err)
@@ -451,7 +451,7 @@ func TestSessionRequests_ContextTier(t *testing.T) {
 	})
 
 	t.Run("resume includes contextTier in JSON when set", func(t *testing.T) {
-		req := resumeSessionRequest{SessionID: "s1", ContextTier: "default"}
+		req := resumeSessionRequest{SessionID: "s1", ContextTier: ContextTierDefault}
 		data, err := json.Marshal(req)
 		if err != nil {
 			t.Fatalf("Failed to marshal: %v", err)

go/types.go

@@ -859,6 +859,16 @@ type LargeToolOutputConfig struct {
 	OutputDirectory string `json:"outputDir,omitempty"`
 }
 
+// ContextTier identifies a context window tier for models that support tiered context windows.
+type ContextTier string
+
+const (
+	// ContextTierDefault is the default context tier with standard context window size.
+	ContextTierDefault ContextTier = "default"
+	// ContextTierLongContext is the extended context tier with a larger context window.
+	ContextTierLongContext ContextTier = "long_context"
+)
+
 // SessionFsCapabilities declares optional provider capabilities.
 type SessionFsCapabilities struct {
 	// Sqlite indicates whether the provider supports SQLite query/exists operations.
@@ -895,8 +905,8 @@ type SessionConfig struct {
 	// Use ReasoningSummaryNone to suppress summary output regardless of whether reasoning is enabled.
 	ReasoningSummary ReasoningSummary
 	// ContextTier pins the session to a context window tier for models that support it.
-	// Valid values: "default", "long_context".
-	ContextTier string
+	// Use ContextTierDefault or ContextTierLongContext for the currently known tiers.
+	ContextTier ContextTier
 	// ConfigDirectory overrides the default configuration directory location.
 	// When specified, the session will use this directory for storing config and state.
 	ConfigDirectory string
@@ -1298,8 +1308,8 @@ type ResumeSessionConfig struct {
 	// Use ReasoningSummaryNone to suppress summary output regardless of whether reasoning is enabled.
 	ReasoningSummary ReasoningSummary
 	// ContextTier pins the session to a context window tier for models that support it.
-	// Valid values: "default", "long_context".
-	ContextTier string
+	// Use ContextTierDefault or ContextTierLongContext for the currently known tiers.
+	ContextTier ContextTier
 	// OnPermissionRequest is an optional handler for permission requests from the server.
 	// When nil, permission requests are surfaced as events and left pending for the
 	// consumer to resolve via pending permission RPCs.
@@ -1660,7 +1670,7 @@ type createSessionRequest struct {
 	ClientName                         string                                 `json:"clientName,omitempty"`
 	ReasoningEffort                    string                                 `json:"reasoningEffort,omitempty"`
 	ReasoningSummary                   ReasoningSummary                       `json:"reasoningSummary,omitempty"`
-	ContextTier                        string                                 `json:"contextTier,omitempty"`
+	ContextTier                        ContextTier                            `json:"contextTier,omitempty"`
 	Tools                              []Tool                                 `json:"tools,omitempty"`
 	SystemMessage                      *SystemMessageConfig                   `json:"systemMessage,omitempty"`
 	AvailableTools                     []string                               `json:"availableTools"`
@@ -1738,7 +1748,7 @@ type resumeSessionRequest struct {
 	Model                              string                                 `json:"model,omitempty"`
 	ReasoningEffort                    string                                 `json:"reasoningEffort,omitempty"`
 	ReasoningSummary                   ReasoningSummary                       `json:"reasoningSummary,omitempty"`
-	ContextTier                        string                                 `json:"contextTier,omitempty"`
+	ContextTier                        ContextTier                            `json:"contextTier,omitempty"`
 	Tools                              []Tool                                 `json:"tools,omitempty"`
 	SystemMessage                      *SystemMessageConfig                   `json:"systemMessage,omitempty"`
 	AvailableTools                     []string                               `json:"availableTools"`