Expose context tier in SDK set model APIs

dotnet/src/Session.cs

@@ -1576,14 +1576,40 @@ public async Task AbortAsync(CancellationToken cancellationToken = default)
     /// <code>
     /// await session.SetModelAsync("gpt-4.1");
     /// await session.SetModelAsync("claude-sonnet-4.6", "high");
+    /// await session.SetModelAsync("gpt-4.1", new SetModelOptions { ContextTier = ContextTier.LongContext });
     /// </code>
     /// </example>
-    public async Task SetModelAsync(string model, string? reasoningEffort, ModelCapabilitiesOverride? modelCapabilities = null, CancellationToken cancellationToken = default)
+    public Task SetModelAsync(string model, string? reasoningEffort, ModelCapabilitiesOverride? modelCapabilities = null, CancellationToken cancellationToken = default)
+    {
+        return SetModelAsync(
+            model,
+            new SetModelOptions
+            {
+                ReasoningEffort = reasoningEffort,
+                ModelCapabilities = modelCapabilities,
+            },
+            cancellationToken);
+    }
+
+    /// <summary>
+    /// Changes the model for this session.
+    /// The new model takes effect for the next message. Conversation history is preserved.
+    /// </summary>
+    /// <param name="model">Model ID to switch to (e.g., "gpt-4.1").</param>
+    /// <param name="options">Settings for the new model.</param>
+    /// <param name="cancellationToken">Optional cancellation token.</param>
+    public async Task SetModelAsync(string model, SetModelOptions options, CancellationToken cancellationToken = default)
     {
         ArgumentNullException.ThrowIfNull(model);
         ThrowIfDisposed();
 
-        await Rpc.Model.SwitchToAsync(model, reasoningEffort, reasoningSummary: null, modelCapabilities: modelCapabilities, cancellationToken: cancellationToken);
+        await Rpc.Model.SwitchToAsync(
+            model,
+            options.ReasoningEffort,
+            options.ReasoningSummary,
+            options.ModelCapabilities,
+            options.ContextTier,
+            cancellationToken);
     }
 
     /// <summary>
@@ -1593,7 +1619,7 @@ public Task SetModelAsync(string model, CancellationToken cancellationToken = de
     {
         ThrowIfDisposed();
 
-        return SetModelAsync(model, reasoningEffort: null, modelCapabilities: null, cancellationToken);
+        return SetModelAsync(model, new SetModelOptions(), cancellationToken);
     }
 
     /// <summary>

dotnet/src/Types.cs

@@ -2449,6 +2449,33 @@ public override void Write(Utf8JsonWriter writer, ContextTier value, JsonSeriali
     }
 }
 
+/// <summary>
+/// Optional settings for <see cref="CopilotSession.SetModelAsync(string, SetModelOptions, CancellationToken)"/>.
+/// </summary>
+public struct SetModelOptions
+{
+    /// <summary>
+    /// Reasoning effort level for the new model.
+    /// </summary>
+    public string? ReasoningEffort { get; set; }
+
+    /// <summary>
+    /// Reasoning summary mode for models that support configurable reasoning summaries.
+    /// </summary>
+    /// <remarks>
+    /// Use <see cref="ReasoningSummary.None"/> to suppress summary output regardless of whether reasoning is enabled.
+    /// </remarks>
+    public ReasoningSummary? ReasoningSummary { get; set; }
+
+    /// <summary>
+    /// Context window tier for models that support it.
+    /// </summary>
+    public ContextTier? ContextTier { get; set; }
+
+    /// <summary>Per-property overrides for model capabilities, deep-merged over runtime defaults.</summary>
+    public ModelCapabilitiesOverride? ModelCapabilities { get; set; }
+}
+
 /// <summary>
 /// Shared configuration properties for creating or resuming a Copilot session.
 /// Use <see cref="SessionConfig"/> when creating a new session, or
@@ -3556,6 +3583,7 @@ public sealed class SystemMessageTransformRpcResponse
 [JsonSerializable(typeof(SessionListFilter))]
 [JsonSerializable(typeof(SectionOverride))]
 [JsonSerializable(typeof(SessionMetadata))]
+[JsonSerializable(typeof(SetModelOptions))]
 [JsonSerializable(typeof(SetForegroundSessionResponse))]
 [JsonSerializable(typeof(SystemMessageConfig))]
 [JsonSerializable(typeof(ToolBinaryResult))]

dotnet/test/Unit/SerializationTests.cs

@@ -87,7 +87,7 @@ public void SendMessageRequest_CanSerializeRequestHeaders_WithSdkOptions()
             ("Mode", "enqueue"),
             ("RequestHeaders", new Dictionary<string, string> { ["X-Trace"] = "trace-value" }));
 
-        var json = JsonSerializer.Serialize(request, requestType, options);
+        var json = JsonSerializer.Serialize(request, requestType!, options);
         using var document = JsonDocument.Parse(json);
         var root = document.RootElement;
         Assert.Equal("session-id", root.GetProperty("sessionId").GetString());
@@ -221,6 +221,25 @@ public void SessionRequests_CanSerializeContextTier_WithSdkOptions()
         Assert.Equal("default", resumeDocument.RootElement.GetProperty("contextTier").GetString());
     }
 
+    [Fact]
+    public void ModelSwitchToRequest_CanSerializeContextTier_WithSdkOptions()
+    {
+        var options = GetSerializerOptions();
+        var requestType = typeof(CopilotClient).Assembly.GetType("GitHub.Copilot.Rpc.ModelSwitchToRequest");
+        Assert.NotNull(requestType);
+        var request = CreateInternalRequest(
+            requestType!,
+            ("ModelId", "gpt-4.1"),
+            ("ReasoningSummary", ReasoningSummary.Detailed),
+            ("ContextTier", ContextTier.LongContext));
+
+        var json = JsonSerializer.Serialize(request, requestType, options);
+        using var document = JsonDocument.Parse(json);
+        Assert.Equal("gpt-4.1", document.RootElement.GetProperty("modelId").GetString());
+        Assert.Equal("detailed", document.RootElement.GetProperty("reasoningSummary").GetString());
+        Assert.Equal("long_context", document.RootElement.GetProperty("contextTier").GetString());
+    }
+
     [Fact]
     public void SessionRequests_CanSerializePluginDirectoriesAndLargeOutput_WithSdkOptions()
     {

go/session.go

@@ -1495,6 +1495,9 @@ type SetModelOptions struct {
 	// ReasoningSummary sets the reasoning summary mode for the new model.
 	// 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.
+	// Use ContextTierDefault or ContextTierLongContext for the currently known tiers.
+	ContextTier *ContextTier
 	// ModelCapabilities overrides individual model capabilities resolved by the runtime.
 	// Only non-nil fields are applied over the runtime-resolved capabilities.
 	ModelCapabilities *rpc.ModelCapabilitiesOverride
@@ -1516,6 +1519,7 @@ func (s *Session) SetModel(ctx context.Context, model string, opts *SetModelOpti
 	if opts != nil {
 		params.ReasoningEffort = opts.ReasoningEffort
 		params.ReasoningSummary = opts.ReasoningSummary
+		params.ContextTier = opts.ContextTier
 		params.ModelCapabilities = opts.ModelCapabilities
 	}
 	_, err := s.RPC.Model.SwitchTo(ctx, params)