diff --git a/dotnet/src/Generated/Rpc.cs b/dotnet/src/Generated/Rpc.cs
index feded0531..88463e73b 100644
--- a/dotnet/src/Generated/Rpc.cs
+++ b/dotnet/src/Generated/Rpc.cs
@@ -2396,7 +2396,7 @@ public sealed class ModelCapabilitiesOverride
 [Experimental(Diagnostics.Experimental)]
 internal sealed class ModelSwitchToRequest
 {
-    /// <summary>Explicit context tier for the selected model. `"default"` / `"long_context"` pin the tier; `null` clears any previous explicit choice; `undefined` leaves the existing tier untouched.</summary>
+    /// <summary>Explicit context tier for the selected model. `"default"` / `"long_context"` apply the requested tier; omit this field to use normal model behavior with no explicit tier.</summary>
     [JsonPropertyName("contextTier")]
     public ContextTier? ContextTier { get; set; }
 
@@ -13851,7 +13851,7 @@ public async Task<CurrentModel> GetCurrentAsync(CancellationToken cancellationTo
     /// <param name="reasoningEffort">Reasoning effort level to use for the model. "none" disables reasoning.</param>
     /// <param name="reasoningSummary">Reasoning summary mode to request for supported model clients.</param>
     /// <param name="modelCapabilities">Override individual model capabilities resolved by the runtime.</param>
-    /// <param name="contextTier">Explicit context tier for the selected model. `"default"` / `"long_context"` pin the tier; `null` clears any previous explicit choice; `undefined` leaves the existing tier untouched.</param>
+    /// <param name="contextTier">Explicit context tier for the selected model. `"default"` / `"long_context"` apply the requested tier; omit this field to use normal model behavior with no explicit tier.</param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>The model identifier active on the session after the switch.</returns>
     public async Task<ModelSwitchToResult> SwitchToAsync(string modelId, string? reasoningEffort = null, ReasoningSummary? reasoningSummary = null, ModelCapabilitiesOverride? modelCapabilities = null, ContextTier? contextTier = null, CancellationToken cancellationToken = default)
diff --git a/dotnet/src/Session.cs b/dotnet/src/Session.cs
index 4140cf6e3..bd369867f 100644
--- a/dotnet/src/Session.cs
+++ b/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>
diff --git a/dotnet/src/Types.cs b/dotnet/src/Types.cs
index 99fa08c2b..7a2ad2951 100644
--- a/dotnet/src/Types.cs
+++ b/dotnet/src/Types.cs
@@ -2390,6 +2390,34 @@ public sealed class CloudSessionOptions
     public CloudSessionRepository? Repository { get; set; }
 }
 
+/// <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>
+    /// Explicit context window tier for models that support it.
+    /// Leave unset to use normal model behavior with no explicit tier.
+    /// </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
diff --git a/go/rpc/zrpc.go b/go/rpc/zrpc.go
index af2ac3fdd..a760a715b 100644
--- a/go/rpc/zrpc.go
+++ b/go/rpc/zrpc.go
@@ -2640,9 +2640,8 @@ type ModelsListRequest struct {
 // Experimental: ModelSwitchToRequest is part of an experimental API and may change or be
 // removed.
 type ModelSwitchToRequest struct {
-	// Explicit context tier for the selected model. `"default"` / `"long_context"` pin the
-	// tier; `null` clears any previous explicit choice; `undefined` leaves the existing tier
-	// untouched.
+	// Explicit context tier for the selected model. `"default"` / `"long_context"` apply the
+	// requested tier; omit this field to use normal model behavior with no explicit tier.
 	ContextTier *ContextTier `json:"contextTier,omitempty"`
 	// Override individual model capabilities resolved by the runtime
 	ModelCapabilities *ModelCapabilitiesOverride `json:"modelCapabilities,omitempty"`
diff --git a/go/session.go b/go/session.go
index 11b1a3d65..6e68631eb 100644
--- a/go/session.go
+++ b/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 explicitly selects a context window tier for models that support it.
+	// Leave nil to use normal model behavior with no explicit tier.
+	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)
diff --git a/go/session_test.go b/go/session_test.go
index 405d7bf7c..b1c36d175 100644
--- a/go/session_test.go
+++ b/go/session_test.go
@@ -1,13 +1,20 @@
 package copilot
 
 import (
+	"bufio"
+	"context"
 	"encoding/json"
 	"fmt"
+	"io"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
+
+	"github.com/github/copilot-sdk/go/internal/jsonrpc2"
+	"github.com/github/copilot-sdk/go/rpc"
 )
 
 // newTestSession creates a session with an event channel and starts the consumer goroutine.
@@ -30,6 +37,136 @@ func ptr[T any](value T) *T {
 	return &value
 }
 
+func TestSession_SetModelForwardsContextTier(t *testing.T) {
+	tier := ContextTierLongContext
+	params := captureSetModelRequest(t, &SetModelOptions{ContextTier: &tier})
+
+	if params["sessionId"] != "session-1" {
+		t.Fatalf("expected sessionId session-1, got %v", params["sessionId"])
+	}
+	if params["modelId"] != "gpt-4.1" {
+		t.Fatalf("expected modelId gpt-4.1, got %v", params["modelId"])
+	}
+	if params["contextTier"] != "long_context" {
+		t.Fatalf("expected contextTier long_context, got %v", params["contextTier"])
+	}
+}
+
+func TestSession_SetModelOmitsContextTierWhenUnset(t *testing.T) {
+	params := captureSetModelRequest(t, nil)
+
+	if _, ok := params["contextTier"]; ok {
+		t.Fatalf("expected contextTier to be omitted, got %v", params["contextTier"])
+	}
+}
+
+func captureSetModelRequest(t *testing.T, opts *SetModelOptions) map[string]any {
+	t.Helper()
+
+	stdinR, stdinW := io.Pipe()
+	stdoutR, stdoutW := io.Pipe()
+	defer stdinR.Close()
+	defer stdinW.Close()
+	defer stdoutR.Close()
+	defer stdoutW.Close()
+
+	client := jsonrpc2.NewClient(stdinW, stdoutR)
+	client.Start()
+	defer client.Stop()
+
+	paramsCh := make(chan map[string]any, 1)
+	errCh := make(chan error, 1)
+
+	go func() {
+		frame, err := readTestJSONRPCFrame(stdinR)
+		if err != nil {
+			errCh <- err
+			return
+		}
+
+		var request struct {
+			ID     json.RawMessage `json:"id"`
+			Method string          `json:"method"`
+			Params map[string]any  `json:"params"`
+		}
+		if err := json.Unmarshal(frame, &request); err != nil {
+			errCh <- err
+			return
+		}
+		if request.Method != "session.model.switchTo" {
+			errCh <- fmt.Errorf("expected session.model.switchTo, got %s", request.Method)
+			return
+		}
+
+		paramsCh <- request.Params
+
+		response := map[string]any{
+			"jsonrpc": "2.0",
+			"id":      json.RawMessage(request.ID),
+			"result":  map[string]any{},
+		}
+		data, err := json.Marshal(response)
+		if err != nil {
+			errCh <- err
+			return
+		}
+		if _, err := fmt.Fprintf(stdoutW, "Content-Length: %d\r\n\r\n%s", len(data), data); err != nil {
+			errCh <- err
+			return
+		}
+	}()
+
+	session := &Session{
+		SessionID: "session-1",
+		client:    client,
+		RPC:       rpc.NewSessionRpc(client, "session-1"),
+	}
+	if err := session.SetModel(context.Background(), "gpt-4.1", opts); err != nil {
+		t.Fatalf("SetModel failed: %v", err)
+	}
+
+	select {
+	case params := <-paramsCh:
+		return params
+	case err := <-errCh:
+		t.Fatal(err)
+	case <-time.After(2 * time.Second):
+		t.Fatal("timed out waiting for session.model.switchTo request")
+	}
+	return nil
+}
+
+func readTestJSONRPCFrame(r io.Reader) ([]byte, error) {
+	reader := bufio.NewReader(r)
+	var contentLength int
+	for {
+		line, err := reader.ReadString('\n')
+		if err != nil {
+			return nil, err
+		}
+		line = strings.TrimSpace(line)
+		if line == "" {
+			break
+		}
+		name, value, ok := strings.Cut(line, ":")
+		if !ok {
+			return nil, fmt.Errorf("invalid header line %q", line)
+		}
+		if name == "Content-Length" {
+			contentLength, err = strconv.Atoi(strings.TrimSpace(value))
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+	if contentLength == 0 {
+		return nil, fmt.Errorf("missing Content-Length header")
+	}
+	data := make([]byte, contentLength)
+	_, err := io.ReadFull(reader, data)
+	return data, err
+}
+
 func TestSession_On(t *testing.T) {
 	t.Run("multiple handlers all receive events", func(t *testing.T) {
 		session, cleanup := newTestSession()
diff --git a/nodejs/package-lock.json b/nodejs/package-lock.json
index b18907204..46a3088b4 100644
--- a/nodejs/package-lock.json
+++ b/nodejs/package-lock.json
@@ -9,7 +9,7 @@
             "version": "0.0.0-dev",
             "license": "MIT",
             "dependencies": {
-                "@github/copilot": "^1.0.57-3",
+                "@github/copilot": "^1.0.57-4",
                 "vscode-jsonrpc": "^8.2.1",
                 "zod": "^4.3.6"
             },
@@ -663,9 +663,9 @@
             }
         },
         "node_modules/@github/copilot": {
-            "version": "1.0.57-3",
-            "resolved": "https://registry.npmjs.org/@github/copilot/-/copilot-1.0.57-3.tgz",
-            "integrity": "sha512-Yo0ebyTybaoZI1TsCOxlzwEn6jxHR2eiiZkTXLMtj2wnXj/TZZWUGCWQo/LltvFPjOjLbe0TR6pzmzKNr3v0VA==",
+            "version": "1.0.57-4",
+            "resolved": "https://registry.npmjs.org/@github/copilot/-/copilot-1.0.57-4.tgz",
+            "integrity": "sha512-EkMOKTbxPEMa8L3Q/Heex+Q3qIWh0sqY6998weX3QeeVG8X9lzZw+XH8yN/ITBjrP9jcPwW+isbLU18l9m7s7A==",
             "license": "SEE LICENSE IN LICENSE.md",
             "dependencies": {
                 "detect-libc": "^2.1.2"
@@ -674,20 +674,20 @@
                 "copilot": "npm-loader.js"
             },
             "optionalDependencies": {
-                "@github/copilot-darwin-arm64": "1.0.57-3",
-                "@github/copilot-darwin-x64": "1.0.57-3",
-                "@github/copilot-linux-arm64": "1.0.57-3",
-                "@github/copilot-linux-x64": "1.0.57-3",
-                "@github/copilot-linuxmusl-arm64": "1.0.57-3",
-                "@github/copilot-linuxmusl-x64": "1.0.57-3",
-                "@github/copilot-win32-arm64": "1.0.57-3",
-                "@github/copilot-win32-x64": "1.0.57-3"
+                "@github/copilot-darwin-arm64": "1.0.57-4",
+                "@github/copilot-darwin-x64": "1.0.57-4",
+                "@github/copilot-linux-arm64": "1.0.57-4",
+                "@github/copilot-linux-x64": "1.0.57-4",
+                "@github/copilot-linuxmusl-arm64": "1.0.57-4",
+                "@github/copilot-linuxmusl-x64": "1.0.57-4",
+                "@github/copilot-win32-arm64": "1.0.57-4",
+                "@github/copilot-win32-x64": "1.0.57-4"
             }
         },
         "node_modules/@github/copilot-darwin-arm64": {
-            "version": "1.0.57-3",
-            "resolved": "https://registry.npmjs.org/@github/copilot-darwin-arm64/-/copilot-darwin-arm64-1.0.57-3.tgz",
-            "integrity": "sha512-1i0D7EByvaIl71aT60jWE/Rd7chdrcs7uETRp9ZO0dmW74ScWC0fF7qywFfiYY81PCeuHJoKfq0iMcuwt3fb5w==",
+            "version": "1.0.57-4",
+            "resolved": "https://registry.npmjs.org/@github/copilot-darwin-arm64/-/copilot-darwin-arm64-1.0.57-4.tgz",
+            "integrity": "sha512-NI5FOzEPuvUONclGW818PToyG9EeBeb33E0OzLsCL8649uJketMzr9gjHWAHbF5C67bRF/xZ/DxvrhrEYunWKw==",
             "cpu": [
                 "arm64"
             ],
@@ -701,9 +701,9 @@
             }
         },
         "node_modules/@github/copilot-darwin-x64": {
-            "version": "1.0.57-3",
-            "resolved": "https://registry.npmjs.org/@github/copilot-darwin-x64/-/copilot-darwin-x64-1.0.57-3.tgz",
-            "integrity": "sha512-s7EuHBRMhaOX+BPR3ys+3OgndNy53csI688Nsi52GiKypx1Ma287ESHEvK9IWZCqtzq3ApxsEdyX8q7enhGstg==",
+            "version": "1.0.57-4",
+            "resolved": "https://registry.npmjs.org/@github/copilot-darwin-x64/-/copilot-darwin-x64-1.0.57-4.tgz",
+            "integrity": "sha512-wvpTb0SLF3wxGOxtUDBLZgePkVxkuw3Bu3nMuazemeKVx03hsGjvvYz8rwAqEEnkonElx3PlADu5eaC6QO//zg==",
             "cpu": [
                 "x64"
             ],
@@ -717,9 +717,9 @@
             }
         },
         "node_modules/@github/copilot-linux-arm64": {
-            "version": "1.0.57-3",
-            "resolved": "https://registry.npmjs.org/@github/copilot-linux-arm64/-/copilot-linux-arm64-1.0.57-3.tgz",
-            "integrity": "sha512-b/061/sNsj/DVO8cHor75H9iV7rMXHPFdeJhL4grn8TU1ooIpGDCmBafWIYcE6oJGJX88ryLVWOraRS63KSSaA==",
+            "version": "1.0.57-4",
+            "resolved": "https://registry.npmjs.org/@github/copilot-linux-arm64/-/copilot-linux-arm64-1.0.57-4.tgz",
+            "integrity": "sha512-4RCCM0PGssfUCZAKxIBLaNXEelW6lw9px0GwCuz7XGvFEYmZmGyC4ArqlrM51alZWnR7MRoaxbRdRmEV2AVdqQ==",
             "cpu": [
                 "arm64"
             ],
@@ -733,9 +733,9 @@
             }
         },
         "node_modules/@github/copilot-linux-x64": {
-            "version": "1.0.57-3",
-            "resolved": "https://registry.npmjs.org/@github/copilot-linux-x64/-/copilot-linux-x64-1.0.57-3.tgz",
-            "integrity": "sha512-E1faDFGzdlCw3FSh5JlBfBxHQhp9YGk9Qx+a0ZXtTZxQ3WiPoy8DfTaoc6HlT1sZo8dGPvfmJfmCHg52jhmfyA==",
+            "version": "1.0.57-4",
+            "resolved": "https://registry.npmjs.org/@github/copilot-linux-x64/-/copilot-linux-x64-1.0.57-4.tgz",
+            "integrity": "sha512-BZhp1lYZb/Y/GEb42OdblEHasHdnmwm7Qg7BeR797w34/m7/0m9k//kqggEU1qdVPTvii4SXjdYoelf7CphjRw==",
             "cpu": [
                 "x64"
             ],
@@ -749,9 +749,9 @@
             }
         },
         "node_modules/@github/copilot-linuxmusl-arm64": {
-            "version": "1.0.57-3",
-            "resolved": "https://registry.npmjs.org/@github/copilot-linuxmusl-arm64/-/copilot-linuxmusl-arm64-1.0.57-3.tgz",
-            "integrity": "sha512-cAOSAtlPXZuJv+zuVkt/YXIwbSoi5aYppxfau3tYDPpiTO/9YAwQYWqXz+GWuqWe3hegzXe6bCeyGDsunhqRtg==",
+            "version": "1.0.57-4",
+            "resolved": "https://registry.npmjs.org/@github/copilot-linuxmusl-arm64/-/copilot-linuxmusl-arm64-1.0.57-4.tgz",
+            "integrity": "sha512-jZ/RpgbYODgvyDzhgUYBNZzMy8r3MRso2Z8THpRCEYVawScQ7+DM9WWe/2+R8WZ0ipDa+omfm+dFSOPCYSVVug==",
             "cpu": [
                 "arm64"
             ],
@@ -765,9 +765,9 @@
             }
         },
         "node_modules/@github/copilot-linuxmusl-x64": {
-            "version": "1.0.57-3",
-            "resolved": "https://registry.npmjs.org/@github/copilot-linuxmusl-x64/-/copilot-linuxmusl-x64-1.0.57-3.tgz",
-            "integrity": "sha512-jLmUpc8jAT5LH6SQncbyjTEU/bcsM7JrGKUfhV0PeJAU5Z7sOuZekeeVxA/Lq4yI+aBod9cE8kFjB3+KXYK0DQ==",
+            "version": "1.0.57-4",
+            "resolved": "https://registry.npmjs.org/@github/copilot-linuxmusl-x64/-/copilot-linuxmusl-x64-1.0.57-4.tgz",
+            "integrity": "sha512-PXcMGlk9wTPIFAmzfFQreM5azF1V6k4lD4fb2ySg5iYcmtCLV+HG2FlcVwZnJUrNa0da46pmROjzJXT4QPV5Yw==",
             "cpu": [
                 "x64"
             ],
@@ -781,9 +781,9 @@
             }
         },
         "node_modules/@github/copilot-win32-arm64": {
-            "version": "1.0.57-3",
-            "resolved": "https://registry.npmjs.org/@github/copilot-win32-arm64/-/copilot-win32-arm64-1.0.57-3.tgz",
-            "integrity": "sha512-RRP9+EUdcKz7X12n3Z78hwFTLwCqVi7RML/Mfr30cCvKk0ypMS3Nnlo780exMj2Ba1Da2RHv08DvMvUttqBbFA==",
+            "version": "1.0.57-4",
+            "resolved": "https://registry.npmjs.org/@github/copilot-win32-arm64/-/copilot-win32-arm64-1.0.57-4.tgz",
+            "integrity": "sha512-ChvV8EDHUkiKcCXdrJQksbLrOMcVnl0xerRRMHNHFMGReBfK8+/TyiKgEhw/R3qRI5NnBv7QvBDRhp0KHZ7rqQ==",
             "cpu": [
                 "arm64"
             ],
@@ -797,9 +797,9 @@
             }
         },
         "node_modules/@github/copilot-win32-x64": {
-            "version": "1.0.57-3",
-            "resolved": "https://registry.npmjs.org/@github/copilot-win32-x64/-/copilot-win32-x64-1.0.57-3.tgz",
-            "integrity": "sha512-SYvCUrtIJzhHuz2XKGDpY5mmCVbaUjF64ZtG/m2zBKsnO9j/Aa9aXw9p3LAk6bbKlXIrHFadE6PTsi0YbOmT9w==",
+            "version": "1.0.57-4",
+            "resolved": "https://registry.npmjs.org/@github/copilot-win32-x64/-/copilot-win32-x64-1.0.57-4.tgz",
+            "integrity": "sha512-xMWYYUL+5HaYy+ddoL6IHVkvLyk2TauXvYUl7b73vqp0elRF2SwEZLFWfYts3L2+wcEkCnYWeOO7lSnT24+FuQ==",
             "cpu": [
                 "x64"
             ],
diff --git a/nodejs/package.json b/nodejs/package.json
index a88fc9021..fb9a591a6 100644
--- a/nodejs/package.json
+++ b/nodejs/package.json
@@ -56,7 +56,7 @@
     "author": "GitHub",
     "license": "MIT",
     "dependencies": {
-        "@github/copilot": "^1.0.57-3",
+        "@github/copilot": "^1.0.57-4",
         "vscode-jsonrpc": "^8.2.1",
         "zod": "^4.3.6"
     },
diff --git a/nodejs/samples/package-lock.json b/nodejs/samples/package-lock.json
index 44e290a09..c427cd47b 100644
--- a/nodejs/samples/package-lock.json
+++ b/nodejs/samples/package-lock.json
@@ -18,7 +18,7 @@
             "version": "0.0.0-dev",
             "license": "MIT",
             "dependencies": {
-                "@github/copilot": "^1.0.57-3",
+                "@github/copilot": "^1.0.57-4",
                 "vscode-jsonrpc": "^8.2.1",
                 "zod": "^4.3.6"
             },
diff --git a/nodejs/src/generated/rpc.ts b/nodejs/src/generated/rpc.ts
index 7e9a86da8..9c6ea74df 100644
--- a/nodejs/src/generated/rpc.ts
+++ b/nodejs/src/generated/rpc.ts
@@ -4961,10 +4961,7 @@ export interface ModelSwitchToRequest {
   reasoningEffort?: string;
   reasoningSummary?: ReasoningSummary;
   modelCapabilities?: ModelCapabilitiesOverride;
-  /**
-   * Explicit context tier for the selected model. `"default"` / `"long_context"` pin the tier; `null` clears any previous explicit choice; `undefined` leaves the existing tier untouched.
-   */
-  contextTier?: ContextTier | null;
+  contextTier?: ContextTier;
 }
 /**
  * The model identifier active on the session after the switch.
diff --git a/nodejs/src/session.ts b/nodejs/src/session.ts
index b7b9c217a..854a1a0d4 100644
--- a/nodejs/src/session.ts
+++ b/nodejs/src/session.ts
@@ -30,6 +30,7 @@ import type {
     MessageOptions,
     PermissionHandler,
     PermissionRequest,
+    ContextTier,
     ReasoningEffort,
     ReasoningSummary,
     ModelCapabilitiesOverride,
@@ -1210,6 +1211,7 @@ export class CopilotSession {
         options?: {
             reasoningEffort?: ReasoningEffort;
             reasoningSummary?: ReasoningSummary;
+            contextTier?: ContextTier;
             modelCapabilities?: ModelCapabilitiesOverride;
         }
     ): Promise<void> {
diff --git a/nodejs/test/client.test.ts b/nodejs/test/client.test.ts
index 3a1e83460..657ec7c9c 100644
--- a/nodejs/test/client.test.ts
+++ b/nodejs/test/client.test.ts
@@ -1004,6 +1004,7 @@ describe("CopilotClient", () => {
         await session.setModel("claude-sonnet-4.6", {
             reasoningEffort: "high",
             reasoningSummary: "detailed",
+            contextTier: "long_context",
         });
 
         expect(spy).toHaveBeenCalledWith("session.model.switchTo", {
@@ -1011,6 +1012,7 @@ describe("CopilotClient", () => {
             modelId: "claude-sonnet-4.6",
             reasoningEffort: "high",
             reasoningSummary: "detailed",
+            contextTier: "long_context",
         });
 
         spy.mockRestore();
diff --git a/python/copilot/generated/rpc.py b/python/copilot/generated/rpc.py
index 2969d7b6c..12bff9e7f 100644
--- a/python/copilot/generated/rpc.py
+++ b/python/copilot/generated/rpc.py
@@ -15460,9 +15460,8 @@ class ModelSwitchToRequest:
     """Model identifier to switch to"""
 
     context_tier: ContextTier | None = None
-    """Explicit context tier for the selected model. `"default"` / `"long_context"` pin the
-    tier; `null` clears any previous explicit choice; `undefined` leaves the existing tier
-    untouched.
+    """Explicit context tier for the selected model. `"default"` / `"long_context"` apply the
+    requested tier; omit this field to use normal model behavior with no explicit tier.
     """
     model_capabilities: ModelCapabilitiesOverride | None = None
     """Override individual model capabilities resolved by the runtime"""
diff --git a/python/copilot/session.py b/python/copilot/session.py
index f9bbb24c3..7e57bd39b 100644
--- a/python/copilot/session.py
+++ b/python/copilot/session.py
@@ -55,6 +55,9 @@
     UIElicitationSchemaType,
     UIHandlePendingElicitationRequest,
 )
+from .generated.rpc import (
+    ContextTier as _RpcContextTier,
+)
 from .generated.rpc import ModelCapabilitiesOverride as _RpcModelCapabilitiesOverride
 from .generated.session_events import (
     AssistantMessageData,
@@ -2394,6 +2397,7 @@ async def set_model(
         *,
         reasoning_effort: str | None = None,
         reasoning_summary: ReasoningSummary | None = None,
+        context_tier: ContextTier | None = None,
         model_capabilities: ModelCapabilitiesOverride | None = None,
     ) -> None:
         """
@@ -2409,6 +2413,8 @@ async def set_model(
             reasoning_summary: Optional reasoning summary mode for supported
                 models. Use "none" to suppress summary output regardless of
                 whether reasoning is enabled.
+            context_tier: Optional context window tier for supported models.
+                Omit to use normal model behavior with no explicit tier.
             model_capabilities: Override individual model capabilities resolved by the runtime.
 
         Raises:
@@ -2434,6 +2440,7 @@ async def set_model(
                     if reasoning_summary is not None
                     else None
                 ),
+                context_tier=(_RpcContextTier(context_tier) if context_tier is not None else None),
                 model_capabilities=rpc_caps,
             )
         )
diff --git a/python/test_client.py b/python/test_client.py
index b1c687204..502d410ab 100644
--- a/python/test_client.py
+++ b/python/test_client.py
@@ -1132,10 +1132,15 @@ async def mock_request(method, params, **kwargs):
                 return await original_request(method, params, **kwargs)
 
             client._client.request = mock_request
-            await session.set_model("gpt-4.1", reasoning_summary="detailed")
+            await session.set_model(
+                "gpt-4.1",
+                reasoning_summary="detailed",
+                context_tier="long_context",
+            )
             assert captured["session.model.switchTo"]["sessionId"] == session.session_id
             assert captured["session.model.switchTo"]["modelId"] == "gpt-4.1"
             assert captured["session.model.switchTo"]["reasoningSummary"] == "detailed"
+            assert captured["session.model.switchTo"]["contextTier"] == "long_context"
         finally:
             await client.force_stop()
 
diff --git a/rust/src/generated/api_types.rs b/rust/src/generated/api_types.rs
index 2bf1ac841..d70291a79 100644
--- a/rust/src/generated/api_types.rs
+++ b/rust/src/generated/api_types.rs
@@ -4283,7 +4283,7 @@ pub struct ModelsListRequest {
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct ModelSwitchToRequest {
-    /// Explicit context tier for the selected model. `"default"` / `"long_context"` pin the tier; `null` clears any previous explicit choice; `undefined` leaves the existing tier untouched.
+    /// Explicit context tier for the selected model. `"default"` / `"long_context"` apply the requested tier; omit this field to use normal model behavior with no explicit tier.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub context_tier: Option<ContextTier>,
     /// Override individual model capabilities resolved by the runtime
diff --git a/rust/src/session.rs b/rust/src/session.rs
index 6a1e4f92c..6fc7a1857 100644
--- a/rust/src/session.rs
+++ b/rust/src/session.rs
@@ -524,8 +524,8 @@ impl Session {
             model_id: model.to_string(),
             reasoning_effort: opts.reasoning_effort,
             reasoning_summary: opts.reasoning_summary,
+            context_tier: opts.context_tier,
             model_capabilities: opts.model_capabilities,
-            ..ModelSwitchToRequest::default()
         };
         self.rpc().model().switch_to(request).await?;
         Ok(())
diff --git a/rust/src/types.rs b/rust/src/types.rs
index e4b9d48e2..ce01259f5 100644
--- a/rust/src/types.rs
+++ b/rust/src/types.rs
@@ -14,6 +14,8 @@ use serde_json::Value;
 
 use crate::canvas::{CanvasDeclaration, CanvasHandler};
 use crate::generated::api_types::OpenCanvasInstance;
+/// Context window tier for models that support tiered context windows.
+pub use crate::generated::session_events::ContextTier;
 use crate::generated::session_events::ReasoningSummary;
 use crate::handler::{
     AutoModeSwitchHandler, ElicitationHandler, ExitPlanModeHandler, PermissionHandler,
@@ -3267,6 +3269,9 @@ pub struct SetModelOptions {
     /// [`ReasoningSummary::None`] to suppress summary output regardless of
     /// whether reasoning is enabled.
     pub reasoning_summary: Option<ReasoningSummary>,
+    /// Explicit context window tier for the new model. Leave unset to use
+    /// normal model behavior with no explicit tier.
+    pub context_tier: Option<ContextTier>,
     /// Override individual model capabilities resolved by the runtime. Only
     /// fields set on the override are applied; the rest fall back to the
     /// runtime-resolved values for the model.
@@ -3286,6 +3291,12 @@ impl SetModelOptions {
         self
     }
 
+    /// Set [`context_tier`](Self::context_tier).
+    pub fn with_context_tier(mut self, tier: ContextTier) -> Self {
+        self.context_tier = Some(tier);
+        self
+    }
+
     /// Set [`model_capabilities`](Self::model_capabilities).
     pub fn with_model_capabilities(
         mut self,
diff --git a/rust/tests/session_test.rs b/rust/tests/session_test.rs
index 786ba97de..7b0400bf3 100644
--- a/rust/tests/session_test.rs
+++ b/rust/tests/session_test.rs
@@ -21,7 +21,7 @@ use github_copilot_sdk::types::{
     ElicitationResult, ExitPlanModeData, ExtensionInfo, MessageOptions, RequestId, SessionConfig,
     SessionId, SetModelOptions, Tool, ToolInvocation, ToolResult,
 };
-use github_copilot_sdk::{Client, tool};
+use github_copilot_sdk::{Client, ContextTier, tool};
 use serde_json::Value;
 use tokio::io::{AsyncWrite, AsyncWriteExt, duplex};
 use tokio::time::timeout;
@@ -1290,7 +1290,8 @@ async fn set_model_sends_switch_to_request() {
                     "claude-sonnet-4",
                     Some(
                         SetModelOptions::default()
-                            .with_reasoning_summary(ReasoningSummary::Detailed),
+                            .with_reasoning_summary(ReasoningSummary::Detailed)
+                            .with_context_tier(ContextTier::LongContext),
                     ),
                 )
                 .await
@@ -1302,6 +1303,7 @@ async fn set_model_sends_switch_to_request() {
     assert_eq!(request["method"], "session.model.switchTo");
     assert_eq!(request["params"]["modelId"], "claude-sonnet-4");
     assert_eq!(request["params"]["reasoningSummary"], "detailed");
+    assert_eq!(request["params"]["contextTier"], "long_context");
     server
         .respond(
             &request,
diff --git a/test/harness/package-lock.json b/test/harness/package-lock.json
index 34ee930ba..e011bca05 100644
--- a/test/harness/package-lock.json
+++ b/test/harness/package-lock.json
@@ -9,7 +9,7 @@
       "version": "1.0.0",
       "license": "ISC",
       "devDependencies": {
-        "@github/copilot": "^1.0.57-3",
+        "@github/copilot": "^1.0.57-4",
         "@modelcontextprotocol/sdk": "^1.26.0",
         "@types/node": "^25.3.3",
         "@types/node-forge": "^1.3.14",
@@ -464,9 +464,9 @@
       }
     },
     "node_modules/@github/copilot": {
-      "version": "1.0.57-3",
-      "resolved": "https://registry.npmjs.org/@github/copilot/-/copilot-1.0.57-3.tgz",
-      "integrity": "sha512-Yo0ebyTybaoZI1TsCOxlzwEn6jxHR2eiiZkTXLMtj2wnXj/TZZWUGCWQo/LltvFPjOjLbe0TR6pzmzKNr3v0VA==",
+      "version": "1.0.57-4",
+      "resolved": "https://registry.npmjs.org/@github/copilot/-/copilot-1.0.57-4.tgz",
+      "integrity": "sha512-EkMOKTbxPEMa8L3Q/Heex+Q3qIWh0sqY6998weX3QeeVG8X9lzZw+XH8yN/ITBjrP9jcPwW+isbLU18l9m7s7A==",
       "dev": true,
       "license": "SEE LICENSE IN LICENSE.md",
       "dependencies": {
@@ -476,20 +476,20 @@
         "copilot": "npm-loader.js"
       },
       "optionalDependencies": {
-        "@github/copilot-darwin-arm64": "1.0.57-3",
-        "@github/copilot-darwin-x64": "1.0.57-3",
-        "@github/copilot-linux-arm64": "1.0.57-3",
-        "@github/copilot-linux-x64": "1.0.57-3",
-        "@github/copilot-linuxmusl-arm64": "1.0.57-3",
-        "@github/copilot-linuxmusl-x64": "1.0.57-3",
-        "@github/copilot-win32-arm64": "1.0.57-3",
-        "@github/copilot-win32-x64": "1.0.57-3"
+        "@github/copilot-darwin-arm64": "1.0.57-4",
+        "@github/copilot-darwin-x64": "1.0.57-4",
+        "@github/copilot-linux-arm64": "1.0.57-4",
+        "@github/copilot-linux-x64": "1.0.57-4",
+        "@github/copilot-linuxmusl-arm64": "1.0.57-4",
+        "@github/copilot-linuxmusl-x64": "1.0.57-4",
+        "@github/copilot-win32-arm64": "1.0.57-4",
+        "@github/copilot-win32-x64": "1.0.57-4"
       }
     },
     "node_modules/@github/copilot-darwin-arm64": {
-      "version": "1.0.57-3",
-      "resolved": "https://registry.npmjs.org/@github/copilot-darwin-arm64/-/copilot-darwin-arm64-1.0.57-3.tgz",
-      "integrity": "sha512-1i0D7EByvaIl71aT60jWE/Rd7chdrcs7uETRp9ZO0dmW74ScWC0fF7qywFfiYY81PCeuHJoKfq0iMcuwt3fb5w==",
+      "version": "1.0.57-4",
+      "resolved": "https://registry.npmjs.org/@github/copilot-darwin-arm64/-/copilot-darwin-arm64-1.0.57-4.tgz",
+      "integrity": "sha512-NI5FOzEPuvUONclGW818PToyG9EeBeb33E0OzLsCL8649uJketMzr9gjHWAHbF5C67bRF/xZ/DxvrhrEYunWKw==",
       "cpu": [
         "arm64"
       ],
@@ -504,9 +504,9 @@
       }
     },
     "node_modules/@github/copilot-darwin-x64": {
-      "version": "1.0.57-3",
-      "resolved": "https://registry.npmjs.org/@github/copilot-darwin-x64/-/copilot-darwin-x64-1.0.57-3.tgz",
-      "integrity": "sha512-s7EuHBRMhaOX+BPR3ys+3OgndNy53csI688Nsi52GiKypx1Ma287ESHEvK9IWZCqtzq3ApxsEdyX8q7enhGstg==",
+      "version": "1.0.57-4",
+      "resolved": "https://registry.npmjs.org/@github/copilot-darwin-x64/-/copilot-darwin-x64-1.0.57-4.tgz",
+      "integrity": "sha512-wvpTb0SLF3wxGOxtUDBLZgePkVxkuw3Bu3nMuazemeKVx03hsGjvvYz8rwAqEEnkonElx3PlADu5eaC6QO//zg==",
       "cpu": [
         "x64"
       ],
@@ -521,9 +521,9 @@
       }
     },
     "node_modules/@github/copilot-linux-arm64": {
-      "version": "1.0.57-3",
-      "resolved": "https://registry.npmjs.org/@github/copilot-linux-arm64/-/copilot-linux-arm64-1.0.57-3.tgz",
-      "integrity": "sha512-b/061/sNsj/DVO8cHor75H9iV7rMXHPFdeJhL4grn8TU1ooIpGDCmBafWIYcE6oJGJX88ryLVWOraRS63KSSaA==",
+      "version": "1.0.57-4",
+      "resolved": "https://registry.npmjs.org/@github/copilot-linux-arm64/-/copilot-linux-arm64-1.0.57-4.tgz",
+      "integrity": "sha512-4RCCM0PGssfUCZAKxIBLaNXEelW6lw9px0GwCuz7XGvFEYmZmGyC4ArqlrM51alZWnR7MRoaxbRdRmEV2AVdqQ==",
       "cpu": [
         "arm64"
       ],
@@ -538,9 +538,9 @@
       }
     },
     "node_modules/@github/copilot-linux-x64": {
-      "version": "1.0.57-3",
-      "resolved": "https://registry.npmjs.org/@github/copilot-linux-x64/-/copilot-linux-x64-1.0.57-3.tgz",
-      "integrity": "sha512-E1faDFGzdlCw3FSh5JlBfBxHQhp9YGk9Qx+a0ZXtTZxQ3WiPoy8DfTaoc6HlT1sZo8dGPvfmJfmCHg52jhmfyA==",
+      "version": "1.0.57-4",
+      "resolved": "https://registry.npmjs.org/@github/copilot-linux-x64/-/copilot-linux-x64-1.0.57-4.tgz",
+      "integrity": "sha512-BZhp1lYZb/Y/GEb42OdblEHasHdnmwm7Qg7BeR797w34/m7/0m9k//kqggEU1qdVPTvii4SXjdYoelf7CphjRw==",
       "cpu": [
         "x64"
       ],
@@ -555,9 +555,9 @@
       }
     },
     "node_modules/@github/copilot-linuxmusl-arm64": {
-      "version": "1.0.57-3",
-      "resolved": "https://registry.npmjs.org/@github/copilot-linuxmusl-arm64/-/copilot-linuxmusl-arm64-1.0.57-3.tgz",
-      "integrity": "sha512-cAOSAtlPXZuJv+zuVkt/YXIwbSoi5aYppxfau3tYDPpiTO/9YAwQYWqXz+GWuqWe3hegzXe6bCeyGDsunhqRtg==",
+      "version": "1.0.57-4",
+      "resolved": "https://registry.npmjs.org/@github/copilot-linuxmusl-arm64/-/copilot-linuxmusl-arm64-1.0.57-4.tgz",
+      "integrity": "sha512-jZ/RpgbYODgvyDzhgUYBNZzMy8r3MRso2Z8THpRCEYVawScQ7+DM9WWe/2+R8WZ0ipDa+omfm+dFSOPCYSVVug==",
       "cpu": [
         "arm64"
       ],
@@ -572,9 +572,9 @@
       }
     },
     "node_modules/@github/copilot-linuxmusl-x64": {
-      "version": "1.0.57-3",
-      "resolved": "https://registry.npmjs.org/@github/copilot-linuxmusl-x64/-/copilot-linuxmusl-x64-1.0.57-3.tgz",
-      "integrity": "sha512-jLmUpc8jAT5LH6SQncbyjTEU/bcsM7JrGKUfhV0PeJAU5Z7sOuZekeeVxA/Lq4yI+aBod9cE8kFjB3+KXYK0DQ==",
+      "version": "1.0.57-4",
+      "resolved": "https://registry.npmjs.org/@github/copilot-linuxmusl-x64/-/copilot-linuxmusl-x64-1.0.57-4.tgz",
+      "integrity": "sha512-PXcMGlk9wTPIFAmzfFQreM5azF1V6k4lD4fb2ySg5iYcmtCLV+HG2FlcVwZnJUrNa0da46pmROjzJXT4QPV5Yw==",
       "cpu": [
         "x64"
       ],
@@ -589,9 +589,9 @@
       }
     },
     "node_modules/@github/copilot-win32-arm64": {
-      "version": "1.0.57-3",
-      "resolved": "https://registry.npmjs.org/@github/copilot-win32-arm64/-/copilot-win32-arm64-1.0.57-3.tgz",
-      "integrity": "sha512-RRP9+EUdcKz7X12n3Z78hwFTLwCqVi7RML/Mfr30cCvKk0ypMS3Nnlo780exMj2Ba1Da2RHv08DvMvUttqBbFA==",
+      "version": "1.0.57-4",
+      "resolved": "https://registry.npmjs.org/@github/copilot-win32-arm64/-/copilot-win32-arm64-1.0.57-4.tgz",
+      "integrity": "sha512-ChvV8EDHUkiKcCXdrJQksbLrOMcVnl0xerRRMHNHFMGReBfK8+/TyiKgEhw/R3qRI5NnBv7QvBDRhp0KHZ7rqQ==",
       "cpu": [
         "arm64"
       ],
@@ -606,9 +606,9 @@
       }
     },
     "node_modules/@github/copilot-win32-x64": {
-      "version": "1.0.57-3",
-      "resolved": "https://registry.npmjs.org/@github/copilot-win32-x64/-/copilot-win32-x64-1.0.57-3.tgz",
-      "integrity": "sha512-SYvCUrtIJzhHuz2XKGDpY5mmCVbaUjF64ZtG/m2zBKsnO9j/Aa9aXw9p3LAk6bbKlXIrHFadE6PTsi0YbOmT9w==",
+      "version": "1.0.57-4",
+      "resolved": "https://registry.npmjs.org/@github/copilot-win32-x64/-/copilot-win32-x64-1.0.57-4.tgz",
+      "integrity": "sha512-xMWYYUL+5HaYy+ddoL6IHVkvLyk2TauXvYUl7b73vqp0elRF2SwEZLFWfYts3L2+wcEkCnYWeOO7lSnT24+FuQ==",
       "cpu": [
         "x64"
       ],
diff --git a/test/harness/package.json b/test/harness/package.json
index 50d242a7f..5e7d110a7 100644
--- a/test/harness/package.json
+++ b/test/harness/package.json
@@ -11,7 +11,7 @@
     "test": "vitest run"
   },
   "devDependencies": {
-    "@github/copilot": "^1.0.57-3",
+    "@github/copilot": "^1.0.57-4",
     "@modelcontextprotocol/sdk": "^1.26.0",
     "@types/node": "^25.3.3",
     "@types/node-forge": "^1.3.14",