all SDKs: add EnableExperimentalMode to session create/resume wire with mode-aware defaults

dotnet/src/Client.cs

@@ -896,6 +896,7 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig config, Cance
                 toolFilter.ExcludedTools,
                 config.Provider,
                 config.EnableSessionTelemetry,
+                config.IsExperimentalMode,
                 config.OnPermissionRequest != null ? true : null,
                 config.OnUserInputRequest != null ? true : null,
                 config.OnExitPlanModeRequest != null ? true : null,
@@ -1091,6 +1092,7 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
                 toolFilter.ExcludedTools,
                 config.Provider,
                 config.EnableSessionTelemetry,
+                config.IsExperimentalMode,
                 config.OnPermissionRequest != null ? true : null,
                 config.OnUserInputRequest != null ? true : null,
                 config.OnExitPlanModeRequest != null ? true : null,
@@ -2283,6 +2285,7 @@ internal record CreateSessionRequest(
         IList<string>? ExcludedTools,
         ProviderConfig? Provider,
         bool? EnableSessionTelemetry,
+        bool? IsExperimentalMode,
         bool? RequestPermission,
         bool? RequestUserInput,
         bool? RequestExitPlanMode,
@@ -2369,6 +2372,7 @@ internal record ResumeSessionRequest(
         IList<string>? ExcludedTools,
         ProviderConfig? Provider,
         bool? EnableSessionTelemetry,
+        bool? IsExperimentalMode,
         bool? RequestPermission,
         bool? RequestUserInput,
         bool? RequestExitPlanMode,

dotnet/src/Types.cs

@@ -2474,6 +2474,7 @@ protected SessionConfigBase(SessionConfigBase? other)
         OnUserInputRequest = other.OnUserInputRequest;
         Provider = other.Provider;
         EnableSessionTelemetry = other.EnableSessionTelemetry;
+        IsExperimentalMode = other.IsExperimentalMode;
         SkipCustomInstructions = other.SkipCustomInstructions;
         CustomAgentsLocalOnly = other.CustomAgentsLocalOnly;
         CoauthorEnabled = other.CoauthorEnabled;
@@ -2639,6 +2640,17 @@ protected SessionConfigBase(SessionConfigBase? other)
     /// </summary>
     public bool? EnableSessionTelemetry { get; set; }
 
+    /// <summary>
+    /// Overrides the session's experimental feature-flag tier resolution.
+    /// </summary>
+    /// <remarks>
+    /// Set to <see langword="true"/> to force-enable the experimental tier for this
+    /// session, or <see langword="false"/> to resolve feature flags as if
+    /// experimental were off. Leave <see langword="null"/> to inherit the runtime
+    /// process defaults unchanged.
+    /// </remarks>
+    public bool? IsExperimentalMode { get; set; }
+
     /// <summary>
     /// When <see langword="true"/>, suppresses loading of custom instruction files
     /// (e.g. <c>.github/copilot-instructions.md</c>, <c>AGENTS.md</c>) from the working directory.

dotnet/test/Unit/CloneTests.cs

@@ -76,6 +76,7 @@ public void SessionConfig_Clone_CopiesAllProperties()
             WorkingDirectory = "/workspace",
             Streaming = true,
             EnableSessionTelemetry = false,
+            IsExperimentalMode = true,
             EnableOnDemandInstructionDiscovery = true,
             IncludeSubAgentStreamingEvents = false,
             McpServers = new Dictionary<string, McpServerConfig> { ["server1"] = new McpStdioServerConfig { Command = "echo" } },
@@ -115,6 +116,7 @@ public void SessionConfig_Clone_CopiesAllProperties()
         Assert.Equal(original.WorkingDirectory, clone.WorkingDirectory);
         Assert.Equal(original.Streaming, clone.Streaming);
         Assert.Equal(original.EnableSessionTelemetry, clone.EnableSessionTelemetry);
+        Assert.Equal(original.IsExperimentalMode, clone.IsExperimentalMode);
         Assert.Equal(original.EnableOnDemandInstructionDiscovery, clone.EnableOnDemandInstructionDiscovery);
         Assert.Equal(original.IncludeSubAgentStreamingEvents, clone.IncludeSubAgentStreamingEvents);
         Assert.Equal(original.McpServers.Count, clone.McpServers!.Count);
@@ -355,6 +357,19 @@ public void ResumeSessionConfig_Clone_CopiesEnableSessionTelemetry()
         Assert.False(clone.EnableSessionTelemetry);
     }
 
+    [Fact]
+    public void ResumeSessionConfig_Clone_CopiesIsExperimentalMode()
+    {
+        var original = new ResumeSessionConfig
+        {
+            IsExperimentalMode = true,
+        };
+
+        var clone = original.Clone();
+
+        Assert.True(clone.IsExperimentalMode);
+    }
+
     [Fact]
     public void ResumeSessionConfig_Clone_CopiesContinuePendingWork()
     {
@@ -440,6 +455,26 @@ public void ResumeSessionConfig_Clone_PreservesEnableSessionTelemetryDefault()
         Assert.Null(clone.EnableSessionTelemetry);
     }
 
+    [Fact]
+    public void SessionConfig_Clone_PreservesIsExperimentalModeDefault()
+    {
+        var original = new SessionConfig();
+
+        var clone = original.Clone();
+
+        Assert.Null(clone.IsExperimentalMode);
+    }
+
+    [Fact]
+    public void ResumeSessionConfig_Clone_PreservesIsExperimentalModeDefault()
+    {
+        var original = new ResumeSessionConfig();
+
+        var clone = original.Clone();
+
+        Assert.Null(clone.IsExperimentalMode);
+    }
+
     [Fact]
     public void SessionConfig_Clone_CopiesEnableOnDemandInstructionDiscovery()
     {

dotnet/test/Unit/SerializationTests.cs

@@ -299,6 +299,40 @@ public void ResumeSessionRequest_CanSerializeEnableSessionTelemetry_WithSdkOptio
         Assert.False(root.GetProperty("enableSessionTelemetry").GetBoolean());
     }
 
+    [Fact]
+    public void SessionRequests_CanSerializeIsExperimentalMode_WithSdkOptions()
+    {
+        var options = GetSerializerOptions();
+
+        var createRequestType = GetNestedType(typeof(CopilotClient), "CreateSessionRequest");
+        var createRequest = CreateInternalRequest(
+            createRequestType,
+            ("SessionId", "session-id"),
+            ("IsExperimentalMode", false));
+        var createRoot = JsonDocument.Parse(JsonSerializer.Serialize(createRequest, createRequestType, options)).RootElement;
+        Assert.False(createRoot.GetProperty("isExperimentalMode").GetBoolean());
+
+        var createRequestOmitted = CreateInternalRequest(
+            createRequestType,
+            ("SessionId", "session-id"));
+        var createOmittedRoot = JsonDocument.Parse(JsonSerializer.Serialize(createRequestOmitted, createRequestType, options)).RootElement;
+        Assert.False(createOmittedRoot.TryGetProperty("isExperimentalMode", out _));
+
+        var resumeRequestType = GetNestedType(typeof(CopilotClient), "ResumeSessionRequest");
+        var resumeRequest = CreateInternalRequest(
+            resumeRequestType,
+            ("SessionId", "session-id"),
+            ("IsExperimentalMode", true));
+        var resumeRoot = JsonDocument.Parse(JsonSerializer.Serialize(resumeRequest, resumeRequestType, options)).RootElement;
+        Assert.True(resumeRoot.GetProperty("isExperimentalMode").GetBoolean());
+
+        var resumeRequestOmitted = CreateInternalRequest(
+            resumeRequestType,
+            ("SessionId", "session-id"));
+        var resumeOmittedRoot = JsonDocument.Parse(JsonSerializer.Serialize(resumeRequestOmitted, resumeRequestType, options)).RootElement;
+        Assert.False(resumeOmittedRoot.TryGetProperty("isExperimentalMode", out _));
+    }
+
     [Fact]
     public void CreateSessionRequest_CanSerializeEnableOnDemandInstructionDiscovery_WithSdkOptions()
     {

go/client.go

@@ -629,6 +629,7 @@ func (c *Client) CreateSession(ctx context.Context, config *SessionConfig) (*Ses
 	req.ToolFilterPrecedence = precedence
 	req.Provider = config.Provider
 	req.EnableSessionTelemetry = config.EnableSessionTelemetry
+	req.IsExperimentalMode = config.IsExperimentalMode
 	req.SkipCustomInstructions = config.SkipCustomInstructions
 	req.CustomAgentsLocalOnly = config.CustomAgentsLocalOnly
 	req.CoauthorEnabled = config.CoauthorEnabled
@@ -923,6 +924,7 @@ func (c *Client) ResumeSessionWithOptions(ctx context.Context, sessionID string,
 	req.Tools = config.Tools
 	req.Provider = config.Provider
 	req.EnableSessionTelemetry = config.EnableSessionTelemetry
+	req.IsExperimentalMode = config.IsExperimentalMode
 	req.SkipCustomInstructions = config.SkipCustomInstructions
 	req.CustomAgentsLocalOnly = config.CustomAgentsLocalOnly
 	req.CoauthorEnabled = config.CoauthorEnabled

go/client_test.go

@@ -1242,6 +1242,63 @@ func TestCreateSessionRequest_RequestMCPApps(t *testing.T) {
 	})
 }
 
+func TestSessionRequests_IsExperimentalMode(t *testing.T) {
+	t.Run("create forwards isExperimentalMode when explicitly false", func(t *testing.T) {
+		req := createSessionRequest{
+			IsExperimentalMode: Bool(false),
+		}
+		data, err := json.Marshal(req)
+		if err != nil {
+			t.Fatalf("Failed to marshal: %v", err)
+		}
+		var m map[string]any
+		if err := json.Unmarshal(data, &m); err != nil {
+			t.Fatalf("Failed to unmarshal: %v", err)
+		}
+		if m["isExperimentalMode"] != false {
+			t.Errorf("Expected isExperimentalMode to be false, got %v", m["isExperimentalMode"])
+		}
+	})
+
+	t.Run("create omits isExperimentalMode when unset", func(t *testing.T) {
+		req := createSessionRequest{}
+		data, _ := json.Marshal(req)
+		var m map[string]any
+		json.Unmarshal(data, &m)
+		if _, ok := m["isExperimentalMode"]; ok {
+			t.Error("Expected isExperimentalMode to be omitted when not set")
+		}
+	})
+
+	t.Run("resume forwards isExperimentalMode when explicitly true", func(t *testing.T) {
+		req := resumeSessionRequest{
+			SessionID:          "s1",
+			IsExperimentalMode: Bool(true),
+		}
+		data, err := json.Marshal(req)
+		if err != nil {
+			t.Fatalf("Failed to marshal: %v", err)
+		}
+		var m map[string]any
+		if err := json.Unmarshal(data, &m); err != nil {
+			t.Fatalf("Failed to unmarshal: %v", err)
+		}
+		if m["isExperimentalMode"] != true {
+			t.Errorf("Expected isExperimentalMode to be true, got %v", m["isExperimentalMode"])
+		}
+	})
+
+	t.Run("resume omits isExperimentalMode when unset", func(t *testing.T) {
+		req := resumeSessionRequest{SessionID: "s1"}
+		data, _ := json.Marshal(req)
+		var m map[string]any
+		json.Unmarshal(data, &m)
+		if _, ok := m["isExperimentalMode"]; ok {
+			t.Error("Expected isExperimentalMode to be omitted when not set")
+		}
+	})
+}
+
 func TestResumeSessionRequest_RequestMCPApps(t *testing.T) {
 	t.Run("sends requestMcpApps flag when EnableMCPApps is set", func(t *testing.T) {
 		req := resumeSessionRequest{

go/types.go

@@ -980,6 +980,12 @@ type SessionConfig struct {
 	// regardless of this setting. This is independent of the OpenTelemetry
 	// configuration in ClientOptions.Telemetry.
 	EnableSessionTelemetry *bool
+	// IsExperimentalMode, when non-nil, overrides the session's experimental
+	// feature-flag tier resolution. Use Bool(true) to force-enable the
+	// experimental tier for this session, Bool(false) to resolve feature flags
+	// as if experimental were off, or nil to inherit the runtime process
+	// defaults unchanged.
+	IsExperimentalMode *bool
 	// SkipCustomInstructions, when non-nil, controls whether the runtime loads
 	// custom instruction files. See also [ClientOptions.Mode] = [ModeEmpty].
 	SkipCustomInstructions *bool
@@ -1295,6 +1301,12 @@ type ResumeSessionConfig struct {
 	// regardless of this setting. This is independent of the OpenTelemetry
 	// configuration in ClientOptions.Telemetry.
 	EnableSessionTelemetry *bool
+	// IsExperimentalMode, when non-nil, overrides the resumed session's
+	// experimental feature-flag tier resolution. Use Bool(true) to force-enable
+	// the experimental tier for this session, Bool(false) to resolve feature
+	// flags as if experimental were off, or nil to inherit the runtime process
+	// defaults unchanged.
+	IsExperimentalMode *bool
 	// SkipCustomInstructions, when non-nil, controls whether the runtime loads
 	// custom instruction files. See also [ClientOptions.Mode] = [ModeEmpty].
 	SkipCustomInstructions *bool
@@ -1690,6 +1702,7 @@ type createSessionRequest struct {
 	ToolFilterPrecedence               *rpc.OptionsUpdateToolFilterPrecedence `json:"toolFilterPrecedence,omitempty"`
 	Provider                           *ProviderConfig                        `json:"provider,omitempty"`
 	EnableSessionTelemetry             *bool                                  `json:"enableSessionTelemetry,omitempty"`
+	IsExperimentalMode                 *bool                                  `json:"isExperimentalMode,omitempty"`
 	SkipCustomInstructions             *bool                                  `json:"skipCustomInstructions,omitempty"`
 	CustomAgentsLocalOnly              *bool                                  `json:"customAgentsLocalOnly,omitempty"`
 	CoauthorEnabled                    *bool                                  `json:"coauthorEnabled,omitempty"`
@@ -1768,6 +1781,7 @@ type resumeSessionRequest struct {
 	ToolFilterPrecedence               *rpc.OptionsUpdateToolFilterPrecedence `json:"toolFilterPrecedence,omitempty"`
 	Provider                           *ProviderConfig                        `json:"provider,omitempty"`
 	EnableSessionTelemetry             *bool                                  `json:"enableSessionTelemetry,omitempty"`
+	IsExperimentalMode                 *bool                                  `json:"isExperimentalMode,omitempty"`
 	SkipCustomInstructions             *bool                                  `json:"skipCustomInstructions,omitempty"`
 	CustomAgentsLocalOnly              *bool                                  `json:"customAgentsLocalOnly,omitempty"`
 	CoauthorEnabled                    *bool                                  `json:"coauthorEnabled,omitempty"`

java/src/main/java/com/github/copilot/SessionRequestBuilder.java

@@ -114,6 +114,7 @@ static CreateSessionRequest buildCreateRequest(SessionConfig config, String sess
         request.setExcludedTools(config.getExcludedTools());
         request.setProvider(config.getProvider());
         config.getEnableSessionTelemetry().ifPresent(request::setEnableSessionTelemetry);
+        config.getIsExperimentalMode().ifPresent(request::setIsExperimentalMode);
         if (config.getOnUserInputRequest() != null) {
             request.setRequestUserInput(true);
         }
@@ -225,6 +226,7 @@ static ResumeSessionRequest buildResumeRequest(String sessionId, ResumeSessionCo
         request.setExcludedTools(config.getExcludedTools());
         request.setProvider(config.getProvider());
         config.getEnableSessionTelemetry().ifPresent(request::setEnableSessionTelemetry);
+        config.getIsExperimentalMode().ifPresent(request::setIsExperimentalMode);
         if (config.getOnUserInputRequest() != null) {
             request.setRequestUserInput(true);
         }

java/src/main/java/com/github/copilot/rpc/CreateSessionRequest.java

@@ -164,6 +164,10 @@ public final class CreateSessionRequest {
     @JsonProperty("requestMcpApps")
     private Boolean requestMcpApps;
 
+    @JsonProperty("isExperimentalMode")
+    @JsonInclude(JsonInclude.Include.NON_NULL)
+    private Boolean isExperimentalMode;
+
     @JsonProperty("requestExitPlanMode")
     private Boolean requestExitPlanMode;
 
@@ -759,6 +763,30 @@ public void clearRequestMcpApps() {
         this.requestMcpApps = null;
     }
 
+    /**
+     * Gets the isExperimentalMode flag.
+     *
+     * @return the flag
+     */
+    public Boolean getIsExperimentalMode() {
+        return isExperimentalMode;
+    }
+
+    /**
+     * Sets the isExperimentalMode flag.
+     *
+     * @param isExperimentalMode
+     *            the flag
+     */
+    public void setIsExperimentalMode(boolean isExperimentalMode) {
+        this.isExperimentalMode = isExperimentalMode;
+    }
+
+    /** Clears the isExperimentalMode setting, reverting to the default behavior. */
+    public void clearIsExperimentalMode() {
+        this.isExperimentalMode = null;
+    }
+
     /** Gets the requestExitPlanMode flag. @return the flag */
     public Boolean getRequestExitPlanMode() {
         return requestExitPlanMode;