github · ellismg · Jun 22, 2026 · Jun 22, 2026 · Jun 23, 2026
@@ -1012,7 +1012,8 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig config, Cance
                 ExtensionInfo: config.ExtensionInfo,
                 Providers: config.Providers,
                 Models: config.Models,
-                ToolFilterPrecedence: toolFilter.ToolFilterPrecedence);
+                ToolFilterPrecedence: toolFilter.ToolFilterPrecedence,
+                ExpAssignments: config.ExpAssignments);
 
             var rpcTimestamp = Stopwatch.GetTimestamp();
 
@@ -1213,7 +1214,8 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
                 OpenCanvases: config.OpenCanvases,
                 Providers: config.Providers,
                 Models: config.Models,
-                ToolFilterPrecedence: toolFilter.ToolFilterPrecedence);
+                ToolFilterPrecedence: toolFilter.ToolFilterPrecedence,
+                ExpAssignments: config.ExpAssignments);
 
             var rpcTimestamp = Stopwatch.GetTimestamp();
             var response = await InvokeRpcAsync<ResumeSessionResponse>(
@@ -2411,7 +2413,8 @@ internal record CreateSessionRequest(
         ExtensionInfo? ExtensionInfo = null,
         IList<NamedProviderConfig>? Providers = null,
         IList<ProviderModelConfig>? Models = null,
-        OptionsUpdateToolFilterPrecedence? ToolFilterPrecedence = null);
+        OptionsUpdateToolFilterPrecedence? ToolFilterPrecedence = null,
+        [property: JsonPropertyName("expAssignments")] JsonElement? ExpAssignments = null);
 #pragma warning restore GHCP001
 
     internal record ToolDefinition(
@@ -2506,7 +2509,8 @@ internal record ResumeSessionRequest(
         IList<OpenCanvasInstance>? OpenCanvases = null,
         IList<NamedProviderConfig>? Providers = null,
         IList<ProviderModelConfig>? Models = null,
-        OptionsUpdateToolFilterPrecedence? ToolFilterPrecedence = null);
+        OptionsUpdateToolFilterPrecedence? ToolFilterPrecedence = null,
+        [property: JsonPropertyName("expAssignments")] JsonElement? ExpAssignments = null);
 #pragma warning restore GHCP001
 
     internal record ResumeSessionResponse(

@@ -3043,6 +3043,23 @@ protected SessionConfigBase(SessionConfigBase? other)
     /// </summary>
     public RemoteSessionMode? RemoteSession { get; set; }
 
+    /// <summary>
+    /// ExP assignment ("flight") data injected by a trusted integrator, in the
+    /// same JSON shape the Copilot CLI fetches from the experimentation service
+    /// (<c>CopilotExpAssignmentResponse</c>). When provided, the runtime feeds it
+    /// into the same feature-flag path as CLI-fetched assignments and stamps it
+    /// onto telemetry and the CAPI request header. When unset, the session does
+    /// not block on ExP. Intended for out-of-process integrators that fetch ExP
+    /// data themselves; malformed payloads are dropped by the runtime (fail-open).
+    /// Serialized on the wire as <c>expAssignments</c>.
+    /// </summary>
+    /// <remarks>
+    /// This is an internal/trusted-integrator option and is hidden from editor
+    /// completion. It is not part of the broadly advertised public surface.
+    /// </remarks>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public JsonElement? ExpAssignments { get; set; }
+
 #pragma warning disable GHCP001
     /// <summary>
     /// Canvas declarations advertised by this connection. The runtime forwards

@@ -442,6 +442,60 @@ public void SessionRequests_OmitMemory_WhenUnset()
         Assert.False(resumeDocument.RootElement.TryGetProperty("memory", out _));
     }
 
+    [Fact]
+    public void SessionRequests_CanSerializeExpAssignments_WithSdkOptions()
+    {
+        var options = GetSerializerOptions();
+
+        using var createAssignments = JsonDocument.Parse("""{"Configs":[{"Id":"exp-create"}]}""");
+        var createRequestType = GetNestedType(typeof(CopilotClient), "CreateSessionRequest");
+        var createRequest = CreateInternalRequest(
+            createRequestType,
+            ("SessionId", "session-id"),
+            ("ExpAssignments", createAssignments.RootElement.Clone()));
+
+        var createJson = JsonSerializer.Serialize(createRequest, createRequestType, options);
+        using var createDocument = JsonDocument.Parse(createJson);
+        var createRoot = createDocument.RootElement;
+        Assert.Equal("exp-create", createRoot.GetProperty("expAssignments").GetProperty("Configs")[0].GetProperty("Id").GetString());
+
+        using var resumeAssignments = JsonDocument.Parse("""{"Configs":[{"Id":"exp-resume"}]}""");
+        var resumeRequestType = GetNestedType(typeof(CopilotClient), "ResumeSessionRequest");
+        var resumeRequest = CreateInternalRequest(
+            resumeRequestType,
+            ("SessionId", "session-id"),
+            ("ExpAssignments", resumeAssignments.RootElement.Clone()));
+
+        var resumeJson = JsonSerializer.Serialize(resumeRequest, resumeRequestType, options);
+        using var resumeDocument = JsonDocument.Parse(resumeJson);
+        var resumeRoot = resumeDocument.RootElement;
+        Assert.Equal("exp-resume", resumeRoot.GetProperty("expAssignments").GetProperty("Configs")[0].GetProperty("Id").GetString());
+    }
+
+    [Fact]
+    public void SessionRequests_OmitExpAssignments_WhenUnset()
+    {
+        var options = GetSerializerOptions();
+
+        var createRequestType = GetNestedType(typeof(CopilotClient), "CreateSessionRequest");
+        var createRequest = CreateInternalRequest(
+            createRequestType,
+            ("SessionId", "session-id"));
+
+        var createJson = JsonSerializer.Serialize(createRequest, createRequestType, options);
+        using var createDocument = JsonDocument.Parse(createJson);
+        Assert.False(createDocument.RootElement.TryGetProperty("expAssignments", out _));
+
+        var resumeRequestType = GetNestedType(typeof(CopilotClient), "ResumeSessionRequest");
+        var resumeRequest = CreateInternalRequest(
+            resumeRequestType,
+            ("SessionId", "session-id"));
+
+        var resumeJson = JsonSerializer.Serialize(resumeRequest, resumeRequestType, options);
+        using var resumeDocument = JsonDocument.Parse(resumeJson);
+        Assert.False(resumeDocument.RootElement.TryGetProperty("expAssignments", out _));
+    }
+
     [Fact]
     public void CreateSessionRequest_CanSerializeEnableSessionTelemetry_WithSdkOptions()
     {

@@ -711,6 +711,7 @@ func (c *Client) CreateSession(ctx context.Context, config *SessionConfig) (*Ses
 	req.RequestCanvasRenderer = config.RequestCanvasRenderer
 	req.RequestExtensions = config.RequestExtensions
 	req.ExtensionSDKPath = config.ExtensionSDKPath
+	req.ExpAssignments = config.ExpAssignments
 
 	if len(config.Commands) > 0 {
 		cmds := make([]wireCommand, 0, len(config.Commands))
@@ -1051,6 +1052,7 @@ func (c *Client) ResumeSessionWithOptions(ctx context.Context, sessionID string,
 	req.RequestCanvasRenderer = config.RequestCanvasRenderer
 	req.RequestExtensions = config.RequestExtensions
 	req.ExtensionSDKPath = config.ExtensionSDKPath
+	req.ExpAssignments = config.ExpAssignments
 	if config.OnPermissionRequest != nil {
 		req.RequestPermission = Bool(true)
 	}

@@ -2220,3 +2220,85 @@ func TestStartCLIServer_StderrFieldSet(t *testing.T) {
 		t.Error("expected Stderr to be *truncbuffer.TruncBuffer after assignment")
 	}
 }
+
+func TestCreateSessionRequest_ExpAssignments(t *testing.T) {
+	assignments := map[string]any{
+		"Parameters":        map[string]any{"copilot_exp_flag": "treatment"},
+		"AssignmentContext": "ctx-123",
+	}
+
+	t.Run("includes expAssignments in JSON when set", func(t *testing.T) {
+		req := createSessionRequest{ExpAssignments: assignments}
+		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)
+		}
+		got, ok := m["expAssignments"].(map[string]any)
+		if !ok {
+			t.Fatalf("Expected expAssignments to be an object, got %v", m["expAssignments"])
+		}
+		if got["AssignmentContext"] != "ctx-123" {
+			t.Errorf("Expected AssignmentContext 'ctx-123', got %v", got["AssignmentContext"])
+		}
+	})
+
+	t.Run("omits expAssignments from JSON when nil", func(t *testing.T) {
+		req := createSessionRequest{}
+		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 _, ok := m["expAssignments"]; ok {
+			t.Error("Expected expAssignments to be omitted when nil")
+		}
+	})
+}
+
+func TestResumeSessionRequest_ExpAssignments(t *testing.T) {
+	assignments := map[string]any{
+		"Parameters":        map[string]any{"copilot_exp_flag": "treatment"},
+		"AssignmentContext": "ctx-456",
+	}
+
+	t.Run("includes expAssignments in JSON when set", func(t *testing.T) {
+		req := resumeSessionRequest{SessionID: "s1", ExpAssignments: assignments}
+		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)
+		}
+		got, ok := m["expAssignments"].(map[string]any)
+		if !ok {
+			t.Fatalf("Expected expAssignments to be an object, got %v", m["expAssignments"])
+		}
+		if got["AssignmentContext"] != "ctx-456" {
+			t.Errorf("Expected AssignmentContext 'ctx-456', got %v", got["AssignmentContext"])
+		}
+	})
+
+	t.Run("omits expAssignments from JSON when nil", func(t *testing.T) {
+		req := resumeSessionRequest{SessionID: "s1"}
+		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 _, ok := m["expAssignments"]; ok {
+			t.Error("Expected expAssignments to be omitted when nil")
+		}
+	})
+}
@@ -1134,6 +1134,18 @@ type SessionConfig struct {
 	CanvasHandler CanvasHandler `json:"-"`
 	// ExtensionInfo identifies the stable extension providing this session's canvases.
 	ExtensionInfo *ExtensionInfo
+	// ExpAssignments injects ExP assignment ("flight") data for this session,
+	// in the same JSON shape the Copilot CLI fetches from the experimentation
+	// service (CopilotExpAssignmentResponse). When supplied, the runtime feeds
+	// it into the same feature-flag path as CLI-fetched assignments and stamps
+	// it onto telemetry and the CAPI request header. When absent, the session
+	// does not block on ExP. Malformed payloads are dropped by the runtime
+	// (fail-open).
+	//
+	// Internal: ExpAssignments is part of the SDK's internal API surface,
+	// intended for trusted out-of-process integrators, and is not intended for
+	// general external use.
+	ExpAssignments any
 }
 
 // ToolDefer controls whether a tool may be deferred (loaded lazily via tool
@@ -1529,6 +1541,14 @@ type ResumeSessionConfig struct {
 	CanvasHandler CanvasHandler `json:"-"`
 	// ExtensionInfo identifies the stable extension providing this session's canvases.
 	ExtensionInfo *ExtensionInfo
+	// ExpAssignments injects ExP assignment ("flight") data on resume. See
+	// SessionConfig.ExpAssignments. Re-supply on resume so the runtime
+	// re-applies the assignments after a CLI process restart.
+	//
+	// Internal: ExpAssignments is part of the SDK's internal API surface,
+	// intended for trusted out-of-process integrators, and is not intended for
+	// general external use.
+	ExpAssignments any
 }
 type ProviderConfig struct {
 	// Type is the provider type: "openai", "azure", or "anthropic". Defaults to "openai".
@@ -1879,6 +1899,7 @@ type createSessionRequest struct {
 	RequestExtensions                  *bool                                  `json:"requestExtensions,omitempty"`
 	ExtensionSDKPath                   *string                                `json:"extensionSdkPath,omitempty"`
 	ExtensionInfo                      *ExtensionInfo                         `json:"extensionInfo,omitempty"`
+	ExpAssignments                     any                                    `json:"expAssignments,omitempty"`
 	Traceparent                        string                                 `json:"traceparent,omitempty"`
 	Tracestate                         string                                 `json:"tracestate,omitempty"`
 }
@@ -1963,6 +1984,7 @@ type resumeSessionRequest struct {
 	RequestExtensions                  *bool                                  `json:"requestExtensions,omitempty"`
 	ExtensionSDKPath                   *string                                `json:"extensionSdkPath,omitempty"`
 	ExtensionInfo                      *ExtensionInfo                         `json:"extensionInfo,omitempty"`
+	ExpAssignments                     any                                    `json:"expAssignments,omitempty"`
 	Traceparent                        string                                 `json:"traceparent,omitempty"`
 	Tracestate                         string                                 `json:"tracestate,omitempty"`
 }

@@ -177,6 +177,7 @@ static CreateSessionRequest buildCreateRequest(SessionConfig config, String sess
         request.setGitHubToken(config.getGitHubToken());
         request.setRemoteSession(config.getRemoteSession());
         request.setCloud(config.getCloud());
+        request.setExpAssignments(config.getExpAssignments());
 
         return request;
     }
@@ -294,6 +295,7 @@ static ResumeSessionRequest buildResumeRequest(String sessionId, ResumeSessionCo
         }
         request.setGitHubToken(config.getGitHubToken());
         request.setRemoteSession(config.getRemoteSession());
+        request.setExpAssignments(config.getExpAssignments());
 
         return request;
     }

@@ -10,6 +10,7 @@
 
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.JsonNode;
 
 import com.github.copilot.CopilotExperimental;
 
@@ -195,6 +196,9 @@ public final class CreateSessionRequest {
     @JsonProperty("cloud")
     private CloudSessionOptions cloud;
 
+    @JsonProperty("expAssignments")
+    private JsonNode expAssignments;
+
     /** Gets the model name. @return the model */
     public String getModel() {
         return model;
@@ -884,4 +888,16 @@ public CloudSessionOptions getCloud() {
     public void setCloud(CloudSessionOptions cloud) {
         this.cloud = cloud;
     }
+
+    /** Gets the ExP assignment data. @return the ExP assignment data */
+    public JsonNode getExpAssignments() {
+        return expAssignments;
+    }
+
+    /**
+     * Sets the ExP assignment data. @param expAssignments the ExP assignment data
+     */
+    public void setExpAssignments(JsonNode expAssignments) {
+        this.expAssignments = expAssignments;
+    }
 }
@@ -12,6 +12,7 @@
 
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonIgnore;
+import com.fasterxml.jackson.databind.JsonNode;
 
 import com.github.copilot.CopilotExperimental;
 import com.github.copilot.generated.SessionEvent;
@@ -95,6 +96,7 @@ public class ResumeSessionConfig {
     private boolean enableMcpApps;
     private String gitHubToken;
     private String remoteSession;
+    private JsonNode expAssignments;
 
     /**
      * Gets the AI model to use.
@@ -1609,6 +1611,30 @@ public ResumeSessionConfig setRemoteSession(String remoteSession) {
         return this;
     }
 
+    /**
+     * Gets the ExP assignment ("flight") data injected by a trusted integrator.
+     *
+     * @return the ExP assignment data, or {@code null} if not set
+     */
+    public JsonNode getExpAssignments() {
+        return expAssignments;
+    }
+
+    /**
+     * Sets ExP assignment ("flight") data injected by a trusted integrator.
+     * <p>
+     * See {@link SessionConfig#setExpAssignments(JsonNode)} for details. The
+     * runtime supports injecting ExP assignments on resume as well as create.
+     *
+     * @param expAssignments
+     *            the opaque ExP assignment data
+     * @return this config for method chaining
+     */
+    public ResumeSessionConfig setExpAssignments(JsonNode expAssignments) {
+        this.expAssignments = expAssignments;
+        return this;
+    }
+
     /**
      * Creates a shallow clone of this {@code ResumeSessionConfig} instance.
      * <p>
@@ -1676,6 +1702,7 @@ public ResumeSessionConfig clone() {
         copy.enableMcpApps = this.enableMcpApps;
         copy.gitHubToken = this.gitHubToken;
         copy.remoteSession = this.remoteSession;
+        copy.expAssignments = this.expAssignments;
         return copy;
     }
 }