Expose exp_assignments injection on session create/resume across all SDKs (#1750)

* Add exp_assignments injection to session create/resume config

Expose ExP assignment ("flight") data on the SDK's session-open and
session-resume paths so an out-of-process integrator can inject the same
CopilotExpAssignmentResponse payload the CLI fetches itself. The runtime
already accepts expAssignments on the wire, but the hand-written
SessionCreateWire / SessionResumeWire structs (and their public configs)
did not carry it.

- SessionConfig / ResumeSessionConfig: add doc-hidden exp_assignments
  field (serde_json::Value) plus a doc-hidden with_exp_assignments builder
- SessionCreateWire / SessionResumeWire: add exp_assignments, serialized
  as camelCase expAssignments and omitted when None
- Forward the field through both into_wire paths
- Unit tests asserting expAssignments is emitted on create and resume and
  omitted when unset

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

* Expose exp_assignments injection across Node, Python, Go, .NET, Java SDKs

Mirror the Rust SDK change in the remaining five SDKs so out-of-process
integrators can inject ExP ("flight") assignment data into session create
and resume. Adds an internal/trusted-integrator config field that forwards
to the wire key `expAssignments` (omitted when unset), in the opaque JSON
shape of `CopilotExpAssignmentResponse`:

- Node: `expAssignments?: Record<string, unknown>` on `SessionConfigBase`
  (`@internal`), forwarded in the inline session.create/session.resume
  payloads in client.ts.
- Python: `exp_assignments: dict[str, Any] | None = None` kwarg on
  `create_session`/`resume_session`, mapped to `payload["expAssignments"]`.
- Go: `ExpAssignments any` on `SessionConfig`/`ResumeSessionConfig`
  (documented Internal:), forwarded into the create/resume wire structs
  with `json:"expAssignments,omitempty"`.
- .NET: `JsonElement? ExpAssignments` on `SessionConfigBase`
  (`[EditorBrowsable(Never)]`), wired through the internal
  CreateSessionRequest/ResumeSessionRequest records.
- Java: `JsonNode expAssignments` field + fluent setter/getter on
  SessionConfig/ResumeSessionConfig, mapped through
  CreateSessionRequest/ResumeSessionRequest in SessionRequestBuilder.

Each language gains create+resume serialization tests asserting the field
serializes to `expAssignments` when set and is omitted when unset.

Part of github/github-app epic #7452; mirrors the runtime contract added
in github/copilot-agent-runtime#9955.

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

* Fix .NET clone dropping ExpAssignments + add clone regression coverage

The SessionConfigBase copy constructor (used by SessionConfig.Clone() and
ResumeSessionConfig.Clone()) did not copy the newly added ExpAssignments
property, so cloning a config silently dropped it and it would not be
forwarded on create/resume. Copy it alongside the other base properties.

Cross-language audit of every config copy/clone path:
- .NET: had the gap (fixed here).
- Java: SessionConfig/ResumeSessionConfig clone() already copy expAssignments (safe).
- Rust: SessionConfig/ResumeSessionConfig derive Clone, no manual impl (safe).
- Node: createSession/resumeSession normalize via { ...defaults, ...config }
  spread, which preserves all own properties (safe).
- Go: create/resume requests assign req.ExpAssignments = config.ExpAssignments
  directly from the config pointer (safe).
- Python: exp_assignments is an explicit kwarg mapped straight into the
  payload, no intermediate dict copy (safe).

Add clone regression tests for .NET, Java, and Rust (the three with explicit
Clone()/clone() methods) that set expAssignments, clone, and assert it
survives and is forwarded on the resulting create/resume request. The .NET
tests fail against the pre-fix copy constructor. Node's existing forward test
already exercises the spread-normalization copy path.

Switch the two new Java expAssignments tests off the deprecated
buildCreateRequest(SessionConfig) overload to the current
buildCreateRequest(SessionConfig, String) form to avoid new CodeQL debt.

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

---------

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

Author: ellismg

dotnet/src/Client.cs

@@ -1003,7 +1003,8 @@ 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();
 
@@ -1204,7 +1205,8 @@ 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>(
@@ -2444,7 +2446,8 @@ 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(
@@ -2539,7 +2542,8 @@ 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(

dotnet/src/Types.cs

@@ -2688,6 +2688,7 @@ protected SessionConfigBase(SessionConfigBase? other)
         CreateSessionFsProvider = other.CreateSessionFsProvider;
         GitHubToken = other.GitHubToken;
         RemoteSession = other.RemoteSession;
+        ExpAssignments = other.ExpAssignments;
 #pragma warning disable GHCP001
         Canvases = other.Canvases is not null ? [.. other.Canvases] : null;
         RequestCanvasRenderer = other.RequestCanvasRenderer;
@@ -3064,6 +3065,23 @@ 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

dotnet/test/Unit/SerializationTests.cs

@@ -446,6 +446,97 @@ 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 SessionConfigClone_PreservesExpAssignments()
+    {
+        using var assignments = JsonDocument.Parse("""{"Configs":[{"Id":"exp-create"}]}""");
+
+        var config = new SessionConfig
+        {
+            SessionId = "session-id",
+            ExpAssignments = assignments.RootElement.Clone(),
+        };
+
+        var clone = config.Clone();
+
+        Assert.True(clone.ExpAssignments.HasValue);
+        Assert.Equal(
+            "exp-create",
+            clone.ExpAssignments!.Value.GetProperty("Configs")[0].GetProperty("Id").GetString());
+    }
+
+    [Fact]
+    public void ResumeSessionConfigClone_PreservesExpAssignments()
+    {
+        using var assignments = JsonDocument.Parse("""{"Configs":[{"Id":"exp-resume"}]}""");
+
+        var config = new ResumeSessionConfig
+        {
+            ExpAssignments = assignments.RootElement.Clone(),
+        };
+
+        var clone = config.Clone();
+
+        Assert.True(clone.ExpAssignments.HasValue);
+        Assert.Equal(
+            "exp-resume",
+            clone.ExpAssignments!.Value.GetProperty("Configs")[0].GetProperty("Id").GetString());
+    }
+
     [Fact]
     public void CreateSessionRequest_CanSerializeEnableSessionTelemetry_WithSdkOptions()
     {

go/client.go

@@ -703,6 +703,7 @@ 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))
@@ -1043,6 +1044,7 @@ 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)
 	}

go/client_test.go

@@ -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")
+		}
+	})
+}

go/types.go

@@ -1147,6 +1147,18 @@ 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
@@ -1542,6 +1554,14 @@ 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".
@@ -1892,6 +1912,7 @@ 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"`
 }
@@ -1976,6 +1997,7 @@ 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"`
 }

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

@@ -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;
     }

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

@@ -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;
+    }
 }