Strongly type expAssignments session config across all SDKs (#2033)

* Strongly type expAssignments session config across all SDKs

Replace the opaque JSON typing of the internal `expAssignments`
session-config field with a strongly-typed `CopilotExpAssignmentResponse`
(plus `ExpConfigEntry`) in every SDK, mirroring the runtime contract.

Wire keys remain PascalCase (Features, Flights, Configs, Id, Parameters,
...), optional fields are omitted when null, and the field keeps its
internal/hidden posture in each language.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 43d7a02b-08ff-4e7d-a8c0-afa6dfbe94b0

* Address review: normalize required exp-assignment fields

- Go: add MarshalJSON to CopilotExpAssignmentResponse/ExpConfigEntry so
  nil Features/Flights/Configs/Parameters serialize as []/{} instead of
  null, matching the Python/Rust/.NET reference behavior; add a test.
- Java: default AssignmentContext to "" so the required field is not
  dropped by NON_NULL when unset.
- .NET: tighten ExpConfigEntry.Parameters value type from JsonNode? to
  JsonValue? to constrain values to JSON scalars.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 43d7a02b-08ff-4e7d-a8c0-afa6dfbe94b0

* Default ExpConfigEntry.Id to "" in Java

The Id field is required by the wire contract, but defaulting to null let
class-level NON_NULL drop the key for a zero-value entry. Default it to ""
so the required key is always emitted, matching the Go and .NET defaults.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 43d7a02b-08ff-4e7d-a8c0-afa6dfbe94b0

* Fix stale "opaque JSON" comment in Python exp wiring

The expAssignments path now serializes a concrete CopilotExpAssignmentResponse,
so drop the outdated "opaque JSON" wording from the adjacent comments.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 43d7a02b-08ff-4e7d-a8c0-afa6dfbe94b0

* Fix nightly rustfmt import grouping in types.rs tests

Move `use std::collections::HashMap;` into the std import block so
`cargo fmt --check` with the nightly group_imports config passes in CI.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 43d7a02b-08ff-4e7d-a8c0-afa6dfbe94b0

---------

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

Author: devm33

dotnet/src/Client.cs

@@ -2758,7 +2758,7 @@ internal record CreateSessionRequest(
         IList<NamedProviderConfig>? Providers = null,
         IList<ProviderModelConfig>? Models = null,
         OptionsUpdateToolFilterPrecedence? ToolFilterPrecedence = null,
-        [property: JsonPropertyName("expAssignments")] JsonElement? ExpAssignments = null,
+        [property: JsonPropertyName("expAssignments")] CopilotExpAssignmentResponse? ExpAssignments = null,
         [property: JsonPropertyName("enableManagedSettings")] bool? EnableManagedSettings = null,
         bool? EnableGitHubTelemetryForwarding = null);
 #pragma warning restore GHCP001
@@ -2864,7 +2864,7 @@ internal record ResumeSessionRequest(
         IList<NamedProviderConfig>? Providers = null,
         IList<ProviderModelConfig>? Models = null,
         OptionsUpdateToolFilterPrecedence? ToolFilterPrecedence = null,
-        [property: JsonPropertyName("expAssignments")] JsonElement? ExpAssignments = null,
+        [property: JsonPropertyName("expAssignments")] CopilotExpAssignmentResponse? ExpAssignments = null,
         [property: JsonPropertyName("enableManagedSettings")] bool? EnableManagedSettings = null,
         bool? EnableGitHubTelemetryForwarding = null);
 #pragma warning restore GHCP001

dotnet/src/Types.cs

@@ -2833,6 +2833,64 @@ public struct SetModelOptions
     public ModelCapabilitiesOverride? ModelCapabilities { get; set; }
 }
 
+/// <summary>
+/// A single configuration entry in a <see cref="CopilotExpAssignmentResponse"/>.
+/// Each entry carries an identifier and a bag of typed parameter values.
+/// </summary>
+public sealed class ExpConfigEntry
+{
+    /// <summary>Identifier of the configuration entry.</summary>
+    [JsonPropertyName("Id")]
+    public string Id { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Parameter values keyed by parameter name. Each value is a scalar string,
+    /// number, boolean, or <c>null</c>.
+    /// </summary>
+    [JsonPropertyName("Parameters")]
+    public IDictionary<string, JsonValue?> Parameters { get; set; } = new Dictionary<string, JsonValue?>();
+}
+
+/// <summary>
+/// ExP ("flight") assignment data, in the same JSON shape the Copilot CLI
+/// fetches from the experimentation service. Property names serialize as
+/// PascalCase (<c>Features</c>, <c>Flights</c>, ...) to match the on-the-wire
+/// contract consumed by the runtime.
+/// </summary>
+public sealed class CopilotExpAssignmentResponse
+{
+    /// <summary>Enabled feature names.</summary>
+    [JsonPropertyName("Features")]
+    public IList<string> Features { get; set; } = new List<string>();
+
+    /// <summary>Assigned flights keyed by flight name.</summary>
+    [JsonPropertyName("Flights")]
+    public IDictionary<string, string> Flights { get; set; } = new Dictionary<string, string>();
+
+    /// <summary>Configuration entries carrying typed parameter values.</summary>
+    [JsonPropertyName("Configs")]
+    public IList<ExpConfigEntry> Configs { get; set; } = new List<ExpConfigEntry>();
+
+    /// <summary>Opaque parameter-group payload passed through untouched. Optional.</summary>
+    [JsonPropertyName("ParameterGroups")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public JsonNode? ParameterGroups { get; set; }
+
+    /// <summary>Version of the flighting configuration. Optional.</summary>
+    [JsonPropertyName("FlightingVersion")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public int? FlightingVersion { get; set; }
+
+    /// <summary>Impression identifier for the assignment. Optional.</summary>
+    [JsonPropertyName("ImpressionId")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public string? ImpressionId { get; set; }
+
+    /// <summary>Assignment context string forwarded to CAPI and telemetry.</summary>
+    [JsonPropertyName("AssignmentContext")]
+    public string AssignmentContext { get; set; } = string.Empty;
+}
+
 /// <summary>
 /// Shared configuration properties for creating or resuming a Copilot session.
 /// Use <see cref="SessionConfig"/> when creating a new session, or
@@ -3339,7 +3397,7 @@ protected SessionConfigBase(SessionConfigBase? other)
     /// completion. It is not part of the broadly advertised public surface.
     /// </remarks>
     [EditorBrowsable(EditorBrowsableState.Never)]
-    public JsonElement? ExpAssignments { get; set; }
+    public CopilotExpAssignmentResponse? ExpAssignments { get; set; }
 
     /// <summary>
     /// Opt-in: when <c>true</c>, the runtime self-fetches enterprise managed
@@ -4041,6 +4099,8 @@ public sealed class SystemMessageTransformRpcResponse
 [JsonSerializable(typeof(AutoModeSwitchRequest))]
 [JsonSerializable(typeof(AutoModeSwitchResponse))]
 [JsonSerializable(typeof(CustomAgentConfig))]
+[JsonSerializable(typeof(CopilotExpAssignmentResponse))]
+[JsonSerializable(typeof(ExpConfigEntry))]
 [JsonSerializable(typeof(ExitPlanModeRequest))]
 [JsonSerializable(typeof(ExitPlanModeResult))]
 [JsonSerializable(typeof(GetAuthStatusResponse))]

dotnet/test/Unit/SerializationTests.cs

@@ -3,6 +3,7 @@
  *--------------------------------------------------------------------------------------------*/
 
 using Xunit;
+using System.Collections.Generic;
 using System.Text.Json;
 #if !NET8_0_OR_GREATER
 using System.Runtime.Serialization;
@@ -488,24 +489,28 @@ 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()));
+            ("ExpAssignments", new CopilotExpAssignmentResponse
+            {
+                Configs = new List<ExpConfigEntry> { new() { Id = "exp-create" } },
+            }));
 
         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()));
+            ("ExpAssignments", new CopilotExpAssignmentResponse
+            {
+                Configs = new List<ExpConfigEntry> { new() { Id = "exp-resume" } },
+            }));
 
         var resumeJson = JsonSerializer.Serialize(resumeRequest, resumeRequestType, options);
         using var resumeDocument = JsonDocument.Parse(resumeJson);
@@ -540,38 +545,36 @@ public void SessionRequests_OmitExpAssignments_WhenUnset()
     [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(),
+            ExpAssignments = new CopilotExpAssignmentResponse
+            {
+                Configs = new List<ExpConfigEntry> { new() { Id = "exp-create" } },
+            },
         };
 
         var clone = config.Clone();
 
-        Assert.True(clone.ExpAssignments.HasValue);
-        Assert.Equal(
-            "exp-create",
-            clone.ExpAssignments!.Value.GetProperty("Configs")[0].GetProperty("Id").GetString());
+        Assert.NotNull(clone.ExpAssignments);
+        Assert.Equal("exp-create", clone.ExpAssignments!.Configs[0].Id);
     }
 
     [Fact]
     public void ResumeSessionConfigClone_PreservesExpAssignments()
     {
-        using var assignments = JsonDocument.Parse("""{"Configs":[{"Id":"exp-resume"}]}""");
-
         var config = new ResumeSessionConfig
         {
-            ExpAssignments = assignments.RootElement.Clone(),
+            ExpAssignments = new CopilotExpAssignmentResponse
+            {
+                Configs = new List<ExpConfigEntry> { new() { Id = "exp-resume" } },
+            },
         };
 
         var clone = config.Clone();
 
-        Assert.True(clone.ExpAssignments.HasValue);
-        Assert.Equal(
-            "exp-resume",
-            clone.ExpAssignments!.Value.GetProperty("Configs")[0].GetProperty("Id").GetString());
+        Assert.NotNull(clone.ExpAssignments);
+        Assert.Equal("exp-resume", clone.ExpAssignments!.Configs[0].Id);
     }
 
     [Fact]

go/client_test.go

@@ -3181,9 +3181,13 @@ func TestStartCLIServer_StderrFieldSet(t *testing.T) {
 }
 
 func TestCreateSessionRequest_ExpAssignments(t *testing.T) {
-	assignments := map[string]any{
-		"Parameters":        map[string]any{"copilot_exp_flag": "treatment"},
-		"AssignmentContext": "ctx-123",
+	assignments := &CopilotExpAssignmentResponse{
+		Features: []string{"copilot_exp_flag"},
+		Flights:  map[string]string{"copilot_exp_flag": "treatment"},
+		Configs: []ExpConfigEntry{
+			{ID: "cfg-1", Parameters: map[string]ExpFlagValue{"threshold": 5, "enabled": true}},
+		},
+		AssignmentContext: "ctx-123",
 	}
 
 	t.Run("includes expAssignments in JSON when set", func(t *testing.T) {
@@ -3221,10 +3225,50 @@ func TestCreateSessionRequest_ExpAssignments(t *testing.T) {
 	})
 }
 
+func TestCopilotExpAssignmentResponse_MarshalNormalizesNilCollections(t *testing.T) {
+	// A response left with zero-value collections must still serialize the
+	// required fields as JSON arrays/objects, not null, so the runtime does not
+	// treat the payload as malformed.
+	data, err := json.Marshal(&CopilotExpAssignmentResponse{AssignmentContext: "ctx"})
+	if err != nil {
+		t.Fatalf("Failed to marshal: %v", err)
+	}
+	var m map[string]json.RawMessage
+	if err := json.Unmarshal(data, &m); err != nil {
+		t.Fatalf("Failed to unmarshal: %v", err)
+	}
+	for _, tc := range []struct{ key, want string }{
+		{"Features", "[]"},
+		{"Flights", "{}"},
+		{"Configs", "[]"},
+		{"AssignmentContext", `"ctx"`},
+	} {
+		if got := string(m[tc.key]); got != tc.want {
+			t.Errorf("Expected %s to serialize as %s, got %s", tc.key, tc.want, got)
+		}
+	}
+
+	// A nil Parameters map on an entry must likewise serialize as {}.
+	entryData, err := json.Marshal(ExpConfigEntry{ID: "cfg"})
+	if err != nil {
+		t.Fatalf("Failed to marshal entry: %v", err)
+	}
+	if err := json.Unmarshal(entryData, &m); err != nil {
+		t.Fatalf("Failed to unmarshal entry: %v", err)
+	}
+	if got := string(m["Parameters"]); got != "{}" {
+		t.Errorf("Expected Parameters to serialize as {}, got %s", got)
+	}
+}
+
 func TestResumeSessionRequest_ExpAssignments(t *testing.T) {
-	assignments := map[string]any{
-		"Parameters":        map[string]any{"copilot_exp_flag": "treatment"},
-		"AssignmentContext": "ctx-456",
+	assignments := &CopilotExpAssignmentResponse{
+		Features: []string{"copilot_exp_flag"},
+		Flights:  map[string]string{"copilot_exp_flag": "treatment"},
+		Configs: []ExpConfigEntry{
+			{ID: "cfg-1", Parameters: map[string]ExpFlagValue{"copilot_exp_flag": "treatment"}},
+		},
+		AssignmentContext: "ctx-456",
 	}
 
 	t.Run("includes expAssignments in JSON when set", func(t *testing.T) {

go/internal/e2e/client_options_e2e_test.go

@@ -272,7 +272,7 @@ func TestClientOptionsE2E(t *testing.T) {
 			RequestExtensions:     copilot.Bool(true),
 			ExtensionSDKPath:      &extensionSDKPath,
 			ExtensionInfo:         &copilot.ExtensionInfo{Source: "github-app", Name: "go-e2e-extension"},
-			ExpAssignments:        map[string]any{"feature": "enabled"},
+			ExpAssignments:        &copilot.CopilotExpAssignmentResponse{Flights: map[string]string{"feature": "enabled"}, AssignmentContext: "ctx"},
 		})
 		if err != nil {
 			t.Fatalf("CreateSession failed: %v", err)
@@ -342,7 +342,7 @@ func TestClientOptionsE2E(t *testing.T) {
 		if canvas["id"] != "canvas" || canvas["displayName"] != "Canvas" || canvas["description"] != "Canvas description" {
 			t.Fatalf("Expected canvas declaration to be forwarded, got %#v", canvas)
 		}
-		if params["expAssignments"].(map[string]any)["feature"] != "enabled" {
+		if params["expAssignments"].(map[string]any)["Flights"].(map[string]any)["feature"] != "enabled" {
 			t.Fatalf("Expected expAssignments to be forwarded, got %#v", params["expAssignments"])
 		}
 	})
@@ -460,7 +460,7 @@ func TestClientOptionsE2E(t *testing.T) {
 			RequestExtensions:     copilot.Bool(true),
 			ExtensionSDKPath:      &extensionSDKPath,
 			ExtensionInfo:         &copilot.ExtensionInfo{Source: "github-app", Name: "go-e2e-extension"},
-			ExpAssignments:        map[string]any{"resumeFeature": "enabled"},
+			ExpAssignments:        &copilot.CopilotExpAssignmentResponse{Flights: map[string]string{"resumeFeature": "enabled"}, AssignmentContext: "ctx"},
 		})
 		if err != nil {
 			t.Fatalf("ResumeSession failed: %v", err)
@@ -503,7 +503,7 @@ func TestClientOptionsE2E(t *testing.T) {
 		if extensionInfo["source"] != "github-app" || extensionInfo["name"] != "go-e2e-extension" {
 			t.Fatalf("Expected extensionInfo on resume, got %#v", extensionInfo)
 		}
-		if params["expAssignments"].(map[string]any)["resumeFeature"] != "enabled" {
+		if params["expAssignments"].(map[string]any)["Flights"].(map[string]any)["resumeFeature"] != "enabled" {
 			t.Fatalf("Expected resume expAssignments to be forwarded, got %#v", params["expAssignments"])
 		}
 	})