Fixes #250 - All SDKs Validate Session Config Model

dotnet/src/Client.cs

@@ -344,6 +344,20 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig? config = nul
     {
         var connection = await EnsureConnectedAsync(cancellationToken);
 
+        if (!string.IsNullOrEmpty(config?.Model))
+        {
+            // ListModelsAsync caches results after the first call, so this validation has minimal overhead
+            var availableModels = await ListModelsAsync(cancellationToken).ConfigureAwait(false);
+            var validModelIds = new HashSet<string>(availableModels.Select(m => m.Id), StringComparer.OrdinalIgnoreCase);
+
+            if (!validModelIds.Contains(config.Model))
+            {
+                throw new ArgumentException(
+                    $"Invalid model '{config.Model}'. Available models: {string.Join(", ", validModelIds)}",
+                    nameof(config));
+            }
+        }
+
         var hasHooks = config?.Hooks != null && (
             config.Hooks.OnPreToolUse != null ||
             config.Hooks.OnPostToolUse != null ||

dotnet/test/SessionTests.cs

@@ -15,7 +15,7 @@ public class SessionTests(E2ETestFixture fixture, ITestOutputHelper output) : E2
     [Fact]
     public async Task ShouldCreateAndDestroySessions()
     {
-        var session = await Client.CreateSessionAsync(new SessionConfig { Model = "fake-test-model" });
+        var session = await Client.CreateSessionAsync(new SessionConfig { Model = "claude-sonnet-4.5" });
 
         Assert.Matches(@"^[a-f0-9-]+$", session.SessionId);
 
@@ -395,4 +395,19 @@ public async Task Should_Create_Session_With_Custom_Config_Dir()
         Assert.NotNull(assistantMessage);
         Assert.Contains("2", assistantMessage!.Data.Content);
     }
+
+    [Fact]
+    public async Task CreateSessionAsync_WithInvalidModel_ThrowsArgumentException()
+    {
+        var exception = await Assert.ThrowsAsync<ArgumentException>(async () =>
+        {
+            await Client.CreateSessionAsync(new SessionConfig
+            {
+                Model = "INVALID_MODEL_THAT_DOES_NOT_EXIST"
+            });
+        });
+
+        Assert.Contains("Invalid model", exception.Message);
+        Assert.Contains("INVALID_MODEL_THAT_DOES_NOT_EXIST", exception.Message);
+    }
 }

go/client.go

@@ -470,6 +470,30 @@ func (c *Client) CreateSession(ctx context.Context, config *SessionConfig) (*Ses
 	params := make(map[string]any)
 	if config != nil {
 		if config.Model != "" {
+			// Validate model if specified
+			// ListModels caches results after the first call, so this validation has minimal overhead
+			availableModels, err := c.ListModels(ctx)
+			if err != nil {
+				return nil, fmt.Errorf("failed to list models: %w", err)
+			}
+
+			modelLower := strings.ToLower(config.Model)
+			modelFound := false
+			for _, model := range availableModels {
+				if strings.ToLower(model.ID) == modelLower {
+					modelFound = true
+					break
+				}
+			}
+
+			if !modelFound {
+				validIDs := make([]string, len(availableModels))
+				for i, model := range availableModels {
+					validIDs[i] = model.ID
+				}
+				return nil, fmt.Errorf("invalid model '%s'. Available models: %s", config.Model, strings.Join(validIDs, ", "))
+			}
+
 			params["model"] = config.Model
 		}
 		if config.SessionID != "" {

go/client_test.go

@@ -5,9 +5,15 @@ import (
 	"path/filepath"
 	"reflect"
 	"regexp"
+	"strings"
 	"testing"
 )
 
+// containsIgnoreCase checks if a string contains a substring (case-insensitive)
+func containsIgnoreCase(s, substr string) bool {
+	return strings.Contains(strings.ToLower(s), strings.ToLower(substr))
+}
+
 // This file is for unit tests. Where relevant, prefer to add e2e tests in e2e/*.test.go instead
 
 func TestClient_HandleToolCallRequest(t *testing.T) {
@@ -48,6 +54,38 @@ func TestClient_HandleToolCallRequest(t *testing.T) {
 	})
 }
 
+func TestClient_CreateSession_WithInvalidModel(t *testing.T) {
+	cliPath := findCLIPathForTest()
+	if cliPath == "" {
+		t.Skip("CLI not found")
+	}
+
+	client := NewClient(&ClientOptions{CLIPath: cliPath})
+	t.Cleanup(func() { client.ForceStop() })
+
+	err := client.Start(t.Context())
+	if err != nil {
+		t.Fatalf("Failed to start client: %v", err)
+	}
+
+	_, err = client.CreateSession(t.Context(), &SessionConfig{
+		Model: "INVALID_MODEL_THAT_DOES_NOT_EXIST",
+	})
+
+	if err == nil {
+		t.Fatal("Expected error when creating session with invalid model, got nil")
+	}
+
+	errorMsg := err.Error()
+	if !containsIgnoreCase(errorMsg, "invalid model") {
+		t.Errorf("Expected error message to contain 'invalid model', got: %s", errorMsg)
+	}
+
+	if !containsIgnoreCase(errorMsg, "INVALID_MODEL_THAT_DOES_NOT_EXIST") {
+		t.Errorf("Expected error message to contain 'INVALID_MODEL_THAT_DOES_NOT_EXIST', got: %s", errorMsg)
+	}
+}
+
 func TestClient_URLParsing(t *testing.T) {
 	t.Run("should parse port-only URL format", func(t *testing.T) {
 		client := NewClient(&ClientOptions{

go/internal/e2e/session_test.go

@@ -18,7 +18,7 @@ func TestSession(t *testing.T) {
 	t.Run("should create and destroy sessions", func(t *testing.T) {
 		ctx.ConfigureForTest(t)
 
-		session, err := client.CreateSession(t.Context(), &copilot.SessionConfig{Model: "fake-test-model"})
+		session, err := client.CreateSession(t.Context(), &copilot.SessionConfig{Model: "claude-sonnet-4.5"})
 		if err != nil {
 			t.Fatalf("Failed to create session: %v", err)
 		}
@@ -41,8 +41,8 @@ func TestSession(t *testing.T) {
 			t.Errorf("Expected session.start sessionId to match")
 		}
 
-		if messages[0].Data.SelectedModel == nil || *messages[0].Data.SelectedModel != "fake-test-model" {
-			t.Errorf("Expected selectedModel to be 'fake-test-model', got %v", messages[0].Data.SelectedModel)
+		if messages[0].Data.SelectedModel == nil || *messages[0].Data.SelectedModel != "claude-sonnet-4.5" {
+			t.Errorf("Expected selectedModel to be 'claude-sonnet-4.5', got %v", messages[0].Data.SelectedModel)
 		}
 
 		if err := session.Destroy(); err != nil {

nodejs/src/client.ts

@@ -467,6 +467,20 @@ export class CopilotClient {
             }
         }
 
+        // Validate model if specified
+        if (config.model) {
+            // listModels() caches results, so this has minimal overhead
+            const availableModels = await this.listModels();
+            const modelLower = config.model.toLowerCase();
+
+            if (!availableModels.some((m) => m.id.toLowerCase() === modelLower)) {
+                const validIds = availableModels.map((m) => m.id);
+                throw new Error(
+                    `Invalid model '${config.model}'. Available models: ${validIds.join(", ")}`
+                );
+            }
+        }
+
         const response = await this.connection!.sendRequest("session.create", {
             model: config.model,
             sessionId: config.sessionId,

nodejs/test/client.test.ts

@@ -28,6 +28,20 @@ describe("CopilotClient", () => {
         });
     });
 
+    it("throws error when creating session with invalid model", async () => {
+        const client = new CopilotClient({ cliPath: CLI_PATH });
+        await client.start();
+        onTestFinished(() => client.forceStop());
+
+        const error = await client
+            .createSession({ model: "INVALID_MODEL_THAT_DOES_NOT_EXIST" })
+            .catch((e) => e);
+
+        expect(error).toBeInstanceOf(Error);
+        expect(error.message).toContain("Invalid model");
+        expect(error.message).toContain("INVALID_MODEL_THAT_DOES_NOT_EXIST");
+    });
+
     describe("URL parsing", () => {
         it("should parse port-only URL format", () => {
             const client = new CopilotClient({

nodejs/test/e2e/session.test.ts

@@ -8,13 +8,13 @@ describe("Sessions", async () => {
     const { copilotClient: client, openAiEndpoint, homeDir, env } = await createSdkTestContext();
 
     it("should create and destroy sessions", async () => {
-        const session = await client.createSession({ model: "fake-test-model" });
+        const session = await client.createSession({ model: "claude-sonnet-4.5" });
         expect(session.sessionId).toMatch(/^[a-f0-9-]+$/);
 
         expect(await session.getMessages()).toMatchObject([
             {
                 type: "session.start",
-                data: { sessionId: session.sessionId, selectedModel: "fake-test-model" },
+                data: { sessionId: session.sessionId, selectedModel: "claude-sonnet-4.5" },
             },
         ]);
 

python/copilot/client.py

@@ -389,6 +389,19 @@ async def create_session(self, config: Optional[SessionConfig] = None) -> Copilo
 
         cfg = config or {}
 
+        # Validate model if specified
+        model = cfg.get("model")
+        if model:
+            # list_models() caches results, so this has minimal overhead
+            available_models = await self.list_models()
+            model_lower = model.lower()
+
+            if not any(m.id.lower() == model_lower for m in available_models):
+                valid_ids = [m.id for m in available_models]
+                raise ValueError(
+                    f"Invalid model '{model}'. Available models: {', '.join(valid_ids)}"
+                )
+
         tool_defs = []
         tools = cfg.get("tools")
         if tools:

python/e2e/test_session.py

@@ -14,14 +14,14 @@
 
 class TestSessions:
     async def test_should_create_and_destroy_sessions(self, ctx: E2ETestContext):
-        session = await ctx.client.create_session({"model": "fake-test-model"})
+        session = await ctx.client.create_session({"model": "claude-sonnet-4.5"})
         assert session.session_id
 
         messages = await session.get_messages()
         assert len(messages) > 0
         assert messages[0].type.value == "session.start"
         assert messages[0].data.session_id == session.session_id
-        assert messages[0].data.selected_model == "fake-test-model"
+        assert messages[0].data.selected_model == "claude-sonnet-4.5"
 
         await session.destroy()
 
@@ -456,6 +456,13 @@ def on_event(event):
         assistant_message = await get_final_assistant_message(session)
         assert "300" in assistant_message.data.content
 
+    async def test_create_session_with_invalid_model_raises_value_error(self, ctx: E2ETestContext):
+        with pytest.raises(ValueError) as exc_info:
+            await ctx.client.create_session({"model": "INVALID_MODEL_THAT_DOES_NOT_EXIST"})
+
+        assert "Invalid model" in str(exc_info.value)
+        assert "INVALID_MODEL_THAT_DOES_NOT_EXIST" in str(exc_info.value)
+
     async def test_should_create_session_with_custom_config_dir(self, ctx: E2ETestContext):
         import os