Isolate reasoning model switch fixtures

Load the gpt-5.4 catalog in dedicated test contexts so model-switch assertions exercise the selected reasoning-capable model across SDKs.

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

Copilot-Session: 9b80f6cb-b851-4598-8294-f4431c6f3143

Author: ellismg

dotnet/test/E2E/SessionE2ETests.cs

@@ -611,7 +611,10 @@ public async Task Should_Set_Model_On_Existing_Session()
     [Fact]
     public async Task Should_Set_Model_With_ReasoningEffort()
     {
-        var session = await CreateSessionAsync();
+        await using var isolatedCtx = await E2ETestContext.CreateAsync();
+        await isolatedCtx.ConfigureForTestAsync("session", nameof(Should_Set_Model_With_ReasoningEffort));
+        var isolatedClient = isolatedCtx.CreateClient();
+        await using var session = await isolatedCtx.CreateSessionAsync(isolatedClient);
 
         var modelChangedTask = TestHelper.GetNextEventOfTypeAsync<SessionModelChangeEvent>(session);
 

go/internal/e2e/session_e2e_test.go

@@ -1047,7 +1047,12 @@ func getSystemMessage(exchange testharness.ParsedHttpExchange) string {
 }
 
 func TestSetModelWithReasoningEffortE2E(t *testing.T) {
+	t.Run("should set model with reasoningeffort", runSetModelWithReasoningEffortE2E)
+}
+
+func runSetModelWithReasoningEffortE2E(t *testing.T) {
 	ctx := testharness.NewTestContext(t)
+	ctx.ConfigureForTest(t)
 	client := ctx.NewClient()
 	t.Cleanup(func() { client.ForceStop() })
 

nodejs/test/e2e/session.e2e.test.ts

@@ -964,15 +964,21 @@ describe("Send Blocking Behavior", async () => {
         expect(event.data.newModel).toBe("gpt-4.1");
     });
 
-    it("should set model with reasoningEffort", async () => {
-        await using session = await client.createSession({ onPermissionRequest: approveAll });
+    describe("reasoning effort model switch (isolated to avoid models cache contamination)", async () => {
+        const { copilotClient: reasoningClient } = await createSdkTestContext();
 
-        const modelChangePromise = getNextEventOfType(session, "session.model_change");
+        it("should set model with reasoningEffort", async () => {
+            await using session = await reasoningClient.createSession({
+                onPermissionRequest: approveAll,
+            });
 
-        await session.setModel("gpt-5.4", { reasoningEffort: "high" });
+            const modelChangePromise = getNextEventOfType(session, "session.model_change");
 
-        const event = await modelChangePromise;
-        expect(event.data.newModel).toBe("gpt-5.4");
-        expect(event.data.reasoningEffort).toBe("high");
+            await session.setModel("gpt-5.4", { reasoningEffort: "high" });
+
+            const event = await modelChangePromise;
+            expect(event.data.newModel).toBe("gpt-5.4");
+            expect(event.data.reasoningEffort).toBe("high");
+        });
     });
 });

python/e2e/test_session_e2e.py

@@ -679,27 +679,36 @@ async def test_should_set_model_with_reasoning_effort(self, ctx: E2ETestContext)
         """Test that setModel passes reasoningEffort and it appears in the model_change event."""
         import asyncio
 
-        session = await ctx.client.create_session(
-            on_permission_request=PermissionHandler.approve_all
-        )
+        isolated_ctx = E2ETestContext()
+        await isolated_ctx.setup()
+        try:
+            await isolated_ctx.configure_for_test(
+                "session", "should_set_model_with_reasoningeffort"
+            )
+            session = await isolated_ctx.client.create_session(
+                on_permission_request=PermissionHandler.approve_all
+            )
 
-        model_change_event = asyncio.get_event_loop().create_future()
+            model_change_event = asyncio.get_event_loop().create_future()
 
-        def on_event(event):
-            if model_change_event.done():
-                return
+            def on_event(event):
+                if model_change_event.done():
+                    return
 
-            match event.data:
-                case SessionModelChangeData() as data:
-                    model_change_event.set_result(data)
+                match event.data:
+                    case SessionModelChangeData() as data:
+                        model_change_event.set_result(data)
 
-        session.on(on_event)
+            session.on(on_event)
 
-        await session.set_model("gpt-5.4", reasoning_effort="high")
+            await session.set_model("gpt-5.4", reasoning_effort="high")
 
-        data = await asyncio.wait_for(model_change_event, timeout=30)
-        assert data.new_model == "gpt-5.4"
-        assert data.reasoning_effort == "high"
+            data = await asyncio.wait_for(model_change_event, timeout=30)
+            assert data.new_model == "gpt-5.4"
+            assert data.reasoning_effort == "high"
+            await session.disconnect()
+        finally:
+            await isolated_ctx.teardown()
 
     async def test_should_accept_blob_attachments(self, ctx: E2ETestContext):
         # Write the image to disk so the model can view it

test/snapshots/session/should_set_model_with_reasoningeffort.yaml

@@ -1,5 +1,6 @@
 models:
   - claude-sonnet-4.5
+  - gpt-5.4
 conversations:
   - messages:
       - role: system