Fix flaky pending-messages-modified E2E test across SDKs

The pending-messages-modified event-fidelity test was failing intermittently in

the C# SDK CI leg. The root cause was that this test was the only one in its

fixture not using the standard `SendAndWaitAsync` + event-collector pattern;

it went through a custom helper that did two independently-timed awaits and

used an `async void` local function for backfilling existing messages.

Refactor the test to the standard pattern in C#, Node, Python, and Go, and

while here, replace the `async void` (and its JS analog,`new Promise(async

(...) => ...)`) with proper `async Task` / `async` functions drained

deterministically. The C# helper now has zero `async void` methods.

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

Author: stephentoub

dotnet/test/E2E/EventFidelityE2ETests.cs

@@ -136,22 +136,26 @@ await session.SendAndWaitAsync(new MessageOptions
     public async Task Should_Emit_Pending_Messages_Modified_Event_When_Message_Queue_Changes()
     {
         var session = await CreateSessionAsync();
-        var pendingMessagesModified = TestHelper.GetNextEventOfTypeAsync<PendingMessagesModifiedEvent>(
-            session,
-            static _ => true,
-            timeout: TimeSpan.FromSeconds(60),
-            timeoutDescription: "pending_messages.modified event");
+        var events = new List<SessionEvent>();
+        session.On<SessionEvent>(evt => { lock (events) { events.Add(evt); } });
 
-        await session.SendAsync(new MessageOptions
+        // Use SendAndWaitAsync + a single event collector to match the pattern
+        // of every other test in this fixture (and the Rust E2E equivalent).
+        // The earlier SendAsync + GetFinalAssistantMessageAsync split relied
+        // on a custom helper with an async-void backfill and required juggling
+        // two independently-timed awaits, which has been observed to flake in
+        // CI.
+        var answer = await session.SendAndWaitAsync(new MessageOptions
         {
             Prompt = "What is 9+9? Reply with just the number.",
-        });
+        }, timeout: TimeSpan.FromSeconds(120));
 
-        var answer = await TestHelper.GetFinalAssistantMessageAsync(session);
-        var pendingEvent = await pendingMessagesModified;
+        PendingMessagesModifiedEvent? pendingEvent;
+        lock (events) { pendingEvent = events.OfType<PendingMessagesModifiedEvent>().FirstOrDefault(); }
 
         Assert.NotNull(pendingEvent);
-        Assert.Contains("18", answer?.Data.Content ?? string.Empty);
+        Assert.NotNull(answer);
+        Assert.Contains("18", answer!.Data.Content);
 
         await session.DisposeAsync();
     }

dotnet/test/Harness/TestHelper.cs

@@ -22,7 +22,7 @@ public static class TestHelper
         using var cts = new CancellationTokenSource(timeout ?? DefaultEventTimeout);
 
         // Both `finalAssistantMessage` and `sawIdle` are set from two threads — the
-        // subscription callback (CLI read loop) and CheckExistingMessages (RPC reply).
+        // subscription callback (CLI read loop) and CheckExistingMessagesAsync (RPC reply).
         // We complete only once we've observed both, regardless of which path saw which.
         var stateLock = new object();
         AssistantMessageEvent? finalAssistantMessage = null;
@@ -59,14 +59,29 @@ void TryComplete()
         });
 
         // Backfill from already-delivered messages so we don't lose events that arrived
-        // between SendAsync returning and the subscription being installed.
-        CheckExistingMessages();
+        // between SendAsync returning and the subscription being installed. Run it
+        // concurrently with the live subscription, but keep the Task observable so any
+        // exception is propagated through tcs (not the unobserved-task handler) and so
+        // we can drain it deterministically below.
+        var backfill = CheckExistingMessagesAsync();
 
-        cts.Token.Register(() => tcs.TrySetException(new TimeoutException("Timeout waiting for assistant message")));
+        using var registration = cts.Token.Register(
+            static state => ((TaskCompletionSource<AssistantMessageEvent>)state!).TrySetException(
+                new TimeoutException("Timeout waiting for assistant message")),
+            tcs);
 
-        return await tcs.Task;
+        try
+        {
+            return await tcs.Task;
+        }
+        finally
+        {
+            // Drain the backfill before our `using` scopes (cts, subscription) dispose.
+            // Any exception was already routed through tcs above, so swallow here.
+            try { await backfill.ConfigureAwait(false); } catch { }
+        }
 
-        async void CheckExistingMessages()
+        async Task CheckExistingMessagesAsync()
         {
             try
             {

go/internal/e2e/event_fidelity_e2e_test.go

@@ -6,7 +6,6 @@ import (
 	"strings"
 	"sync"
 	"testing"
-	"time"
 
 	copilot "github.com/github/copilot-sdk/go"
 	"github.com/github/copilot-sdk/go/internal/e2e/testharness"
@@ -135,34 +134,40 @@ func TestEventFidelityE2E(t *testing.T) {
 		}
 		t.Cleanup(func() { _ = session.Disconnect() })
 
-		pendingModified := make(chan *copilot.SessionEvent, 1)
+		var mu sync.Mutex
+		var events []copilot.SessionEvent
 		session.On(func(event copilot.SessionEvent) {
-			if _, ok := event.Data.(*copilot.PendingMessagesModifiedData); ok {
-				select {
-				case pendingModified <- &event:
-				default:
-				}
-			}
+			mu.Lock()
+			events = append(events, event)
+			mu.Unlock()
 		})
 
-		if _, err := session.Send(t.Context(), copilot.MessageOptions{
+		// SendAndWait collects everything in one round trip and matches the
+		// pattern of every other test in this file (and the Rust E2E equivalent),
+		// avoiding the split fire-and-forget + helper pattern that previously
+		// made this test prone to flakes.
+		answer, err := session.SendAndWait(t.Context(), copilot.MessageOptions{
 			Prompt: "What is 9+9? Reply with just the number.",
-		}); err != nil {
-			t.Fatalf("Send failed: %v", err)
+		})
+		if err != nil {
+			t.Fatalf("SendAndWait failed: %v", err)
 		}
 
-		select {
-		case evt := <-pendingModified:
-			if evt == nil {
-				t.Error("Expected a non-nil pending_messages.modified event")
+		snapshot := snapshotEventFidelityEvents(&mu, &events)
+
+		var pendingEvent *copilot.SessionEvent
+		for i := range snapshot {
+			if _, ok := snapshot[i].Data.(*copilot.PendingMessagesModifiedData); ok {
+				pendingEvent = &snapshot[i]
+				break
 			}
-		case <-time.After(60 * time.Second):
-			t.Fatal("Timed out waiting for pending_messages.modified event")
+		}
+		if pendingEvent == nil {
+			t.Error("Expected to observe a pending_messages.modified event")
 		}
 
-		answer, err := testharness.GetFinalAssistantMessage(t.Context(), session)
-		if err != nil {
-			t.Fatalf("Failed to get final assistant message: %v", err)
+		if answer == nil {
+			t.Fatal("Expected SendAndWait to return an assistant message")
 		}
 		if ad, ok := answer.Data.(*copilot.AssistantMessageData); !ok || !strings.Contains(ad.Content, "18") {
 			t.Errorf("Expected answer to contain '18', got %v", answer.Data)

nodejs/test/e2e/event_fidelity.e2e.test.ts

@@ -7,7 +7,6 @@ import { join } from "path";
 import { describe, expect, it } from "vitest";
 import { SessionEvent, approveAll } from "../../src/index.js";
 import { createSdkTestContext } from "./harness/sdkTestContext";
-import { getFinalAssistantMessage, getNextEventOfType } from "./harness/sdkTestHelper.js";
 
 describe("Event Fidelity", async () => {
     const { copilotClient: client, workDir } = await createSdkTestContext();
@@ -178,17 +177,20 @@ describe("Event Fidelity", async () => {
 
     it("should emit pending messages modified event when message queue changes", async () => {
         const session = await client.createSession({ onPermissionRequest: approveAll });
+        const events: SessionEvent[] = [];
+        session.on((event) => {
+            events.push(event);
+        });
 
-        const pendingModifiedP = getNextEventOfType(session, "pending_messages.modified");
-
-        void session.send({
+        // sendAndWait collects everything in one round trip and matches the
+        // pattern of every other test in this file (and the Rust E2E equivalent),
+        // avoiding the split fire-and-forget + helper pattern that previously
+        // made this test prone to flakes.
+        const answer = await session.sendAndWait({
             prompt: "What is 9+9? Reply with just the number.",
         });
 
-        const [pendingEvent, answer] = await Promise.all([
-            pendingModifiedP,
-            getFinalAssistantMessage(session),
-        ]);
+        const pendingEvent = events.find((e) => e.type === "pending_messages.modified");
 
         expect(pendingEvent).toBeDefined();
         expect(answer?.data.content).toContain("18");

nodejs/test/e2e/harness/sdkTestHelper.ts

@@ -8,50 +8,46 @@ export async function getFinalAssistantMessage(
     session: CopilotSession,
     { alreadyIdle = false }: { alreadyIdle?: boolean } = {}
 ): Promise<AssistantMessageEvent> {
-    // We don't know whether the answer has already arrived or not, so race both possibilities
-    return new Promise<AssistantMessageEvent>(async (resolve, reject) => {
-        getFutureFinalResponse(session).then(resolve).catch(reject);
-        getExistingFinalResponse(session, alreadyIdle)
-            .then((msg) => {
-                if (msg) {
-                    resolve(msg);
-                }
-            })
-            .catch(reject);
-    });
+    // Start listening for the answer immediately so we don't miss any events that arrive
+    // between the existing-messages RPC starting and the subscription being installed.
+    const futurePromise = getFutureFinalResponse(session);
+    // We may end up returning from the existing-messages path; attach a noop handler so
+    // the unawaited future-response rejection doesn't surface as an unhandled rejection.
+    futurePromise.catch(() => {});
+
+    const existing = await getExistingFinalResponse(session, alreadyIdle);
+    if (existing) {
+        return existing;
+    }
+    return futurePromise;
 }
 
-function getExistingFinalResponse(
+async function getExistingFinalResponse(
     session: CopilotSession,
     alreadyIdle: boolean = false
 ): Promise<AssistantMessageEvent | undefined> {
-    return new Promise<AssistantMessageEvent | undefined>(async (resolve, reject) => {
-        const messages = await session.getEvents();
-        const finalUserMessageIndex = messages.findLastIndex((m) => m.type === "user.message");
-        const currentTurnMessages =
-            finalUserMessageIndex < 0 ? messages : messages.slice(finalUserMessageIndex);
+    const messages = await session.getEvents();
+    const finalUserMessageIndex = messages.findLastIndex((m) => m.type === "user.message");
+    const currentTurnMessages =
+        finalUserMessageIndex < 0 ? messages : messages.slice(finalUserMessageIndex);
 
-        const currentTurnError = currentTurnMessages.find((m) => m.type === "session.error");
-        if (currentTurnError) {
-            const error = new Error(currentTurnError.data.message);
-            error.stack = currentTurnError.data.stack;
-            reject(error);
-            return;
-        }
+    const currentTurnError = currentTurnMessages.find((m) => m.type === "session.error");
+    if (currentTurnError) {
+        const error = new Error(currentTurnError.data.message);
+        error.stack = currentTurnError.data.stack;
+        throw error;
+    }
 
-        const sessionIdleMessageIndex = alreadyIdle
-            ? currentTurnMessages.length
-            : currentTurnMessages.findIndex((m) => m.type === "session.idle");
-        if (sessionIdleMessageIndex !== -1) {
-            const lastAssistantMessage = currentTurnMessages
-                .slice(0, sessionIdleMessageIndex)
-                .findLast((m) => m.type === "assistant.message");
-            resolve(lastAssistantMessage as AssistantMessageEvent | undefined);
-            return;
-        }
+    const sessionIdleMessageIndex = alreadyIdle
+        ? currentTurnMessages.length
+        : currentTurnMessages.findIndex((m) => m.type === "session.idle");
+    if (sessionIdleMessageIndex !== -1) {
+        return currentTurnMessages
+            .slice(0, sessionIdleMessageIndex)
+            .findLast((m) => m.type === "assistant.message") as AssistantMessageEvent | undefined;
+    }
 
-        resolve(undefined);
-    });
+    return undefined;
 }
 
 function getFutureFinalResponse(session: CopilotSession): Promise<AssistantMessageEvent> {

python/e2e/test_event_fidelity_e2e.py

@@ -2,7 +2,6 @@
 
 from __future__ import annotations
 
-import asyncio
 from pathlib import Path
 
 import pytest
@@ -178,22 +177,19 @@ async def test_should_emit_pending_messages_modified_event_when_message_queue_ch
         session = await ctx.client.create_session(
             on_permission_request=PermissionHandler.approve_all
         )
-        pending_task: asyncio.Future = asyncio.get_event_loop().create_future()
-
-        def on_event(event):
-            if isinstance(event.data, PendingMessagesModifiedData) and not pending_task.done():
-                pending_task.set_result(event)
-
-        unsubscribe = session.on(on_event)
+        events = []
+        unsubscribe = session.on(events.append)
         try:
-            # Fire-and-forget to trigger pending_messages.modified; then wait for it
-            asyncio.ensure_future(session.send("What is 9+9? Reply with just the number."))
-            pending_event = await asyncio.wait_for(pending_task, timeout=60.0)
+            # send_and_wait collects everything in one round trip and matches the
+            # pattern of every other test in this file (and the Rust E2E equivalent),
+            # avoiding the split fire-and-forget + helper pattern that previously
+            # made this test prone to flakes.
+            answer = await session.send_and_wait("What is 9+9? Reply with just the number.")
+
+            pending_event = next(
+                (e for e in events if isinstance(e.data, PendingMessagesModifiedData)), None
+            )
             assert pending_event is not None
-
-            from .testharness.helper import get_final_assistant_message
-
-            answer = await get_final_assistant_message(session, timeout=60.0)
             assert answer is not None
             assert "18" in (answer.data.content or "")
         finally: