fix(a2a-client): watches no longer hang on a silently dead task stream

Hour-long remote tasks were never delivered: a subscribe watch held one SSE stream for up to a day, the remote
sent nothing while its agent worked without text, and once the quiet connection died on the way (no FIN/RST) the
watch waited on it with no error and no log line.

- A subscription lasts at most 5 minutes (SubscriptionSlice), then GetTask and a new subscription: a pause, not a
  failure. A slice ended mid-read counts as ours even when it surfaces as an I/O error.
- The shared handler connects with TCP keepalive (60 s, every 15 s, 4 probes) and HTTP/2 pings.
- Log lines when a watch starts or is restored, when a slice ends on a running task, when a result is delivered or
  kept; the Telegram host's console log carries a UTC timestamp.
- action=status on a settled task ends its watch, so the result isn't delivered a second time.
- A "context busy" refusal of send says nothing was sent, names the watched task in that context and that its
  result will arrive by itself; refusals never touch a watch.

docs/a2a.md

@@ -260,7 +260,7 @@ One tool, `a2a`, with `action` and, depending on it, `agent`, `message`, `contex
 |---|---|
 | `agents` | For every entry, fetches the card (`CardTimeoutSeconds`) and lists name, description, card name, `streaming`/`push` capabilities and skills (id, name, description, up to three examples), or `unreachable (<reason>)` |
 | `send` | Sends `message` and/or `files` to `agent`; `contextId` continues an earlier conversation with it, `taskId` answers its question on that task |
-| `status` | `GetTask` for `taskId`: same result shape as `send` |
+| `status` | `GetTask` for `taskId`: same result shape as `send`; a settled task's watch ends (its outcome is in the conversation now, so it isn't delivered again) |
 | `cancel` | Stops this chat's watch of `taskId`, then `CancelTask` |
 
 **send** calls `SendMessage` with `returnImmediately: true` and `metadata["volcanic.hops"]` (see [Hops](#hops)), then
@@ -314,7 +314,10 @@ because the task already finished), the result says so and carries the task's ou
 or doesn't answer in time, the watch is re-armed and the error ends with `The task is still watched.`
 
 Errors are error tool results with a readable message: unknown agent (with the configured names), `rejected the key
-(401)`, `Could not reach agent ...`, `Agent ... refused: ... (<code>)`, `did not answer in time`. Every single remote
+(401)`, `Could not reach agent ...`, `Agent ... refused: ... (<code>)`, `did not answer in time`. A `send` with a
+`contextId` that the remote refuses as busy (`UnsupportedOperation` mentioning "busy", as ours and VolcanicAgents do)
+says that nothing was sent, names the task this chat watches in that context (`its result will arrive here by
+itself`) and suggests sending again after it or without `contextId`. A refusal never touches a watch. Every single remote
 call is bounded by 60 seconds; an agent that ignores `returnImmediately` and needs longer than that to answer
 `SendMessage` gets `did not answer in time`, and since its task id never arrived, nothing watches that task.
 
@@ -333,14 +336,20 @@ How it learns about updates, chosen when the watch starts:
    body is never trusted. A safety `GetTask` runs every 10 minutes. If the remote refuses the push config, the watch
    subscribes or polls instead.
 2. **subscribe**: the card advertises streaming. A stream that drops or ends before the task settles is reopened after
-   1 s, 5 s, then 15 s; after 3 failures in a row the watch polls instead. A stream is also reopened after a day.
+   1 s, 5 s, then 15 s; after 3 failures in a row the watch polls instead. No stream is held for more than 5 minutes:
+   the watch then reads the task with `GetTask` and subscribes again (a pause, not a failure). Remotes send nothing
+   while their agent works without writing text, and such a quiet stream can die on the way (a proxy or NAT dropping
+   it) without either side noticing; before this slice a watch could wait on a dead stream until `WatchTimeout`.
 3. **poll**: `GetTask` after 15 s, 30 s, 1 min, 2 min, then every 5 min.
 
 Each check reads `agents.json` again: an agent removed from it ends the watch with a `(stopped)` notice; a malformed
 file is treated as a passing failure (the user may be editing it) and the watch keeps going. A watch keeps its
 connection (card and client) between checks and reconnects after a failure or when the agent's entry changes. After
 `WatchTimeout` (1 day by default) from its start the watch gives up with a `(no result)` notice suggesting
-`action=status`. On a settled task it saves files like `send` does and, for push watches, deletes the push config
+`action=status`. The shared handler turns on TCP keepalive (probes after 60 s of silence, every 15 s, 4 tries) and
+HTTP/2 pings, so a dead connection fails instead of hanging. The log has one line when a watch starts or is restored
+(task, agent, chat, source, watch id), when a stream slice ends on a running task (`subscribing again`), when a
+result is delivered or kept, and for every failed check. On a settled task it saves files like `send` does and, for push watches, deletes the push config
 (best effort).
 
 The delivered text is a follow-up prompt:

docs/docker.md

@@ -102,6 +102,7 @@ The compose file maps them into the container.
 
 `TELEGRAM_MODEL` and `LOG_LEVEL` are read by the compose file only, which passes them to the bot as `Telegram__Model`
 and `Logging__LogLevel__Default`; `INSTALL_PLAYWRIGHT` and `CLAUDE_CODE_VERSION` are build arguments (see above).
+Every log line starts with its UTC time (`2026-10-16 13:48:00 UTC info: ...`, `Logging:Console` in `appsettings.json`).
 
 > **Never set `ANTHROPIC_API_KEY`, `ANTHROPIC_AUTH_TOKEN`, `ANTHROPIC_BASE_URL`, `CLAUDE_CODE_OAUTH_TOKEN`,
 > `CLAUDE_CODE_USE_BEDROCK` or `CLAUDE_CODE_USE_VERTEX`.** The Claude Code provider runs on a Claude subscription only

docs/superpowers/specs/2026-10-07-a2a-client-design.md

@@ -101,10 +101,13 @@ Arguments: `action` (`agents` \| `send` \| `status` \| `cancel`), `agent`, `mess
   - still running → starts a watch, returns `{state: working, taskId, contextId, note: "the result will be delivered to
     this conversation when it is ready"}`.
   A message-only reply (the agent answered with a `Message`, no task) is returned as text with its `contextId`.
-- **`status`** `{agent, taskId}` — `GetTask`; same result shape as a finished `send`.
+- **`status`** `{agent, taskId}` — `GetTask`; same result shape as a finished `send`. A settled task's watch ends, so its
+  outcome isn't delivered a second time.
 - **`cancel`** `{agent, taskId}` — `CancelTask`, removes any watch for it.
 
-Errors (network, timeout, 401, JSON-RPC error, unknown agent) become error tool results with a readable message. The
+Errors (network, timeout, 401, JSON-RPC error, unknown agent) become error tool results with a readable message. A
+"context busy" refusal of a `send` with `contextId` names the task this chat watches there and says its result will
+arrive by itself; a refusal never touches a watch. The
 tool never throws into the loop.
 
 ## Watches
@@ -125,7 +128,9 @@ Watch record, persisted in `<workspace>/.volcanic/a2a/pending.json`:
   1. **push**: card has `capabilities.pushNotifications` and `PublicUrl` is set → `CreateTaskPushNotificationConfig`
      with URL `<PublicUrl>/a2a/push/<watchId>` and a random 32-byte token. A safety `GetTask` runs every 10 min.
   2. **subscribe**: card has `streaming` → `SubscribeToTask`; on disconnect, reconnect with backoff, falling back to polling
-     after 3 failures.
+     after 3 failures. A stream is held at most 5 minutes, then `GetTask` and a new subscription (a pause, not a
+     failure): a quiet stream may have died silently on the way (2026-10-16: hour-long tasks never delivered). The
+     shared handler uses TCP keepalive and HTTP/2 pings.
   3. **poll**: `GetTask` with backoff 15 s → 30 s → 1 min → 2 min → 5 min (cap).
 - **Push receiving**: the host wires the server's `A2AServerOptions.OnPush` to `A2AWatchRegistry.Shared.OnPushAsync(watchId,
   token, body)`, which checks the token (constant-time) and returns false for unknown watches or bad tokens (→ 404).

extensions/Volcanic.AgenticCore.Extensions.A2A/A2ATool.cs

@@ -97,7 +97,9 @@ internal sealed class A2ATool(A2AClientSettings settings, string target, A2AWatc
         return await CallAsync(entry, ct, async agent =>
         {
             SendMessageResponse response;
-            using (var bounded = Bounded(ct))
+            try
+            {
+                using var bounded = Bounded(ct);
                 response = await agent.Client.SendMessageAsync(new SendMessageRequest
                 {
                     Message = new A2AMessage
@@ -111,6 +113,11 @@ internal sealed class A2ATool(A2AClientSettings settings, string target, A2AWatc
                     Configuration = new SendMessageConfiguration { ReturnImmediately = true },
                     Metadata = new Dictionary<string, JsonElement> { ["volcanic.hops"] = JsonSerializer.SerializeToElement(inboundHops() + 1) },
                 }, bounded.Token).ConfigureAwait(false);
+            }
+            catch (A2AException ex) when (IsContextBusy(ex) && !string.IsNullOrWhiteSpace(args.ContextId))
+            {
+                throw new InvalidOperationException(BusyContext(entry, args.ContextId!), ex);
+            }
 
             if (response.Task is not { } task)
             {
@@ -150,12 +157,29 @@ internal sealed class A2ATool(A2AClientSettings settings, string target, A2AWatc
         return await CallAsync(entry, ct, async agent =>
         {
             var task = await GetTaskAsync(agent, taskId, ct).ConfigureAwait(false);
-            return A2AResults.IsSettled(task.Status.State)
+            if (!A2AResults.IsSettled(task.Status.State)) return A2AResults.Describe(entry.Name, task, []);
-                ? await DescribeSettledAsync(workspace, entry.Name, agent, task, ct).ConfigureAwait(false)
+            var described = await DescribeSettledAsync(workspace, entry.Name, agent, task, ct).ConfigureAwait(false);
-                : A2AResults.Describe(entry.Name, task, []);
+            // The outcome is in this conversation now: a watch of the task would only deliver it a second time.
+            await registry.StopWatchingAsync(target, workspace, taskId).ConfigureAwait(false);
+            return described;
         }).ConfigureAwait(false);
     }
 
+    /// <summary>The remote refuses a message to a context whose task still runs (our server and VolcanicAgents say "Context busy").</summary>
+    private static bool IsContextBusy(A2AException ex) =>
+        ex.ErrorCode == A2AErrorCode.UnsupportedOperation && ex.Message.Contains("busy", StringComparison.OrdinalIgnoreCase);
+
+    private string BusyContext(RemoteAgentEntry entry, string contextId)
+    {
+        var running = registry.List(target).FirstOrDefault(r => r.Agent == entry.Name && r.ContextId == contextId);
+        return running is null
+            ? $"Agent \"{entry.Name}\" is still working on an earlier task in context {contextId} and refused this message. " +
+              "Nothing was sent. Send it again once that task has ended, or start a new conversation without contextId."
+            : $"Agent \"{entry.Name}\" is still working on task {running.TaskId} in context {contextId} and refused this message. " +
+              "Nothing was sent, and that task is still watched: its result will arrive here by itself. " +
+              "Send this message again after that result, or start a new conversation without contextId.";
+    }
+
     private async Task<string> CancelAsync(string workspace, A2AArgs args, CancellationToken ct)
     {
         var entry = Entry(workspace, args.Agent);

extensions/Volcanic.AgenticCore.Extensions.A2A/A2AWatchRegistry.cs

@@ -1,4 +1,6 @@
 using System.Collections.Concurrent;
+using System.Net;
+using System.Net.Sockets;
 using System.Security.Cryptography;
 using System.Text;
 using System.Text.Json;
@@ -25,15 +27,43 @@ namespace Volcanic.AgenticCore.Extensions.A2A;
 /// </remarks>
 public sealed class A2AWatchRegistry : IAsyncDisposable
 {
-    public static A2AWatchRegistry Shared { get; } = new(TimeProvider.System,
+    public static A2AWatchRegistry Shared { get; } = new(TimeProvider.System, CreateSharedHandler());
-        new SocketsHttpHandler { PooledConnectionLifetime = TimeSpan.FromMinutes(5) });
+
+    /// <summary>
+    /// The shared handler: pooled connections renewed every 5 minutes, and TCP keepalive (plus HTTP/2 pings) so a
+    /// connection that died silently on the way (a proxy or NAT dropping an idle stream) fails instead of hanging.
+    /// </summary>
+    internal static SocketsHttpHandler CreateSharedHandler() => new()
+    {
+        PooledConnectionLifetime = TimeSpan.FromMinutes(5),
+        KeepAlivePingDelay = TimeSpan.FromSeconds(60),
+        KeepAlivePingTimeout = TimeSpan.FromSeconds(20),
+        ConnectCallback = (context, ct) => ConnectWithKeepAliveAsync(context.DnsEndPoint, ct),
+    };
+
+    /// <summary>A TCP connection probed after 60 s of silence, every 15 s, given up after 4 unanswered probes.</summary>
+    internal static async ValueTask<Stream> ConnectWithKeepAliveAsync(DnsEndPoint endPoint, CancellationToken cancellationToken)
+    {
+        var socket = new Socket(SocketType.Stream, ProtocolType.Tcp) { NoDelay = true };
+        try
+        {
+            socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
+            socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime, 60);
+            socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval, 15);
+            socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount, 4);
+            await socket.ConnectAsync(endPoint, cancellationToken).ConfigureAwait(false);
+            return new NetworkStream(socket, ownsSocket: true);
+        }
+        catch
+        {
+            socket.Dispose();
+            throw;
+        }
+    }
 
     public static readonly TimeSpan[] DefaultPollDelays =
         [TimeSpan.FromSeconds(15), TimeSpan.FromSeconds(30), TimeSpan.FromMinutes(1), TimeSpan.FromMinutes(2), TimeSpan.FromMinutes(5)];
 
-    /// <summary>Longest single subscription; the stream is then reopened (a pause, not a failure).</summary>
-    private static readonly TimeSpan MaxSubscription = TimeSpan.FromDays(1);
-
     private static readonly TimeSpan DeleteTimeout = TimeSpan.FromSeconds(10);
 
     /// <summary>After this many failed checks in a row, a subscribe watch polls instead.</summary>
@@ -79,6 +109,13 @@ public sealed class A2AWatchRegistry : IAsyncDisposable
     internal TimeSpan[] ReconnectDelays { get; init; } = [TimeSpan.FromSeconds(1), TimeSpan.FromSeconds(5), TimeSpan.FromSeconds(15)];
     internal TimeSpan SafetyPollInterval { get; init; } = TimeSpan.FromMinutes(10);
 
+    /// <summary>
+    /// Longest single subscription: the watch then reads the task with <c>GetTask</c> and subscribes again (a pause, not
+    /// a failure). Remotes send nothing while their agent works without text, and a stream that went quiet may have died
+    /// on the way without either side noticing, so no stream is trusted for longer than this.
+    /// </summary>
+    internal TimeSpan SubscriptionSlice { get; init; } = TimeSpan.FromMinutes(5);
+
     /// <summary>Bound of one remote call (get, create/delete push config), never past the watch deadline.</summary>
     internal TimeSpan CallTimeout { get; init; } = TimeSpan.FromSeconds(60);
 
@@ -157,6 +194,8 @@ public sealed class A2AWatchRegistry : IAsyncDisposable
             watch.DisposeStop();
             return watch.Record;
         }
+        Logger.LogInformation("A2A: watching task {TaskId} of {Agent} for {Target} by {Source} (watch {WatchId})",
+            task.Id, agent.Entry.Name, target, watch.Record.Source, watchId);
         watch.Loop = Task.Run(() => RunAsync(watch, settings));
         return watch.Record;
     }
@@ -199,6 +238,8 @@ public sealed class A2AWatchRegistry : IAsyncDisposable
                     if (!ReferenceEquals(record, stored)) PendingFile.Upsert(workspace, record);
                     // Checked after the watch is in the map: a StopAsync that set the flag later sees (and stops) it.
                     if (_stopped) { _watches.TryRemove(new(record.WatchId, watch)); watch.DisposeStop(); return count; }
+                    Logger.LogInformation("A2A: restored the watch of task {TaskId} of {Agent} for {Target} by {Source} (watch {WatchId})",
+                        record.TaskId, record.Agent, target, record.Source, record.WatchId);
                     watch.Loop = Task.Run(() => RunAsync(watch, settings));
                     count++;
                 }
@@ -230,6 +271,7 @@ public sealed class A2AWatchRegistry : IAsyncDisposable
             {
                 if (_stopped) break; // shutting down: the rest stay in pending.json for the next start
                 if (!await TryDeliverAsync(target, record.Undelivered!, cancellationToken).ConfigureAwait(false)) { failed = true; continue; }
+                Logger.LogInformation("A2A: delivered the kept result of task {TaskId} of {Agent} to {Target}", record.TaskId, record.Agent, target);
                 using (await LockAsync(workspace).ConfigureAwait(false))
                     PendingFile.Remove(workspace, record.WatchId);
             }
@@ -343,6 +385,9 @@ public sealed class A2AWatchRegistry : IAsyncDisposable
                         (task, pause) = await SubscribeAsync(agent, record.TaskId, deadline, ct).ConfigureAwait(false);
                         if ((task is null || !A2AResults.IsSettled(task.Status.State)) && !pause)
                             throw new IOException("the task stream ended before the task settled");
+                        if (pause && task is not null && !A2AResults.IsSettled(task.Status.State))
+                            Logger.LogInformation("A2A: task {TaskId} of {Agent} is still {State}; subscribing again", record.TaskId, record.Agent,
+                                A2AResults.StateName(task.Status.State));
                     }
                     else
                     {
@@ -415,7 +460,7 @@ public sealed class A2AWatchRegistry : IAsyncDisposable
 
     /// <summary>
     /// Follows the task's stream until it settles or ends, then reads the task. <c>Paused</c>: the stream was ended by
-    /// us (the deadline or <see cref="MaxSubscription"/>), which is not a failure.
+    /// us (the deadline or <see cref="SubscriptionSlice"/>), which is not a failure.
     /// </summary>
     private async Task<(AgentTask? Task, bool Paused)> SubscribeAsync(RemoteAgent agent, string taskId, DateTimeOffset deadline, CancellationToken ct)
     {
@@ -424,7 +469,7 @@ public sealed class A2AWatchRegistry : IAsyncDisposable
         using (var until = CancellationTokenSource.CreateLinkedTokenSource(ct))
         {
             var remaining = deadline - Time.GetUtcNow();
-            until.CancelAfter(remaining <= TimeSpan.Zero ? TimeSpan.Zero : Min(remaining, MaxSubscription));
+            until.CancelAfter(remaining <= TimeSpan.Zero ? TimeSpan.Zero : Min(remaining, SubscriptionSlice));
             try
             {
                 await foreach (var ev in agent.Client.SubscribeToTaskAsync(new SubscribeToTaskRequest { Id = taskId }, until.Token).ConfigureAwait(false))
@@ -436,7 +481,8 @@ public sealed class A2AWatchRegistry : IAsyncDisposable
             catch (A2AException) // e.g. the task is already terminal: read it instead
             {
             }
-            catch (OperationCanceledException) when (!ct.IsCancellationRequested && until.IsCancellationRequested)
+            // Ending a stream mid-read may surface as an I/O error rather than a cancel: ours either way.
+            catch (Exception ex) when (ex is not OutOfMemoryException && !ct.IsCancellationRequested && until.IsCancellationRequested)
             {
                 paused = true;
             }
@@ -488,6 +534,10 @@ public sealed class A2AWatchRegistry : IAsyncDisposable
         {
             // A failed delivery writes nothing: the text is stored, and a cancel meanwhile has removed it for good.
             delivered = await TryDeliverAsync(record.Target, text, CancellationToken.None).ConfigureAwait(false);
+            Logger.LogInformation(delivered
+                    ? "A2A: delivered the result of task {TaskId} of {Agent} to {Target}"
+                    : "A2A: kept the result of task {TaskId} of {Agent} for {Target} until it can be delivered",
+                record.TaskId, record.Agent, record.Target);
             if (delivered)
                 using (await LockAsync(watch.Workspace).ConfigureAwait(false))
                     PendingFile.Remove(watch.Workspace, record.WatchId);

hosts/Volcanic.AgenticCore.Telegram/appsettings.json

@@ -4,7 +4,8 @@
       "Default": "Information",
       "Microsoft.Hosting.Lifetime": "Warning",
       "System.Net.Http": "Warning"
-    }
+    },
+    "Console": { "FormatterName": "simple", "FormatterOptions": { "TimestampFormat": "yyyy-MM-dd HH:mm:ss 'UTC' ", "UseUtcTimestamp": true } }
   },
   "Telegram": {
     "BotToken": "",

tests/Volcanic.AgenticCore.Extensions.Tests/A2A/A2AToolTests.cs

@@ -242,6 +242,49 @@ public class A2AToolTests
         Assert.DoesNotContain(host.UserTexts(), t => t.Contains("[A2A result", StringComparison.Ordinal));
     }
 
+    [Fact]
+    public async Task Status_SettledTask_EndsItsWatch_SoTheResultIsNotDeliveredTwice()
+    {
+        await using var remote = await RemoteAgentServer.StartAsync();
+        remote.Faux.Enqueue(FauxTurn.Say("Answer by status").WithDelay(TimeSpan.FromMilliseconds(300)));
+        RemoteRewriter? rewriter = null;
+        // The watch's stream stays silent (a dead connection): only status can see the task finish.
+        await using var host = new A2AClientHost(remote, new A2AClientSettings { Enabled = true, WaitSeconds = 0 },
+            inner => rewriter = new RemoteRewriter(inner) { SilentSubscribes = true });
+        var session = await host.CreateSessionAsync();
+        var (started, _) = await host.CallToolAsync(session, new { action = "send", agent = "remote", message = "go" });
+        var taskId = Field(started, "task");
+        Assert.Single(host.Registry.List(A2AClientHost.Target));
+        string status;
+        do
+        {
+            await Task.Delay(100);
+            (status, _) = await host.CallToolAsync(session, new { action = "status", agent = "remote", taskId });
+        } while (!status.Contains("Answer by status"));
+        Assert.Empty(host.Registry.List(A2AClientHost.Target));
+        Assert.Empty(PendingFile.Read(host.Workspace));
+        Assert.DoesNotContain(host.UserTexts(), t => t.Contains("[A2A result", StringComparison.Ordinal));
+    }
+
+    [Fact]
+    public async Task Send_BusyContext_SaysWhichTaskRunsAndThatItIsStillWatched()
+    {
+        await using var remote = await RemoteAgentServer.StartAsync();
+        remote.Faux.Enqueue(FauxTurn.Say("first").WithDelay(TimeSpan.FromSeconds(10)));
+        await using var host = new A2AClientHost(remote, new A2AClientSettings { Enabled = true, WaitSeconds = 0 });
+        var session = await host.CreateSessionAsync();
+        var (started, _) = await host.CallToolAsync(session, new { action = "send", agent = "remote", message = "long job" });
+        var taskId = Field(started, "task");
+        var contextId = Field(started, "context");
+        var (refused, isError) = await host.CallToolAsync(session, new { action = "send", agent = "remote", message = "and another", contextId });
+        Assert.True(isError, refused);
+        Assert.Contains($"still working on task {taskId} in context {contextId}", refused);
+        Assert.Contains("still watched", refused);
+        Assert.Contains("without contextId", refused);
+        var watch = Assert.Single(host.Registry.List(A2AClientHost.Target)); // the refusal leaves the running task's watch alone
+        Assert.Equal(taskId, watch.TaskId);
+    }
+
     [Fact]
     public async Task Send_AbortedWhileWaiting_TaskWatched_AndResultArrives()
     {

tests/Volcanic.AgenticCore.Extensions.Tests/A2A/A2AWatchTests.cs

@@ -368,6 +368,54 @@ public sealed class A2AWatchTests : IDisposable
         Assert.Equal(3, http.Subscribes); // three failed subscribes, then polling only
     }
 
+    [Fact]
+    public async Task Watch_SilentStream_SubscribesAgainEachSlice_AndDelivers()
+    {
+        await using var remote = await RemoteAgentServer.StartAsync();
+        remote.Faux.Enqueue(FauxTurn.Say("Finished in silence").WithDelay(TimeSpan.FromMilliseconds(700)));
+        WriteAgents();
+        var delivery = new RecordingDelivery(_ws);
+        var logger = new ListLogger();
+        var http = new RemoteRewriter(remote.Handler) { SilentSubscribes = true };
+        await using var registry = new A2AWatchRegistry(TimeProvider.System, http, logger)
+        {
+            SourceOverride = WatchSources.Subscribe, SubscriptionSlice = TimeSpan.FromMilliseconds(200),
+            ReconnectDelays = [TimeSpan.FromSeconds(30)], // a failure would stall the watch past the test's wait
+        };
+        registry.UseDelivery(delivery);
+        var settings = new A2AClientSettings();
+        using var agent = await registry.Connector(settings).ConnectAsync(AgentsFile.Find(_ws, "remote")!, CancellationToken.None);
+        var task = await StartRemoteTaskAsync(agent);
+        var record = await registry.StartAsync("1", _ws, agent, task, settings, CancellationToken.None);
+        await WaitUntilAsync(() => !delivery.Delivered.IsEmpty, timeoutMs: 10_000);
+        Assert.Contains("Finished in silence", Assert.Single(delivery.Delivered).Text);
+        Assert.True(http.Subscribes >= 2, $"subscribes: {http.Subscribes}"); // each slice ended, the task read, then a new stream
+        Assert.Equal(WatchSources.Subscribe, record.Source); // pauses aren't failures: it never fell back to polling
+        Assert.DoesNotContain(logger.Lines, l => l.Contains("failed", StringComparison.Ordinal));
+        Assert.Contains(logger.Lines, l => l.Contains($"watching task {task.Id} of remote for 1 by subscribe", StringComparison.Ordinal));
+        Assert.Contains(logger.Lines, l => l.Contains($"task {task.Id} of remote is still", StringComparison.Ordinal) && l.Contains("subscribing again", StringComparison.Ordinal));
+        await WaitUntilAsync(() => logger.Lines.Any(l => l.Contains($"delivered the result of task {task.Id} of remote to 1", StringComparison.Ordinal)));
+    }
+
+    [Fact]
+    public async Task SharedHandler_ConnectsWithTcpKeepAlive()
+    {
+        using var listener = new System.Net.Sockets.TcpListener(System.Net.IPAddress.Loopback, 0);
+        listener.Start();
+        var port = ((System.Net.IPEndPoint)listener.LocalEndpoint).Port;
+        var accepted = listener.AcceptSocketAsync();
+        await using var stream = Assert.IsType<System.Net.Sockets.NetworkStream>(
+            await A2AWatchRegistry.ConnectWithKeepAliveAsync(new System.Net.DnsEndPoint("localhost", port), CancellationToken.None));
+        using var server = await accepted;
+        var socket = stream.Socket;
+        Assert.True(socket.Connected);
+        Assert.NotEqual(0, (int)socket.GetSocketOption(System.Net.Sockets.SocketOptionLevel.Socket, System.Net.Sockets.SocketOptionName.KeepAlive)!);
+        Assert.Equal(60, (int)socket.GetSocketOption(System.Net.Sockets.SocketOptionLevel.Tcp, System.Net.Sockets.SocketOptionName.TcpKeepAliveTime)!);
+        Assert.Equal(15, (int)socket.GetSocketOption(System.Net.Sockets.SocketOptionLevel.Tcp, System.Net.Sockets.SocketOptionName.TcpKeepAliveInterval)!);
+        using var handler = A2AWatchRegistry.CreateSharedHandler();
+        Assert.NotNull(handler.ConnectCallback);
+    }
+
     [Fact]
     public async Task Watch_ReusesTheConnectionBetweenChecks()
     {

tests/Volcanic.AgenticCore.Extensions.Tests/A2A/Support/RemoteRewriter.cs

@@ -9,7 +9,8 @@ namespace Volcanic.AgenticCore.Extensions.Tests.A2A.Support;
 
 /// <summary>
 /// Between our client and the remote: counts subscribes and card fetches, can hide the card's streaming capability
-/// (so the client polls), and can answer <c>SendMessage</c> itself with a plain message instead of a task.
+/// (so the client polls), can answer <c>SendMessage</c> itself with a plain message instead of a task, and can answer
+/// <c>SubscribeToTask</c> with a stream that never sends anything (a connection that died silently on the way).
 /// </summary>
 public sealed class RemoteRewriter(HttpMessageHandler inner) : DelegatingHandler(inner)
 {
@@ -21,6 +22,9 @@ public sealed class RemoteRewriter(HttpMessageHandler inner) : DelegatingHandler
     /// <summary>When set, <c>SendMessage</c> is answered with a message carrying this text (the remote never sees it).</summary>
     public string? MessageReply { get; init; }
 
+    /// <summary>When true, every subscribe gets an open stream that stays silent until the client gives up on it.</summary>
+    public volatile bool SilentSubscribes;
+
     /// <summary>When true, every request fails as if the remote were down.</summary>
     public volatile bool Down;
 
@@ -30,7 +34,16 @@ public sealed class RemoteRewriter(HttpMessageHandler inner) : DelegatingHandler
     {
         if (Down) throw new HttpRequestException("remote is down");
         string? body = request.Content is null ? null : await request.Content.ReadAsStringAsync(cancellationToken);
-        if (body is not null && body.Contains("\"SubscribeToTask\"", StringComparison.Ordinal)) Interlocked.Increment(ref _subscribes);
+        if (body is not null && body.Contains("\"SubscribeToTask\"", StringComparison.Ordinal))
+        {
+            Interlocked.Increment(ref _subscribes);
+            if (SilentSubscribes)
+            {
+                var silent = new StreamContent(new SilentStream());
+                silent.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("text/event-stream");
+                return new HttpResponseMessage(HttpStatusCode.OK) { Content = silent };
+            }
+        }
         if (MessageReply is not null && body is not null && body.Contains("\"SendMessage\"", StringComparison.Ordinal))
         {
             var id = JsonNode.Parse(body)!["id"]?.DeepClone();
@@ -57,3 +70,25 @@ public sealed class RemoteRewriter(HttpMessageHandler inner) : DelegatingHandler
         return response;
     }
 }
+
+/// <summary>A response body that never yields a byte; a read ends only when the reader cancels it.</summary>
+internal sealed class SilentStream : Stream
+{
+    public override bool CanRead => true;
+    public override bool CanSeek => false;
+    public override bool CanWrite => false;
+    public override long Length => throw new NotSupportedException();
+    public override long Position { get => throw new NotSupportedException(); set => throw new NotSupportedException(); }
+    public override void Flush() { }
+    public override int Read(byte[] buffer, int offset, int count) => throw new NotSupportedException("async reads only");
+    public override async ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken = default)
+    {
+        await Task.Delay(Timeout.Infinite, cancellationToken);
+        return 0;
+    }
+    public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken) =>
+        ReadAsync(buffer.AsMemory(offset, count), cancellationToken).AsTask();
+    public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException();
+    public override void SetLength(long value) => throw new NotSupportedException();
+    public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();
+}