agent: suppress distress dispatch when the auth failure came from a dead endpoint

Follow-up to 13dd6723: a platform-403 (stopped/retired app, HTML error
page) now skips the DeviceNotRegistered/AuthCertificateRejected distress
report - it would be sent to the very endpoint that just answered with a
platform page, and its classification would be wrong. Counting and the
shutdown thresholds are unchanged, so a dead endpoint still drives the
agent to its soft shutdown instead of retrying forever.

The one-report-per-streak gate moves from "count == 1" to a streak-scoped
flag: an endpoint-unavailable failure no longer consumes the streak's
report slot, so a later genuine 403 from a live backend still dispatches.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Security/AuthFailureTrackerTests.cs

@@ -260,6 +260,60 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Security
             Assert.Contains("agent/config", fakeDistress.Calls[0].Message);
         }
 
+        [Fact]
+        public void RecordFailure_endpoint_unavailable_suppresses_distress_but_still_counts()
+        {
+            var clock = new FakeClock(new DateTime(2026, 4, 21, 10, 0, 0, DateTimeKind.Utc));
+            var fakeDistress = new FakeDistressReporter();
+            var tracker = new AuthFailureTracker(maxFailures: 2, timeoutMinutes: 0, clock, NewLogger(), fakeDistress);
+
+            int events = 0;
+            tracker.ThresholdExceeded += (_, _) => events++;
+
+            // Platform-403s from a stopped endpoint: a distress report cannot reach it and the
+            // DeviceNotRegistered classification would be wrong — nothing must be dispatched.
+            tracker.RecordFailure(403, "agent/config", endpointUnavailable: true);
+            tracker.RecordFailure(403, "agent/register-session", endpointUnavailable: true);
+
+            Assert.Empty(fakeDistress.Calls);
+            // The shutdown thresholds still apply — a dead endpoint must not retry forever.
+            Assert.Equal(2, tracker.ConsecutiveFailures);
+            Assert.Equal(1, events);
+        }
+
+        [Fact]
+        public void RecordFailure_endpoint_unavailable_does_not_consume_streak_report_slot()
+        {
+            var clock = new FakeClock(new DateTime(2026, 4, 21, 10, 0, 0, DateTimeKind.Utc));
+            var fakeDistress = new FakeDistressReporter();
+            var tracker = new AuthFailureTracker(maxFailures: 50, timeoutMinutes: 0, clock, NewLogger(), fakeDistress);
+
+            // The streak opens with a platform-403 (no dispatch), then a live backend answers
+            // with a genuine 403 — that one must still produce the streak's single report.
+            tracker.RecordFailure(403, "agent/config", endpointUnavailable: true);
+            tracker.RecordFailure(403, "agent/config");
+            tracker.RecordFailure(403, "agent/config");
+
+            Assert.Single(fakeDistress.Calls);
+            Assert.Equal(DistressErrorType.DeviceNotRegistered, fakeDistress.Calls[0].ErrorType);
+        }
+
+        [Fact]
+        public void RecordSuccess_rearms_distress_dispatch_for_the_next_streak()
+        {
+            var clock = new FakeClock(new DateTime(2026, 4, 21, 10, 0, 0, DateTimeKind.Utc));
+            var fakeDistress = new FakeDistressReporter();
+            var tracker = new AuthFailureTracker(maxFailures: 50, timeoutMinutes: 0, clock, NewLogger(), fakeDistress);
+
+            tracker.RecordFailure(401, "agent/config");
+            tracker.RecordSuccess();
+            tracker.RecordFailure(403, "agent/config");
+
+            Assert.Equal(2, fakeDistress.Calls.Count);
+            Assert.Equal(DistressErrorType.AuthCertificateRejected, fakeDistress.Calls[0].ErrorType);
+            Assert.Equal(DistressErrorType.DeviceNotRegistered, fakeDistress.Calls[1].ErrorType);
+        }
+
         [Fact]
         public void RecordFailure_without_distress_reporter_is_silent_but_still_tracks()
         {

src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/RemoteConfigService.cs

@@ -155,7 +155,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Configuration
                     // see the AuthCertificateRejected/DeviceNotRegistered distress signal
                     // without 100 s of needless retry delay.
                     _logger.Warning($"Config fetch authentication failed: {ex.Message}");
-                    _authFailureTracker?.RecordFailure(ex.StatusCode, "agent/config");
+                    _authFailureTracker?.RecordFailure(ex.StatusCode, "agent/config", ex.EndpointUnavailable);
                     lastException = ex;
                     LastFetchAuthStatusCode = ex.StatusCode;
                     break;

src/Agent/AutopilotMonitor.Agent.V2.Core/Security/AuthFailureTracker.cs

@@ -46,6 +46,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Security
         private int _consecutiveFailures;
         private DateTime? _firstFailureUtc;
         private int _thresholdFired;           // 0/1 — Interlocked exchange makes the event single-shot
+        private int _distressDispatched;       // 0/1 per failure streak — reset by RecordSuccess
 
         public AuthFailureTracker(
             int maxFailures,
@@ -77,18 +78,27 @@ namespace AutopilotMonitor.Agent.V2.Core.Security
         public void RecordSuccess()
         {
             if (Interlocked.Exchange(ref _consecutiveFailures, 0) == 0) return; // no-op if already zero
+            Interlocked.Exchange(ref _distressDispatched, 0);
             lock (_windowLock) { _firstFailureUtc = null; }
         }
 
         /// <summary>
-        /// Record a 401/403 from the given <paramref name="operation"/>. V1 parity — on the FIRST
-        /// failure this fires a single distress report via the constructor-injected
-        /// <see cref="DistressReporter"/> (401 → AuthCertificateRejected, 403 → DeviceNotRegistered).
-        /// Subsequent failures are logged + counted but do not dispatch further distress signals.
-        /// Emits <see cref="ThresholdExceeded"/> the first time either ceiling is crossed;
-        /// subsequent calls after termination is armed are no-ops.
+        /// Record a 401/403 from the given <paramref name="operation"/>. V1 parity — the first
+        /// qualifying failure of a streak fires a single distress report via the
+        /// constructor-injected <see cref="DistressReporter"/> (401 → AuthCertificateRejected,
+        /// 403 → DeviceNotRegistered). Subsequent failures are logged + counted but do not
+        /// dispatch further distress signals. Emits <see cref="ThresholdExceeded"/> the first
+        /// time either ceiling is crossed; subsequent calls after termination is armed are no-ops.
         /// </summary>
-        public void RecordFailure(int statusCode, string operation)
+        /// <param name="endpointUnavailable">
+        /// True when the 401/403 was a platform-level HTML response (stopped/retired app, edge
+        /// proxy — see <see cref="BackendAuthException.EndpointUnavailable"/>). Suppresses the
+        /// distress dispatch: the report would go to the very endpoint that just answered with a
+        /// platform error page, and its DeviceNotRegistered classification would be wrong anyway.
+        /// Counting and the shutdown thresholds are unaffected. Should a later failure in the
+        /// same streak come from a live backend, that one still dispatches the streak's report.
+        /// </param>
+        public void RecordFailure(int statusCode, string operation, bool endpointUnavailable = false)
         {
             if (Volatile.Read(ref _thresholdFired) == 1) return;
 
@@ -102,10 +112,21 @@ namespace AutopilotMonitor.Agent.V2.Core.Security
                 firstFailureAt = _firstFailureUtc.Value;
             }
 
-            // V1 parity — only the first consecutive failure produces a distress report. Repeat
-            // hits at 2..N use the local log only; the backend already knows from the first
-            // report that the device is unauthorized.
-            if (count == 1 && _distressReporter != null)
+            // V1 parity — only one distress report per failure streak. Repeat hits use the local
+            // log only; the backend already knows from the first report that the device is
+            // unauthorized. Endpoint-unavailable failures never dispatch (nothing to send to) but
+            // also never consume the streak's report slot.
+            if (endpointUnavailable)
+            {
+                if (_distressReporter != null && Volatile.Read(ref _distressDispatched) == 0)
+                {
+                    _logger.Info(
+                        $"AuthFailureTracker: distress report suppressed for {operation} (http {statusCode}) — " +
+                        "endpoint unavailable (platform error page), a report cannot reach a stopped endpoint.");
+                }
+            }
+            else if (_distressReporter != null &&
+                     Interlocked.CompareExchange(ref _distressDispatched, 1, 0) == 0)
             {
                 var distressType = statusCode == 403
                     ? DistressErrorType.DeviceNotRegistered

src/Agent/AutopilotMonitor.Agent.V2.Core/Security/SessionRegistrationHelper.cs

@@ -89,7 +89,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Security
                     // Feed the central auth-failure tracker so the first-failure distress dispatches
                     // and the shutdown threshold advances. V1 parity: no retry on auth-failure —
                     // backend has definitively rejected the cert/token.
-                    authFailureTracker?.RecordFailure(ex.StatusCode, "agent/register-session");
+                    authFailureTracker?.RecordFailure(ex.StatusCode, "agent/register-session", ex.EndpointUnavailable);
                     return SessionRegistrationResult.AuthFailed(ex.StatusCode, ex.Message);
                 }
                 catch (Exception ex)