diff --git a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/PostSessionStartedSignalTests.cs b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/PostSessionStartedSignalTests.cs
new file mode 100644
index 00000000..30443fd9
--- /dev/null
+++ b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/PostSessionStartedSignalTests.cs
@@ -0,0 +1,125 @@
+using System.IO;
+using System.Reflection;
+using AutopilotMonitor.Agent.V2;
+using AutopilotMonitor.Agent.V2.Core.Configuration;
+using AutopilotMonitor.Agent.V2.Core.Logging;
+using AutopilotMonitor.Agent.V2.Core.Security;
+using AutopilotMonitor.Agent.V2.Core.Tests.Harness;
+using AutopilotMonitor.Agent.V2.Core.Tests.Orchestration;
+using AutopilotMonitor.DecisionCore.Signals;
+using AutopilotMonitor.Shared.Models;
+using Xunit;
+
+namespace AutopilotMonitor.Agent.V2.Core.Tests.Program
+{
+    /// <summary>
+    /// Verifies that <see cref="AutopilotMonitor.Agent.V2.Program.PostSessionStartedSignal"/>
+    /// emits the SessionStarted decision signal with the registration metadata payload the
+    /// reducer anchor needs. Plan §M4-V2-parity PR-C.
+    /// </summary>
+    public sealed class PostSessionStartedSignalTests
+    {
+        private static AgentLogger NewLogger(string path)
+            => new AgentLogger(Path.Combine(path, "logs"), AgentLogLevel.Info);
+
+        private static SessionRegistrationResult BuildSucceeded(ValidatorType validatedBy, string? adminAction = null)
+        {
+            var response = new RegisterSessionResponse
+            {
+                SessionId = "session-anon-1",
+                Success = true,
+                Message = "ok",
+                ValidatedBy = validatedBy,
+                AdminAction = adminAction,
+            };
+
+            // SessionRegistrationResult uses a private ctor + public static factories. Call
+            // Succeeded via reflection-free factory — it's public.
+            return SessionRegistrationResult.Succeeded(response);
+        }
+
+        private static AgentConfiguration BuildConfig(bool bootstrap)
+        {
+            return new AgentConfiguration
+            {
+                ApiBaseUrl = "https://example.invalid",
+                SessionId = "session-anon-1",
+                TenantId = "tenant-anon-1",
+                SpoolDirectory = Path.GetTempPath(),
+                LogDirectory = Path.GetTempPath(),
+                LogLevel = AgentLogLevel.Info,
+                UseClientCertAuth = !bootstrap,
+                UseBootstrapTokenAuth = bootstrap,
+                BootstrapToken = bootstrap ? "dummy" : null,
+            };
+        }
+
+        [Fact]
+        public void Posts_session_started_kind_with_default_schema_version()
+        {
+            using var tmp = new TempDirectory();
+            var sink = new FakeSignalIngressSink();
+            var logger = NewLogger(tmp.Path);
+            var result = BuildSucceeded(ValidatorType.AutopilotV1);
+
+            AutopilotMonitor.Agent.V2.Program.PostSessionStartedSignal(sink, result, BuildConfig(bootstrap: false), logger);
+
+            Assert.Single(sink.Posted);
+            var posted = sink.Posted[0];
+            Assert.Equal(DecisionSignalKind.SessionStarted, posted.Kind);
+            Assert.Equal(1, posted.KindSchemaVersion);
+            Assert.Equal("Program.RunAgent", posted.SourceOrigin);
+            Assert.Equal(EvidenceKind.Synthetic, posted.Evidence.Kind);
+            Assert.Equal("register_session_success", posted.Evidence.Identifier);
+        }
+
+        [Fact]
+        public void Payload_includes_registration_metadata_keys()
+        {
+            using var tmp = new TempDirectory();
+            var sink = new FakeSignalIngressSink();
+            var logger = NewLogger(tmp.Path);
+            var result = BuildSucceeded(ValidatorType.CorporateIdentifier);
+
+            AutopilotMonitor.Agent.V2.Program.PostSessionStartedSignal(sink, result, BuildConfig(bootstrap: true), logger);
+
+            var payload = sink.Posted[0].Payload;
+            Assert.NotNull(payload);
+            Assert.Equal("CorporateIdentifier", payload!["validatedBy"]);
+            Assert.Equal("true", payload["isBootstrapSession"]);
+            Assert.Contains("enrollmentType", (System.Collections.Generic.IDictionary<string, string>)payload);
+            Assert.Contains("isHybridJoin", (System.Collections.Generic.IDictionary<string, string>)payload);
+            Assert.Contains("agentVersion", (System.Collections.Generic.IDictionary<string, string>)payload);
+        }
+
+        [Fact]
+        public void Swallows_sink_exceptions()
+        {
+            // The helper is called on the hot startup path — a misbehaving sink must not
+            // crash the agent.
+            using var tmp = new TempDirectory();
+            var sink = new ThrowingSink();
+            var logger = NewLogger(tmp.Path);
+            var result = BuildSucceeded(ValidatorType.Bootstrap);
+
+            var ex = Record.Exception(() =>
+                AutopilotMonitor.Agent.V2.Program.PostSessionStartedSignal(sink, result, BuildConfig(bootstrap: false), logger));
+
+            Assert.Null(ex);
+        }
+
+        private sealed class ThrowingSink : AutopilotMonitor.Agent.V2.Core.Orchestration.ISignalIngressSink
+        {
+            public void Post(
+                DecisionSignalKind kind,
+                System.DateTime occurredAtUtc,
+                string sourceOrigin,
+                Evidence evidence,
+                System.Collections.Generic.IReadOnlyDictionary<string, string>? payload = null,
+                int kindSchemaVersion = 1)
+            {
+                throw new System.InvalidOperationException("ingress unavailable");
+            }
+        }
+    }
+}
diff --git a/src/Agent/AutopilotMonitor.Agent.V2/Program.cs b/src/Agent/AutopilotMonitor.Agent.V2/Program.cs
index 567362bd..3c2cd019 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2/Program.cs
+++ b/src/Agent/AutopilotMonitor.Agent.V2/Program.cs
@@ -16,6 +16,7 @@ using AutopilotMonitor.Agent.V2.Core.Termination;
 using AutopilotMonitor.Agent.V2.Core.Transport.Telemetry;
 using AutopilotMonitor.DecisionCore.Classifiers;
 using AutopilotMonitor.DecisionCore.Engine;
+using AutopilotMonitor.DecisionCore.Signals;
 using AutopilotMonitor.DecisionCore.State;
 using AutopilotMonitor.Shared;
 using AutopilotMonitor.Shared.Models;
@@ -712,6 +713,17 @@ namespace AutopilotMonitor.Agent.V2
                             EmitSystemRebootDetectedEvent(orchestrator, agentConfig, previousExit, logger);
                         }
 
+                        // V2 parity — post SessionStarted so the reducer establishes the session anchor
+                        // (HandleSessionStartedV1 in DecisionEngine.Shared.cs). Skipped on:
+                        //   - WhiteGlove Part-2 resume: EnrollmentOrchestrator.Start already posts
+                        //     SessionRecovered, which triggers HandleWhiteGlovePart1To2Bridge.
+                        //   - Admin preemption: the AdminPreemptionDetected signal below drives the
+                        //     session straight to a terminal stage; SessionStarted first would be noise.
+                        if (!isWhiteGloveResume && string.IsNullOrEmpty(registrationResult.AdminAction))
+                        {
+                            PostSessionStartedSignal(orchestrator.IngressSink, registrationResult, agentConfig, logger);
+                        }
+
                         // V1 parity (MonitoringService.cs:388-413 — the "ADMIN OVERRIDE on startup"
                         // block) — if the register-session response carried an AdminAction the
                         // session was already marked terminal by an operator via the portal. Emit
@@ -950,6 +962,49 @@ namespace AutopilotMonitor.Agent.V2
             }
         }
 
+        /// <summary>
+        /// V2 parity — post a <see cref="DecisionSignalKind.SessionStarted"/> signal with the
+        /// tenant-registered session metadata (EnrollmentType / IsHybridJoin / ValidatedBy) so the
+        /// reducer's session-anchor handler runs. Without this the DecisionState.Stage stays at
+        /// the initial value and subsequent raw signals (ESP / Hello) see an uninitialised session.
+        /// </summary>
+        internal static void PostSessionStartedSignal(
+            ISignalIngressSink ingressSink,
+            SessionRegistrationResult registrationResult,
+            AgentConfiguration agentConfig,
+            AgentLogger logger)
+        {
+            try
+            {
+                var payload = new System.Collections.Generic.Dictionary<string, string>(StringComparer.Ordinal)
+                {
+                    ["enrollmentType"] = EnrollmentRegistryDetector.DetectEnrollmentType(),
+                    ["isHybridJoin"] = EnrollmentRegistryDetector.DetectHybridJoin() ? "true" : "false",
+                    ["validatedBy"] = registrationResult.ValidatedBy.ToString(),
+                    ["agentVersion"] = GetAgentVersion(),
+                    ["isBootstrapSession"] = agentConfig.UseBootstrapTokenAuth ? "true" : "false",
+                };
+
+                var evidence = new Evidence(
+                    kind: EvidenceKind.Synthetic,
+                    identifier: "register_session_success",
+                    summary: "Session registration handshake succeeded; posting SessionStarted anchor for reducer.");
+
+                ingressSink.Post(
+                    kind: DecisionSignalKind.SessionStarted,
+                    occurredAtUtc: DateTime.UtcNow,
+                    sourceOrigin: "Program.RunAgent",
+                    evidence: evidence,
+                    payload: payload);
+
+                logger.Debug($"SessionStarted signal posted (validatedBy={registrationResult.ValidatedBy}, enrollmentType={payload["enrollmentType"]}).");
+            }
+            catch (Exception ex)
+            {
+                logger.Warning($"SessionStarted post failed: {ex.Message}");
+            }
+        }
+
         /// <summary>
         /// V1 parity — fire-and-forget <c>agent_started</c> event emitted after
         /// <see cref="EnrollmentOrchestrator.Start"/>. Carries a snapshot of the boot classification
diff --git a/src/Agent/AutopilotMonitor.Agent.V2/buildcounter.txt b/src/Agent/AutopilotMonitor.Agent.V2/buildcounter.txt
index 29d6383b..257e5632 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2/buildcounter.txt
+++ b/src/Agent/AutopilotMonitor.Agent.V2/buildcounter.txt
@@ -1 +1 @@
-100
+102
diff --git a/src/Agent/AutopilotMonitor.Agent/buildcounter.txt b/src/Agent/AutopilotMonitor.Agent/buildcounter.txt
index 7e179677..e255a855 100644
--- a/src/Agent/AutopilotMonitor.Agent/buildcounter.txt
+++ b/src/Agent/AutopilotMonitor.Agent/buildcounter.txt
@@ -1 +1 @@
-1071
+1072