diff --git a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/InstallModeMergeTests.cs b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/InstallModeMergeTests.cs
index 72f35c11..a2e490b0 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/InstallModeMergeTests.cs
+++ b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/InstallModeMergeTests.cs
@@ -132,10 +132,12 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Program
         }
 
         [Fact]
-        public void First_install_without_wait_arg_uses_agent_default_of_600()
+        public void First_install_without_wait_arg_persists_null_so_runtime_default_applies()
         {
             // Generic bootstrap path: PS1 calls `--install` without --tenant-id-wait,
-            // and there is no persisted config yet. Agent owns the default (600 s).
+            // and there is no persisted config yet. "Not configured" is persisted as null
+            // so BuildAgentConfiguration applies the agent-side default (600 s) — freezing
+            // a number into the file would decouple deployed devices from default changes.
             var merged = AutopilotMonitor.Agent.V2.Program.MergeBootstrapConfig(
                 existing: null,
                 bootstrapTokenArg: null!, bootstrapTokenGiven: false,
@@ -144,20 +146,42 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Program
 
             Assert.Null(merged.BootstrapToken);
             Assert.Null(merged.TenantId);
-            Assert.Equal(600, merged.TenantIdWaitSeconds);
+            Assert.Null(merged.TenantIdWaitSeconds);
         }
 
         [Fact]
-        public void Existing_config_without_wait_field_treated_as_zero()
+        public void Legacy_config_without_wait_field_stays_null_on_plain_reinstall()
         {
-            // BackCompat: pre-fix bootstrap-config.json has no TenantIdWaitSeconds field;
-            // Newtonsoft deserialises that to 0. New install with --tenant-id-wait 600
-            // overrides cleanly; no extra migration logic required.
+            // BackCompat: pre-feature bootstrap-config.json has no TenantIdWaitSeconds
+            // property → nullable deserialisation yields null. A reinstall that does not
+            // pass --tenant-id-wait must NOT turn that into 0 (which would silently
+            // disable the wait); null carries through and the runtime default applies.
             var existing = new BootstrapConfigFile
             {
                 BootstrapToken = "tok-legacy",
                 TenantId = "tenant-legacy",
-                TenantIdWaitSeconds = 0,
+                TenantIdWaitSeconds = null,
+            };
+
+            var merged = AutopilotMonitor.Agent.V2.Program.MergeBootstrapConfig(
+                existing,
+                bootstrapTokenArg: null!, bootstrapTokenGiven: false,
+                tenantIdArg: null!, tenantIdGiven: false,
+                tenantIdWaitSeconds: 0, tenantIdWaitGiven: false);
+
+            Assert.Equal("tok-legacy", merged.BootstrapToken);
+            Assert.Equal("tenant-legacy", merged.TenantId);
+            Assert.Null(merged.TenantIdWaitSeconds);
+        }
+
+        [Fact]
+        public void Legacy_config_without_wait_field_takes_explicit_wait_arg()
+        {
+            var existing = new BootstrapConfigFile
+            {
+                BootstrapToken = "tok-legacy",
+                TenantId = "tenant-legacy",
+                TenantIdWaitSeconds = null,
             };
 
             var merged = AutopilotMonitor.Agent.V2.Program.MergeBootstrapConfig(
@@ -170,5 +194,26 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Program
             Assert.Equal("tenant-legacy", merged.TenantId);
             Assert.Equal(600, merged.TenantIdWaitSeconds);
         }
+
+        [Fact]
+        public void Explicit_zero_opt_out_survives_redeploy_without_wait_arg()
+        {
+            // An admin's deliberate `--tenant-id-wait 0` must not be resurrected to a
+            // wait by a later redeploy that omits the flag.
+            var existing = new BootstrapConfigFile
+            {
+                BootstrapToken = "tok-abc",
+                TenantId = "tenant-xyz",
+                TenantIdWaitSeconds = 0,
+            };
+
+            var merged = AutopilotMonitor.Agent.V2.Program.MergeBootstrapConfig(
+                existing,
+                bootstrapTokenArg: null!, bootstrapTokenGiven: false,
+                tenantIdArg: null!, tenantIdGiven: false,
+                tenantIdWaitSeconds: 0, tenantIdWaitGiven: false);
+
+            Assert.Equal(0, merged.TenantIdWaitSeconds);
+        }
     }
 }
diff --git a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/TenantIdWaitResolutionTests.cs b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/TenantIdWaitResolutionTests.cs
new file mode 100644
index 00000000..68fb4739
--- /dev/null
+++ b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Program/TenantIdWaitResolutionTests.cs
@@ -0,0 +1,90 @@
+using AutopilotMonitor.Agent.V2;
+using AutopilotMonitor.Agent.V2.Core.Configuration;
+using Xunit;
+
+namespace AutopilotMonitor.Agent.V2.Core.Tests.Program
+{
+    /// <summary>
+    /// Pins the TenantIdWaitSeconds resolution order in
+    /// <see cref="AutopilotMonitor.Agent.V2.Program.BuildAgentConfiguration"/>:
+    /// CLI arg &gt; persisted bootstrap-config.json value &gt; agent default (600 s).
+    /// The regression this guards: a legacy bootstrap-config.json written before the
+    /// TenantIdWaitSeconds property existed deserialises to null — that must fall
+    /// through to the 600 s default, NOT be treated as an explicit 0 opt-out
+    /// (which silently disabled the wait and broke Hybrid-AAD-join devices whose
+    /// registry TenantId lands minutes after first boot).
+    /// </summary>
+    public sealed class TenantIdWaitResolutionTests
+    {
+        private static AgentConfiguration Build(BootstrapConfigFile bootstrapConfig, params string[] args) =>
+            AutopilotMonitor.Agent.V2.Program.BuildAgentConfiguration(
+                args: args,
+                tenantId: "tenant-t",
+                sessionId: "session-s",
+                bootstrapConfig: bootstrapConfig,
+                awaitConfig: null);
+
+        [Fact]
+        public void No_cli_and_no_bootstrap_config_uses_default_600()
+        {
+            var cfg = Build(bootstrapConfig: null);
+
+            Assert.Equal(600, cfg.TenantIdWaitSeconds);
+        }
+
+        [Fact]
+        public void Legacy_bootstrap_config_with_null_wait_uses_default_600()
+        {
+            var legacy = new BootstrapConfigFile
+            {
+                BootstrapToken = "tok-legacy",
+                TenantId = "tenant-legacy",
+                TenantIdWaitSeconds = null,
+            };
+
+            var cfg = Build(legacy);
+
+            Assert.Equal(600, cfg.TenantIdWaitSeconds);
+        }
+
+        [Fact]
+        public void Explicit_zero_in_bootstrap_config_disables_wait()
+        {
+            var optedOut = new BootstrapConfigFile { TenantIdWaitSeconds = 0 };
+
+            var cfg = Build(optedOut);
+
+            Assert.Equal(0, cfg.TenantIdWaitSeconds);
+        }
+
+        [Fact]
+        public void Nonzero_bootstrap_config_value_wins_over_default()
+        {
+            var configured = new BootstrapConfigFile { TenantIdWaitSeconds = 1800 };
+
+            var cfg = Build(configured);
+
+            Assert.Equal(1800, cfg.TenantIdWaitSeconds);
+        }
+
+        [Fact]
+        public void Cli_arg_wins_over_bootstrap_config_value()
+        {
+            var configured = new BootstrapConfigFile { TenantIdWaitSeconds = 1800 };
+
+            var cfg = Build(configured, "--tenant-id-wait", "300");
+
+            Assert.Equal(300, cfg.TenantIdWaitSeconds);
+        }
+
+        [Fact]
+        public void Cli_zero_wins_over_bootstrap_config_value()
+        {
+            var configured = new BootstrapConfigFile { TenantIdWaitSeconds = 1800 };
+
+            var cfg = Build(configured, "--tenant-id-wait", "0");
+
+            Assert.Equal(0, cfg.TenantIdWaitSeconds);
+        }
+    }
+}
diff --git a/src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/AgentConfiguration.cs b/src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/AgentConfiguration.cs
index 6c93d107..5b65ccb7 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/AgentConfiguration.cs
+++ b/src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/AgentConfiguration.cs
@@ -272,9 +272,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Configuration
         /// CloudDomainJoin keys and re-probes on every change until either resolution
         /// or this timeout. 0 = no wait, fast-fail on miss (legacy behaviour).
         /// <para>
-        /// Set via the install-time CLI flag <c>--tenant-id-wait &lt;sec&gt;</c>, persisted
-        /// to <c>bootstrap-config.json</c>. The PS1 bootstrap script owns the production
-        /// default; the agent itself never assumes one.
+        /// Set via the CLI flag <c>--tenant-id-wait &lt;sec&gt;</c> or a persisted
+        /// <c>bootstrap-config.json</c> value; when neither supplies a value the agent
+        /// applies its own default of 600 s in BuildAgentConfiguration. The 0 default
+        /// here is only the POCO baseline, not the effective runtime default.
         /// </para>
         /// </summary>
         public int TenantIdWaitSeconds { get; set; } = 0;
diff --git a/src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/BootstrapConfigFile.cs b/src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/BootstrapConfigFile.cs
index 5dd95c1c..cf4d6563 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/BootstrapConfigFile.cs
+++ b/src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/BootstrapConfigFile.cs
@@ -16,8 +16,11 @@ namespace AutopilotMonitor.Agent.V2.Core.Configuration
         /// <summary>
         /// TenantId-wait timeout in seconds, persisted from the <c>--tenant-id-wait</c>
         /// install-time arg. Read by AgentBootstrap when the registry probe finds no
-        /// TenantId on first try. 0 / missing = no wait, legacy fast-fail.
+        /// TenantId on first try. 0 = explicit opt-out (legacy fast-fail).
+        /// Null / missing (files written before this property existed) = not configured —
+        /// the agent-side default (600 s) applies. Nullable so a legacy file without the
+        /// property cannot masquerade as an explicit 0 and silently disable the wait.
         /// </summary>
-        public int TenantIdWaitSeconds { get; set; }
+        public int? TenantIdWaitSeconds { get; set; }
     }
 }
diff --git a/src/Agent/AutopilotMonitor.Agent.V2/Program.InstallMode.cs b/src/Agent/AutopilotMonitor.Agent.V2/Program.InstallMode.cs
index 81045a00..46736fe0 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2/Program.InstallMode.cs
+++ b/src/Agent/AutopilotMonitor.Agent.V2/Program.InstallMode.cs
@@ -135,7 +135,7 @@ namespace AutopilotMonitor.Agent.V2
                         $"{(bootstrapTokenGiven ? "*" : "")}, " +
                         $"tenantId={(string.IsNullOrEmpty(merged.TenantId) ? "no" : "yes")}" +
                         $"{(tenantIdGiven ? "*" : "")}, " +
-                        $"tenantIdWait={merged.TenantIdWaitSeconds}s" +
+                        $"tenantIdWait={(merged.TenantIdWaitSeconds is int w ? $"{w}s" : "default")}" +
                         $"{(tenantIdWaitGiven ? "*" : "")})." +
                         " Star = explicitly set on this --install invocation, others were preserved from prior config.");
                 }
@@ -361,9 +361,12 @@ namespace AutopilotMonitor.Agent.V2
             {
                 BootstrapToken = bootstrapTokenGiven ? bootstrapTokenArg : existing?.BootstrapToken,
                 TenantId = tenantIdGiven ? tenantIdArg : existing?.TenantId,
+                // Not given → carry the prior value through, INCLUDING null. Persisting a
+                // number here would freeze the agent-side default into the file; persisting
+                // null keeps "not configured" distinguishable from an explicit opt-out (0).
                 TenantIdWaitSeconds = tenantIdWaitGiven
                     ? tenantIdWaitSeconds
-                    : (existing?.TenantIdWaitSeconds ?? 600),
+                    : existing?.TenantIdWaitSeconds,
             };
         }
 
diff --git a/src/Agent/AutopilotMonitor.Agent.V2/Program.cs b/src/Agent/AutopilotMonitor.Agent.V2/Program.cs
index af8c5bb8..d4127782 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2/Program.cs
+++ b/src/Agent/AutopilotMonitor.Agent.V2/Program.cs
@@ -339,13 +339,15 @@ namespace AutopilotMonitor.Agent.V2
             // over the agent-side default (600 s). Hybrid-AAD-joined devices typically
             // need ~5 min for the AAD device cert to land in the registry; 600 s leaves
             // headroom. Pass `--tenant-id-wait 0` to opt out (legacy fast-fail).
+            // A bootstrap-config.json whose TenantIdWaitSeconds is null (file predates the
+            // property) is "not configured", NOT an opt-out — the default must still apply.
             var tenantIdWaitRaw = GetArgValue(args, "--tenant-id-wait");
             const int tenantIdWaitDefaultSeconds = 600;
             int tenantIdWaitSeconds = tenantIdWaitDefaultSeconds;
             if (!string.IsNullOrEmpty(tenantIdWaitRaw) && int.TryParse(tenantIdWaitRaw, out var parsedTenantIdWait))
                 tenantIdWaitSeconds = parsedTenantIdWait;
-            else if (bootstrapConfig != null)
-                tenantIdWaitSeconds = bootstrapConfig.TenantIdWaitSeconds;
+            else if (bootstrapConfig?.TenantIdWaitSeconds != null)
+                tenantIdWaitSeconds = bootstrapConfig.TenantIdWaitSeconds.Value;
 
             var useBootstrapTokenAuth = !string.IsNullOrEmpty(bootstrapToken);