diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Persistence/SiteStorageService.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Persistence/SiteStorageService.cs
index fd25814a..910d1b8b 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Persistence/SiteStorageService.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Persistence/SiteStorageService.cs
@@ -229,6 +229,71 @@ public class SiteStorageService
         _logger.LogDebug("Stored deployed config for {Instance}, deploymentId={DeploymentId}", instanceName, deploymentId);
     }
 
+    /// <summary>
+    /// Stores a deployed instance configuration only if the incoming write is strictly
+    /// newer than any already-stored row for the same instance.  If no row exists the
+    /// write is always performed (insert path).  Uses SQLite's conditional
+    /// <c>ON CONFLICT … WHERE excluded.deployed_at &gt; deployed_configurations.deployed_at</c>
+    /// clause so the guard is atomic with no application-level read-modify-write.
+    /// </summary>
+    /// <remarks>
+    /// This is the standby-node write path for replicated configs.  The active-node
+    /// apply path (<see cref="StoreDeployedConfigAsync"/>) remains unguarded and always
+    /// overwrites, because the active node's write is always authoritative.
+    /// <para>
+    /// <paramref name="deployedAtOverride"/> is exposed for testing so that the exact
+    /// <c>deployed_at</c> value can be controlled without sleeping between calls.
+    /// Production callers omit it and receive the default <see cref="DateTimeOffset.UtcNow"/>
+    /// semantics, identical to <see cref="StoreDeployedConfigAsync"/>.
+    /// </para>
+    /// </remarks>
+    /// <param name="instanceName">The unique name of the instance.</param>
+    /// <param name="configJson">The deployed configuration as JSON.</param>
+    /// <param name="deploymentId">The unique deployment identifier.</param>
+    /// <param name="revisionHash">The configuration revision hash for staleness detection.</param>
+    /// <param name="isEnabled">Whether the instance is enabled.</param>
+    /// <param name="deployedAtOverride">
+    /// Optional explicit <c>deployed_at</c> timestamp.  Defaults to
+    /// <see cref="DateTimeOffset.UtcNow"/> when <see langword="null"/>.
+    /// </param>
+    /// <returns>A task that completes when the conditional upsert has been executed.</returns>
+    public async Task StoreDeployedConfigIfNewerAsync(
+        string instanceName,
+        string configJson,
+        string deploymentId,
+        string revisionHash,
+        bool isEnabled,
+        DateTimeOffset? deployedAtOverride = null)
+    {
+        var deployedAt = (deployedAtOverride ?? DateTimeOffset.UtcNow).ToString("O");
+
+        await using var connection = new SqliteConnection(_connectionString);
+        await connection.OpenAsync();
+
+        await using var command = connection.CreateCommand();
+        command.CommandText = @"
+            INSERT INTO deployed_configurations
+                (instance_unique_name, config_json, deployment_id, revision_hash, is_enabled, deployed_at)
+            VALUES (@name, @json, @depId, @hash, @enabled, @deployedAt)
+            ON CONFLICT(instance_unique_name) DO UPDATE SET
+                config_json   = excluded.config_json,
+                deployment_id = excluded.deployment_id,
+                revision_hash = excluded.revision_hash,
+                is_enabled    = excluded.is_enabled,
+                deployed_at   = excluded.deployed_at
+            WHERE excluded.deployed_at > deployed_configurations.deployed_at";
+
+        command.Parameters.AddWithValue("@name", instanceName);
+        command.Parameters.AddWithValue("@json", configJson);
+        command.Parameters.AddWithValue("@depId", deploymentId);
+        command.Parameters.AddWithValue("@hash", revisionHash);
+        command.Parameters.AddWithValue("@enabled", isEnabled ? 1 : 0);
+        command.Parameters.AddWithValue("@deployedAt", deployedAt);
+
+        await command.ExecuteNonQueryAsync();
+        _logger.LogDebug("StoreDeployedConfigIfNewer for {Instance}, deploymentId={DeploymentId}", instanceName, deploymentId);
+    }
+
     /// <summary>
     /// Removes a deployed instance configuration and its static overrides.
     /// </summary>
diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Persistence/SiteStorageServiceTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Persistence/SiteStorageServiceTests.cs
index fe51198c..8adcb42d 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Persistence/SiteStorageServiceTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Persistence/SiteStorageServiceTests.cs
@@ -1,3 +1,4 @@
+using Microsoft.Data.Sqlite;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime.Persistence;
@@ -194,4 +195,112 @@ public class SiteStorageServiceTests : IAsyncLifetime, IDisposable
         Assert.Empty(configs);
         Assert.Empty(overrides);
     }
+
+    // ── Task 13: StoreDeployedConfigIfNewerAsync (guarded standby write) ──
+
+    /// <summary>
+    /// Seeds a deployed_configurations row with an explicit deployed_at timestamp using the same
+    /// "O" format the service uses, so tests can establish deterministic older/newer/equal rows.
+    /// </summary>
+    private async Task SeedDeployedConfigAsync(
+        string instanceName, string configJson, string deploymentId,
+        string revisionHash, bool isEnabled, DateTimeOffset deployedAt)
+    {
+        await using var conn = new SqliteConnection($"Data Source={_dbFile}");
+        await conn.OpenAsync();
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = @"
+            INSERT INTO deployed_configurations
+                (instance_unique_name, config_json, deployment_id, revision_hash, is_enabled, deployed_at)
+            VALUES (@name, @json, @depId, @hash, @enabled, @deployedAt)
+            ON CONFLICT(instance_unique_name) DO UPDATE SET
+                config_json   = excluded.config_json,
+                deployment_id = excluded.deployment_id,
+                revision_hash = excluded.revision_hash,
+                is_enabled    = excluded.is_enabled,
+                deployed_at   = excluded.deployed_at";
+        cmd.Parameters.AddWithValue("@name", instanceName);
+        cmd.Parameters.AddWithValue("@json", configJson);
+        cmd.Parameters.AddWithValue("@depId", deploymentId);
+        cmd.Parameters.AddWithValue("@hash", revisionHash);
+        cmd.Parameters.AddWithValue("@enabled", isEnabled ? 1 : 0);
+        cmd.Parameters.AddWithValue("@deployedAt", deployedAt.ToString("O"));
+        await cmd.ExecuteNonQueryAsync();
+    }
+
+    [Fact]
+    public async Task StoreDeployedConfigIfNewer_NoExistingRow_Inserts()
+    {
+        var at = DateTimeOffset.UtcNow;
+
+        await _storage.StoreDeployedConfigIfNewerAsync(
+            "Pump1", "{\"v\":1}", "dep-001", "sha256:aaa", isEnabled: true, deployedAtOverride: at);
+
+        var configs = await _storage.GetAllDeployedConfigsAsync();
+        Assert.Single(configs);
+        Assert.Equal("Pump1", configs[0].InstanceUniqueName);
+        Assert.Equal("{\"v\":1}", configs[0].ConfigJson);
+        Assert.Equal("dep-001", configs[0].DeploymentId);
+        Assert.Equal("sha256:aaa", configs[0].RevisionHash);
+        Assert.True(configs[0].IsEnabled);
+    }
+
+    [Fact]
+    public async Task StoreDeployedConfigIfNewer_ExistingOlderRow_Overwrites()
+    {
+        var olderAt = new DateTimeOffset(2026, 1, 1, 10, 0, 0, TimeSpan.Zero);
+        var newerAt = new DateTimeOffset(2026, 1, 1, 11, 0, 0, TimeSpan.Zero);
+
+        await SeedDeployedConfigAsync("Pump1", "{\"v\":1}", "dep-001", "sha256:aaa", true, olderAt);
+
+        await _storage.StoreDeployedConfigIfNewerAsync(
+            "Pump1", "{\"v\":2}", "dep-002", "sha256:bbb", isEnabled: false, deployedAtOverride: newerAt);
+
+        var configs = await _storage.GetAllDeployedConfigsAsync();
+        Assert.Single(configs);
+        Assert.Equal("{\"v\":2}", configs[0].ConfigJson);
+        Assert.Equal("dep-002", configs[0].DeploymentId);
+        Assert.Equal("sha256:bbb", configs[0].RevisionHash);
+        Assert.False(configs[0].IsEnabled);
+    }
+
+    [Fact]
+    public async Task StoreDeployedConfigIfNewer_ExistingNewerRow_IsNoop()
+    {
+        var newerAt = new DateTimeOffset(2026, 6, 1, 12, 0, 0, TimeSpan.Zero);
+        var olderAt = new DateTimeOffset(2026, 5, 1, 12, 0, 0, TimeSpan.Zero);
+
+        // Seed the row that is already newer than what the standby would write
+        await SeedDeployedConfigAsync("Pump1", "{\"v\":2}", "dep-002", "sha256:bbb", false, newerAt);
+
+        // Guarded write with an older timestamp — must be a NO-OP
+        await _storage.StoreDeployedConfigIfNewerAsync(
+            "Pump1", "{\"v\":1}", "dep-001", "sha256:aaa", isEnabled: true, deployedAtOverride: olderAt);
+
+        var configs = await _storage.GetAllDeployedConfigsAsync();
+        Assert.Single(configs);
+        // The newer seeded row must survive unchanged
+        Assert.Equal("{\"v\":2}", configs[0].ConfigJson);
+        Assert.Equal("dep-002", configs[0].DeploymentId);
+        Assert.Equal("sha256:bbb", configs[0].RevisionHash);
+        Assert.False(configs[0].IsEnabled);
+    }
+
+    [Fact]
+    public async Task StoreDeployedConfigIfNewer_EqualDeployedAt_IsNoop()
+    {
+        var at = new DateTimeOffset(2026, 3, 15, 9, 30, 0, TimeSpan.Zero);
+
+        await SeedDeployedConfigAsync("Pump1", "{\"v\":1}", "dep-001", "sha256:aaa", true, at);
+
+        // Guarded write with the IDENTICAL timestamp — must be a NO-OP (> not >=)
+        await _storage.StoreDeployedConfigIfNewerAsync(
+            "Pump1", "{\"v\":2}", "dep-002", "sha256:bbb", isEnabled: false, deployedAtOverride: at);
+
+        var configs = await _storage.GetAllDeployedConfigsAsync();
+        Assert.Single(configs);
+        // Original row preserved
+        Assert.Equal("{\"v\":1}", configs[0].ConfigJson);
+        Assert.Equal("dep-001", configs[0].DeploymentId);
+    }
 }