diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts/OpcUaEndpointUrlRewriter.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts/OpcUaEndpointUrlRewriter.cs
index c15217a60..68f98169d 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts/OpcUaEndpointUrlRewriter.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts/OpcUaEndpointUrlRewriter.cs
@@ -46,6 +46,33 @@ public static class OpcUaEndpointUrlRewriter
             return advertisedUrl;
         var builder = new UriBuilder(advertised) { Host = configured.DnsSafeHost, Port = port };
         // AbsoluteUri, not ToString(): ToString() un-escapes, turning an advertised "/x%20y" into "/x y" (#812).
-        return builder.Uri.AbsoluteUri;
+        return RestoreZoneIndex(builder.Uri, configured);
+    }
+
+    /// <summary>
+    ///     Gitea #819 — keep an IPv6 zone index (<c>[fe80::1%eth0]</c>) from the configured host. .NET's <see cref="Uri"/>
+    ///     parses the zone (only <see cref="Uri.DnsSafeHost"/> / <see cref="Uri.IdnHost"/> carry it) but drops it from
+    ///     <see cref="Uri.Host"/>, <see cref="Uri.Authority"/>, <see cref="Uri.AbsoluteUri"/> and <c>ToString()</c>, so
+    ///     any re-serialised URL loses it. The SDK dials <c>new Uri(EndpointUrl).IdnHost</c>, which DOES carry the zone
+    ///     — so the zone is spliced back into the string, verbatim as the operator wrote it (<c>%eth0</c>, <c>%4</c>, or
+    ///     an RFC 6874 <c>%25eth0</c>: not normalised, so the session dials exactly what GetEndpoints just reached).
+    ///     Without it a link-local endpoint is unroutable: the session would dial a zone-less <c>fe80::</c> address.
+    /// </summary>
+    /// <param name="rebuilt">The rewritten URL (configured host, zone already lost).</param>
+    /// <param name="configured">The operator-configured URL.</param>
+    /// <returns><paramref name="rebuilt"/>'s absolute URI with the configured zone index restored.</returns>
+    private static string RestoreZoneIndex(Uri rebuilt, Uri configured)
+    {
+        var absolute = rebuilt.AbsoluteUri;
+        if (configured.HostNameType != UriHostNameType.IPv6) return absolute;
+        var zoned = configured.DnsSafeHost;                    // "fe80::1%eth0" — the only accessor that keeps it
+        if (zoned.IndexOf('%') < 0) return absolute;
+
+        var bracketed = rebuilt.Host;                          // "[fe80::1]"
+        var authorityStart = absolute.IndexOf("://", StringComparison.Ordinal);
+        if (authorityStart < 0) return absolute;
+        var hostAt = absolute.IndexOf(bracketed, authorityStart + 3, StringComparison.OrdinalIgnoreCase);
+        if (hostAt < 0) return absolute;
+        return absolute[..hostAt] + "[" + zoned + "]" + absolute[(hostAt + bracketed.Length)..];
     }
 }
diff --git a/tests/Client/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/Adapters/EndpointSelectorRewriteParityTests.cs b/tests/Client/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/Adapters/EndpointSelectorRewriteParityTests.cs
index 84d861b46..4ba60048a 100644
--- a/tests/Client/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/Adapters/EndpointSelectorRewriteParityTests.cs
+++ b/tests/Client/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/Adapters/EndpointSelectorRewriteParityTests.cs
@@ -42,8 +42,10 @@ public sealed class EndpointSelectorRewriteParityTests
     [InlineData("opc.tcp://plc-a:4840/Server", "opc.tcp://PLC-A:4840/", "opc.tcp://plc-a:4840/Server")]
     // A real rewrite keeps the advertised path's percent-escaping intact.
     [InlineData("opc.tcp://a:4840/x%20y?q=1", "opc.tcp://b:1/", "opc.tcp://b:1/x%20y?q=1")]
-    // An IPv6 zone index on the configured host is dropped (UriBuilder behaviour, same in both copies).
-    [InlineData("opc.tcp://internal-host:4840/OtOpcUa", "opc.tcp://[fe80::1%25eth0]:4841", "opc.tcp://[fe80::1]:4841/OtOpcUa")]
+    // #819: an IPv6 zone index on the configured host is KEPT, verbatim (it used to be dropped — UriBuilder behaviour,
+    // preserved deliberately through #812 — leaving link-local endpoints unroutable).
+    [InlineData("opc.tcp://internal-host:4840/OtOpcUa", "opc.tcp://[fe80::1%25eth0]:4841", "opc.tcp://[fe80::1%25eth0]:4841/OtOpcUa")]
+    [InlineData("opc.tcp://internal-host:4840/OtOpcUa", "opc.tcp://[fe80::1%eth0]:4841", "opc.tcp://[fe80::1%eth0]:4841/OtOpcUa")]
     public void SelectBest_rewrites_exactly_as_before(string advertised, string configured, string expected)
     {
         Select(advertised, configured).ShouldBe(expected);
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientEndpointRewriteTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientEndpointRewriteTests.cs
index 53e9b7a9c..2d27d1f27 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientEndpointRewriteTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientEndpointRewriteTests.cs
@@ -94,6 +94,43 @@ public sealed class OpcUaClientEndpointRewriteTests
             .ShouldBe("opc.tcp://[fd00::1]:54840/OtOpcUa");
     }
 
+    /// <summary>
+    ///     Gitea #819 — a configured IPv6 zone index survives the rewrite verbatim. .NET's <see cref="Uri"/> drops it from
+    ///     every re-serialisation (Host, Authority, AbsoluteUri, ToString), so before #819 a link-local endpoint came out
+    ///     zone-less and unroutable.
+    /// </summary>
+    [Theory]
+    [InlineData("opc.tcp://[fe80::1%eth0]:4841", "opc.tcp://[fe80::1%eth0]:4841/OtOpcUa")]
+    [InlineData("opc.tcp://[fe80::1%4]:4841/", "opc.tcp://[fe80::1%4]:4841/OtOpcUa")]
+    // RFC 6874's encoded delimiter is kept as written, not normalised: the session dials what GetEndpoints reached.
+    [InlineData("opc.tcp://[fe80::1%25eth0]:4841", "opc.tcp://[fe80::1%25eth0]:4841/OtOpcUa")]
+    // Port-less configured URL: the advertised port is kept, the zone still is.
+    [InlineData("opc.tcp://[fe80::1%eth0]", "opc.tcp://[fe80::1%eth0]:4840/OtOpcUa")]
+    public void An_IPv6_zone_index_on_the_configured_host_is_preserved(string configured, string expected)
+    {
+        OpcUaEndpointUrlRewriter.Rewrite("opc.tcp://internal-host:4840/OtOpcUa", configured).ShouldBe(expected);
+    }
+
+    [Fact]
+    public void A_zone_index_reaches_the_host_the_SDK_dials()
+    {
+        // The SDK's TCP transport connects to new Uri(EndpointDescription.EndpointUrl).IdnHost (ConfiguredEndpoint
+        // re-parses the string), so the zone must survive into that accessor — and the advertised path with it.
+        var rewritten = OpcUaEndpointUrlRewriter.Rewrite(
+            "opc.tcp://[fe80::9]:4840/Ua", "opc.tcp://[fe80::1%eth0]:4841");
+        var endpoint = new ConfiguredEndpoint(null, new EndpointDescription { EndpointUrl = rewritten }, EndpointConfiguration.Create());
+        endpoint.EndpointUrl!.IdnHost.ShouldBe("fe80::1%eth0");
+        endpoint.EndpointUrl.Port.ShouldBe(4841);
+        endpoint.EndpointUrl.AbsolutePath.ShouldBe("/Ua");
+    }
+
+    [Fact]
+    public void A_zone_matching_no_op_returns_the_advertised_string_verbatim()
+    {
+        OpcUaEndpointUrlRewriter.Rewrite("opc.tcp://[fe80::1%eth0]:4840/Ua", "opc.tcp://[fe80::1%eth0]:4840/")
+            .ShouldBe("opc.tcp://[fe80::1%eth0]:4840/Ua");
+    }
+
     [Fact]
     public void An_IPv6_advertised_host_is_replaced_by_a_configured_name()
     {