diff --git a/dotnet/src/ZB.MOM.NatsNet.Server/JetStream/JetStreamErrors.GeneratedConstructors.cs b/dotnet/src/ZB.MOM.NatsNet.Server/JetStream/JetStreamErrors.GeneratedConstructors.cs
new file mode 100644
index 0000000..b58c881
--- /dev/null
+++ b/dotnet/src/ZB.MOM.NatsNet.Server/JetStream/JetStreamErrors.GeneratedConstructors.cs
@@ -0,0 +1,147 @@
+// Copyright 2020-2026 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+//
+// Generated constructor surface for JetStream API errors.
+// Source parity: server/jetstream_errors_generated.go
+
+namespace ZB.MOM.NatsNet.Server;
+
+public static partial class JsApiErrors
+{
+    public static JsApiError NewJSAccountResourcesExceededError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(AccountResourcesExceeded);
+    }
+
+    public static JsApiError NewJSAtomicPublishContainsDuplicateMessageError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(AtomicPublishContainsDuplicateMessage);
+    }
+
+    public static JsApiError NewJSAtomicPublishDisabledError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(AtomicPublishDisabled);
+    }
+
+    public static JsApiError NewJSAtomicPublishIncompleteBatchError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(AtomicPublishIncompleteBatch);
+    }
+
+    public static JsApiError NewJSAtomicPublishInvalidBatchCommitError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(AtomicPublishInvalidBatchCommit);
+    }
+
+    public static JsApiError NewJSAtomicPublishInvalidBatchIDError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(AtomicPublishInvalidBatchID);
+    }
+
+    public static JsApiError NewJSAtomicPublishMissingSeqError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(AtomicPublishMissingSeq);
+    }
+
+    public static JsApiError NewJSBadRequestError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(BadRequest);
+    }
+
+    public static JsApiError NewJSClusterIncompleteError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(ClusterIncomplete);
+    }
+
+    public static JsApiError NewJSClusterNotActiveError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(ClusterNotActive);
+    }
+
+    public static JsApiError NewJSClusterNotAssignedError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(ClusterNotAssigned);
+    }
+
+    public static JsApiError NewJSClusterNotAvailError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(ClusterNotAvail);
+    }
+
+    public static JsApiError NewJSClusterNotLeaderError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(ClusterNotLeader);
+    }
+
+    public static JsApiError NewJSClusterPeerNotMemberError(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(ClusterPeerNotMember);
+    }
+
+    public static JsApiError NewJSAtomicPublishTooLargeBatchError(object? size, params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return NewWithTags(AtomicPublishTooLargeBatch, "{size}", size);
+    }
+
+    public static JsApiError NewJSAtomicPublishUnsupportedHeaderBatchError(object? header, params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return NewWithTags(AtomicPublishUnsupportedHeaderBatch, "{header}", header);
+    }
+
+    public static JsApiError NewJSClusterNoPeersError(Exception err, params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return NewWithTags(ClusterNoPeers, "{err}", err);
+    }
+
+}
diff --git a/dotnet/src/ZB.MOM.NatsNet.Server/JetStream/JetStreamErrors.cs b/dotnet/src/ZB.MOM.NatsNet.Server/JetStream/JetStreamErrors.cs
index 8182cf6..bb66826 100644
--- a/dotnet/src/ZB.MOM.NatsNet.Server/JetStream/JetStreamErrors.cs
+++ b/dotnet/src/ZB.MOM.NatsNet.Server/JetStream/JetStreamErrors.cs
@@ -43,7 +43,7 @@ public sealed class JsApiError
 /// Pre-built <see cref="JsApiError"/> instances for all JetStream error codes.
 /// Mirrors the <c>ApiErrors</c> map in server/jetstream_errors_generated.go.
 /// </summary>
-public static class JsApiErrors
+public static partial class JsApiErrors
 {
     public delegate object? ErrorOption();
 
@@ -356,14 +356,10 @@ public static class JsApiErrors
     /// </summary>
     public static JsApiError NewJSRestoreSubscribeFailedError(Exception err, string subject, params ErrorOption[] opts)
     {
-        var overridden = ParseUnless(opts);
-        if (overridden != null)
-            return overridden;
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
 
-        return NewWithTags(
-            RestoreSubscribeFailed,
-            ("{err}", err.Message),
-            ("{subject}", subject));
+        return NewWithTags(RestoreSubscribeFailed, "{subject}", subject, "{err}", err);
     }
 
     /// <summary>
@@ -371,11 +367,10 @@ public static class JsApiErrors
     /// </summary>
     public static JsApiError NewJSStreamRestoreError(Exception err, params ErrorOption[] opts)
     {
-        var overridden = ParseUnless(opts);
-        if (overridden != null)
-            return overridden;
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
 
-        return NewWithTags(StreamRestore, ("{err}", err.Message));
+        return NewWithTags(StreamRestore, "{err}", err);
     }
 
     /// <summary>
@@ -383,20 +378,20 @@ public static class JsApiErrors
     /// </summary>
     public static JsApiError NewJSPeerRemapError(params ErrorOption[] opts)
     {
-        var overridden = ParseUnless(opts);
-        return overridden ?? Clone(PeerRemap);
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(PeerRemap);
     }
 
-    private static JsApiError? ParseUnless(ReadOnlySpan<ErrorOption> opts)
+    private static object? ParseOpts(params ErrorOption[] opts)
     {
-        foreach (var opt in opts)
-        {
-            var value = opt();
-            if (value is JsApiError apiErr)
-                return Clone(apiErr);
-        }
+        object? value = null;
 
-        return null;
+        foreach (var opt in opts)
+            value = opt();
+
+        return value;
     }
 
     private static JsApiError Clone(JsApiError source) => new()
@@ -406,13 +401,43 @@ public static class JsApiErrors
         Description = source.Description,
     };
 
-    private static JsApiError NewWithTags(JsApiError source, params (string key, string value)[] replacements)
+    private static string[] ToReplacerArgs(params object?[] replacements)
+    {
+        var args = new List<string>(replacements.Length);
+        string key = string.Empty;
+
+        for (var i = 0; i < replacements.Length; i++)
+        {
+            if (i % 2 == 0)
+            {
+                key = replacements[i] as string
+                    ?? throw new InvalidOperationException("Replacement keys must be strings.");
+                continue;
+            }
+
+            var value = replacements[i] switch
+            {
+                string s => s,
+                Exception ex => ex.Message,
+                null => string.Empty,
+                var other => other.ToString() ?? string.Empty,
+            };
+
+            args.Add(key);
+            args.Add(value);
+        }
+
+        return args.ToArray();
+    }
+
+    private static JsApiError NewWithTags(JsApiError source, params object?[] replacements)
     {
         var clone = Clone(source);
         var description = clone.Description ?? string.Empty;
+        var args = ToReplacerArgs(replacements);
 
-        foreach (var (key, value) in replacements)
-            description = description.Replace(key, value, StringComparison.Ordinal);
+        for (var i = 0; i + 1 < args.Length; i += 2)
+            description = description.Replace(args[i], args[i + 1], StringComparison.Ordinal);
 
         clone.Description = description;
         return clone;
diff --git a/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/JetStreamErrorsGeneratedConstructorsTests.cs b/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/JetStreamErrorsGeneratedConstructorsTests.cs
new file mode 100644
index 0000000..a0603a0
--- /dev/null
+++ b/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/JetStreamErrorsGeneratedConstructorsTests.cs
@@ -0,0 +1,39 @@
+// Copyright 2020-2026 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+
+using Shouldly;
+
+namespace ZB.MOM.NatsNet.Server.Tests.JetStream;
+
+public sealed class JetStreamErrorsGeneratedConstructorsTests
+{
+    [Fact]
+    public void ConstructorSurface_Group01()
+    {
+        JsApiErrors.NewJSAccountResourcesExceededError().ErrCode.ShouldBe(JsApiErrors.AccountResourcesExceeded.ErrCode);
+        JsApiErrors.NewJSAtomicPublishContainsDuplicateMessageError().ErrCode.ShouldBe(JsApiErrors.AtomicPublishContainsDuplicateMessage.ErrCode);
+        JsApiErrors.NewJSAtomicPublishDisabledError().ErrCode.ShouldBe(JsApiErrors.AtomicPublishDisabled.ErrCode);
+        JsApiErrors.NewJSAtomicPublishIncompleteBatchError().ErrCode.ShouldBe(JsApiErrors.AtomicPublishIncompleteBatch.ErrCode);
+        JsApiErrors.NewJSAtomicPublishInvalidBatchCommitError().ErrCode.ShouldBe(JsApiErrors.AtomicPublishInvalidBatchCommit.ErrCode);
+        JsApiErrors.NewJSAtomicPublishInvalidBatchIDError().ErrCode.ShouldBe(JsApiErrors.AtomicPublishInvalidBatchID.ErrCode);
+        JsApiErrors.NewJSAtomicPublishMissingSeqError().ErrCode.ShouldBe(JsApiErrors.AtomicPublishMissingSeq.ErrCode);
+        JsApiErrors.NewJSBadRequestError().ErrCode.ShouldBe(JsApiErrors.BadRequest.ErrCode);
+        JsApiErrors.NewJSClusterIncompleteError().ErrCode.ShouldBe(JsApiErrors.ClusterIncomplete.ErrCode);
+        JsApiErrors.NewJSClusterNotActiveError().ErrCode.ShouldBe(JsApiErrors.ClusterNotActive.ErrCode);
+        JsApiErrors.NewJSClusterNotAssignedError().ErrCode.ShouldBe(JsApiErrors.ClusterNotAssigned.ErrCode);
+        JsApiErrors.NewJSClusterNotAvailError().ErrCode.ShouldBe(JsApiErrors.ClusterNotAvail.ErrCode);
+        JsApiErrors.NewJSClusterNotLeaderError().ErrCode.ShouldBe(JsApiErrors.ClusterNotLeader.ErrCode);
+        JsApiErrors.NewJSClusterPeerNotMemberError().ErrCode.ShouldBe(JsApiErrors.ClusterPeerNotMember.ErrCode);
+
+        JsApiErrors.NewJSAtomicPublishTooLargeBatchError(512).Description.ShouldBe("atomic publish batch is too large: 512");
+        JsApiErrors.NewJSAtomicPublishUnsupportedHeaderBatchError("Nats-Msg-Id").Description.ShouldBe("atomic publish unsupported header used: Nats-Msg-Id");
+        JsApiErrors.NewJSClusterNoPeersError(new InvalidOperationException("no peers")).Description.ShouldBe("no peers");
+
+        var expected = new JsApiError { Code = 499, ErrCode = 9090, Description = "override" };
+        var fromOverride = JsApiErrors.NewJSAccountResourcesExceededError(JsApiErrors.Unless(expected));
+        fromOverride.Code.ShouldBe(expected.Code);
+        fromOverride.ErrCode.ShouldBe(expected.ErrCode);
+        fromOverride.Description.ShouldBe(expected.Description);
+        ReferenceEquals(fromOverride, expected).ShouldBeFalse();
+    }
+}
diff --git a/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/JetStreamErrorsTests.cs b/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/JetStreamErrorsTests.cs
index 66cc22b..bb153ad 100644
--- a/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/JetStreamErrorsTests.cs
+++ b/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/JetStreamErrorsTests.cs
@@ -1,6 +1,7 @@
 // Copyright 2020-2026 The NATS Authors
 // Licensed under the Apache License, Version 2.0 (the "License");
 
+using System.Reflection;
 using Shouldly;
 
 namespace ZB.MOM.NatsNet.Server.Tests.JetStream;
@@ -97,4 +98,39 @@ public sealed class JetStreamErrorsTests
             JsApiErrors.NewJSPeerRemapError(JsApiErrors.Unless(new Exception("other error"))),
             peerRemap).ShouldBeTrue();
     }
+
+    [Fact]
+    public void ParseOpts_WithUnlessApiErrorOption_ReturnsOverride()
+    {
+        var parseOpts = typeof(JsApiErrors).GetMethod(
+            "ParseOpts",
+            BindingFlags.NonPublic | BindingFlags.Static);
+
+        parseOpts.ShouldNotBeNull();
+
+        var expected = new JsApiError { Code = 401, ErrCode = 2048, Description = "override" };
+        var result = parseOpts.Invoke(
+            null,
+            [new JsApiErrors.ErrorOption[] { JsApiErrors.Unless(expected) }]);
+
+        result.ShouldBeAssignableTo<JsApiError>()
+            .ErrCode.ShouldBe(expected.ErrCode);
+    }
+
+    [Fact]
+    public void ToReplacerArgs_WithStringErrorAndObject_ConvertsToStringValues()
+    {
+        var toReplacerArgs = typeof(JsApiErrors).GetMethod(
+            "ToReplacerArgs",
+            BindingFlags.NonPublic | BindingFlags.Static);
+
+        toReplacerArgs.ShouldNotBeNull();
+
+        var result = toReplacerArgs.Invoke(
+            null,
+            [new object?[] { "{string}", "value", "{error}", new InvalidOperationException("boom"), "{number}", 42 }]);
+
+        result.ShouldBeAssignableTo<string[]>()
+            .ShouldBe(["{string}", "value", "{error}", "boom", "{number}", "42"]);
+    }
 }
diff --git a/porting.db b/porting.db
index 42fb94b..81e4bc6 100644
Binary files a/porting.db and b/porting.db differ
diff --git a/tools/generate-jetstream-errors.sh b/tools/generate-jetstream-errors.sh
new file mode 100755
index 0000000..9e76d78
--- /dev/null
+++ b/tools/generate-jetstream-errors.sh
@@ -0,0 +1,77 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+output_file="$repo_root/dotnet/src/ZB.MOM.NatsNet.Server/JetStream/JetStreamErrors.GeneratedConstructors.cs"
+
+simple_methods=(
+  "NewJSAccountResourcesExceededError|AccountResourcesExceeded"
+  "NewJSAtomicPublishContainsDuplicateMessageError|AtomicPublishContainsDuplicateMessage"
+  "NewJSAtomicPublishDisabledError|AtomicPublishDisabled"
+  "NewJSAtomicPublishIncompleteBatchError|AtomicPublishIncompleteBatch"
+  "NewJSAtomicPublishInvalidBatchCommitError|AtomicPublishInvalidBatchCommit"
+  "NewJSAtomicPublishInvalidBatchIDError|AtomicPublishInvalidBatchID"
+  "NewJSAtomicPublishMissingSeqError|AtomicPublishMissingSeq"
+  "NewJSBadRequestError|BadRequest"
+  "NewJSClusterIncompleteError|ClusterIncomplete"
+  "NewJSClusterNotActiveError|ClusterNotActive"
+  "NewJSClusterNotAssignedError|ClusterNotAssigned"
+  "NewJSClusterNotAvailError|ClusterNotAvail"
+  "NewJSClusterNotLeaderError|ClusterNotLeader"
+  "NewJSClusterPeerNotMemberError|ClusterPeerNotMember"
+)
+
+templated_methods=(
+  "NewJSAtomicPublishTooLargeBatchError|object?|size|AtomicPublishTooLargeBatch|{size}"
+  "NewJSAtomicPublishUnsupportedHeaderBatchError|object?|header|AtomicPublishUnsupportedHeaderBatch|{header}"
+  "NewJSClusterNoPeersError|Exception|err|ClusterNoPeers|{err}"
+)
+
+{
+  cat <<'EOF'
+// Copyright 2020-2026 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+//
+// Generated constructor surface for JetStream API errors.
+// Source parity: server/jetstream_errors_generated.go
+
+namespace ZB.MOM.NatsNet.Server;
+
+public static partial class JsApiErrors
+{
+EOF
+
+  for entry in "${simple_methods[@]}"; do
+    IFS='|' read -r method field <<<"$entry"
+    cat <<EOF
+    public static JsApiError ${method}(params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return Clone(${field});
+    }
+
+EOF
+  done
+
+  for entry in "${templated_methods[@]}"; do
+    IFS='|' read -r method arg_type arg_name field placeholder <<<"$entry"
+    cat <<EOF
+    public static JsApiError ${method}(${arg_type} ${arg_name}, params ErrorOption[] opts)
+    {
+        if (ParseOpts(opts) is JsApiError overridden)
+            return Clone(overridden);
+
+        return NewWithTags(${field}, "${placeholder}", ${arg_name});
+    }
+
+EOF
+  done
+
+  cat <<'EOF'
+}
+EOF
+} >"$output_file"
+
+echo "Generated $output_file"