[Mgmt Generator] Wrong base class (ArmResource instead of TrackedResourceData) in published npm package

[live1206]

Minimal Reproduction Spec

Add the following TypeSpec to the mgmt test project (or a standalone spec) and regenerate:

import "@typespec/http";
import "@typespec/rest";
import "@azure-tools/typespec-azure-core";
import "@azure-tools/typespec-azure-resource-manager";

using TypeSpec.Http;
using TypeSpec.Rest;
using Azure.ResourceManager;

@armProviderNamespace
@service({
  title: "Repro Service",
})
@versioned(Versions)
namespace Repro.Service;

enum Versions {
  @useDependency(Azure.ResourceManager.Versions.v1_0_Preview_1)
  v2024_01_01: "2024-01-01",
}

model ReproResource
  is Azure.ResourceManager.TrackedResource<ReproProperties, false> {
  ...ResourceNameParameter<
    Resource = ReproResource,
    KeyName = "reproName",
    SegmentName = "repros"
  >;

  /** The managed service identities assigned to this resource. */
  #suppress "@azure-tools/typespec-azure-resource-manager/arm-resource-invalid-envelope-property"
  identity?: Azure.ResourceManager.CommonTypes.ManagedServiceIdentity;
}

model ReproProperties {
  provisioningState?: string;
  displayName?: string;
}

@armResourceOperations
interface ReproResources {
  get is ArmResourceRead<ReproResource>;
  createOrUpdate is ArmResourceCreateOrUpdateAsync<ReproResource>;
  delete is ArmResourceDeleteWithoutOkAsync<ReproResource>;
  listByResourceGroup is ArmResourceListByParent<ReproResource>;
  listBySubscription is ArmListBySubscription<ReproResource>;
}

Key triggering pattern

The distinguishing factor is the direct identity? envelope property on the resource model:

#suppress "@azure-tools/typespec-azure-resource-manager/arm-resource-invalid-envelope-property"
identity?: Azure.ResourceManager.CommonTypes.ManagedServiceIdentity;

The local test project's JobResource (databox.tsp) uses the same TrackedResource<T, false> base but does not have a direct identity envelope property, and generates correctly with TrackedResourceData as the base class.

Expected output

public partial class ReproResourceData : TrackedResourceData

Actual output (with published npm package)

public partial class ReproResourceData : ArmResource

---

Bug Description

When regenerating the NewRelic SDK (Azure.ResourceManager.NewRelicObservability) using the published npm package @azure-typespec/http-client-csharp-mgmt@1.0.0-alpha.20260314.2, the generator emits wrong base classes:

1. NewRelicMonitorResourceData : ArmResource — should be : TrackedResourceData
2. NewRelicMonitorResource.Serialization.cs redundantly adds : ArmResource to the partial declaration — should only have : IJsonModel<NewRelicMonitorResourceData>

This causes build error CS0115: no suitable method found to override because ArmResource doesn't have JsonModelWriteCore.

Reproduction

1. Regenerate the NewRelic SDK:

   tsp-client update --output-dir sdk/newrelicobservability/Azure.ResourceManager.NewRelicObservability
   

2. Build:

   dotnet build sdk/newrelicobservability/Azure.ResourceManager.NewRelicObservability/src
   

3. Observe CS0115 errors on NewRelicMonitorResourceData.JsonModelWriteCore

Expected Behavior

NewRelicMonitorResourceData should inherit from TrackedResourceData (matching the TypeSpec definition is Azure.ResourceManager.TrackedResource<MonitorProperties, false>).

Investigation Findings

• The local generator test project (Mgmt-TypeSpec) correctly generates TrackedResourceData for the same pattern (TrackedResource<T, false> in databox.tsp → JobResourceData : TrackedResourceData).
• The bug only reproduces when using the published npm package (@azure-typespec/http-client-csharp-mgmt@1.0.0-alpha.20260314.2), not the local build.
• The bug is likely in the published generator DLL or the @typespec/http-client-csharp@1.0.0-alpha.20260313.1 dependency bundled in the npm package.
• The mgmt emitter does not modify baseModel — the issue originates upstream in the code model generation.

TypeSpec Definition (NewRelic)

model NewRelicMonitorResource
  is Azure.ResourceManager.TrackedResource<MonitorProperties, false> {
  ...ResourceNameParameter<...>
  identity?: Azure.ResourceManager.CommonTypes.ManagedServiceIdentity;
}

Affected Package

• @azure-typespec/http-client-csharp-mgmt@1.0.0-alpha.20260314.2
• SDK: Azure.ResourceManager.NewRelicObservability

[live1206]

Update: Cannot reproduce locally

After extensive investigation, the bug cannot be reproduced even with the exact published npm package DLL (Azure.Generator.Management.dll from @azure-typespec/http-client-csharp-mgmt@1.0.0-alpha.20260314.2).

The minimal repro spec with TrackedResource<T, false> + identity envelope property + @@flattenProperty generates TrackedResourceData correctly in the test project.

The issue appears to be specific to the full NewRelic spec's interaction with the generator — likely triggered by the combination of:

• Extensive @@clientName renames in client.tsp
• @@alternateType overrides (including CommonTypes.Resource.id)
• @@usage(AccountProperties, Usage.input | Usage.output) decorators
• @@flattenProperty on multiple resources
• Custom SubResource model definition in Azure.ResourceManager.Models namespace

To reproduce, run tsp-client update on the NewRelic SDK (sdk/newrelicobservability/Azure.ResourceManager.NewRelicObservability).