[http-client-csharp] @@clientName on discriminated union subtypes is ignored — generates original TypeSpec name

[yungshinlintw]

Bug Report

Emitter Version

@azure-typespec/http-client-csharp v1.0.0-alpha.20260512.2

Description

@@clientName directives for discriminated union subtypes are ignored during code generation. The emitter generates classes using the original TypeSpec model name instead of the @@clientName-specified name.

This is a regression — the GA version of our SDK (generated with an earlier emitter) correctly produced ContentArrayField, ContentStringField, etc. The new emitter produces ArrayField, StringField, etc., ignoring the @@clientName directives.

Repro

TypeSpec models (models.tsp):

@discriminator("type")
model ContentField { ... }

model ArrayField extends ContentField {
  type: ContentFieldType.array;
  valueArray?: ContentField[];
}

model StringField extends ContentField {
  type: ContentFieldType.string;
  valueString?: string;
}
// ... other subtypes of ContentField discriminated union

Client customization (client.tsp) — source: client.tsp L166-L174:

@@clientName(StringField, "ContentStringField", "csharp");
@@clientName(DateField, "ContentDateTimeOffsetField", "csharp");
@@clientName(TimeField, "ContentTimeField", "csharp");
@@clientName(NumberField, "ContentNumberField", "csharp");
@@clientName(IntegerField, "ContentIntegerField", "csharp");
@@clientName(BooleanField, "ContentBooleanField", "csharp");
@@clientName(ArrayField, "ContentArrayField", "csharp");
@@clientName(ObjectField, "ContentObjectField", "csharp");
@@clientName(JsonField, "ContentJsonField", "csharp");

Expected Behavior

The emitter should generate C# classes with the @@clientName-specified names:

• ContentArrayField.cs
• ContentStringField.cs
• ContentDateTimeOffsetField.cs
• etc.

Actual Behavior

The emitter generates classes using the original TypeSpec model names, ignoring @@clientName:

• ArrayField.cs
• StringField.cs
• DateField.cs
• etc.

Impact

This is a breaking change for our published GA SDK (Azure.AI.ContentUnderstanding). Our GA SDK exposes ContentArrayField, ContentStringField, etc. as public types. Regenerating with the new emitter renames these types, breaking:

• All consumer code using these types
• Our customization files (ContentUnderstandingClient.Customizations.cs) which reference these types by their @@clientName names — resulting in 144 compilation errors after regeneration

Merged GA SDK (proof that older emitter correctly applied @@clientName)

• .NET GA SDK PR: Content Understanding GA SDK for DotNet — merged 2026-02-26
  • Generated class (correct name): ContentArrayField.cs
  • Customization referencing these types: ContentUnderstandingClient.Customizations.cs

Context

• TypeSpec @@clientName directives: client.tsp L166-L174 (C#) and client.tsp L207-L215 (Java)
• TypeSpec repo: azure-rest-api-specs-pr (private), PR #27815
• SDK: Azure.AI.ContentUnderstanding
• API version: 2026-07-01-preview (base GA: 2025-11-01)
• The same issue also affects the Java emitter (@azure-tools/typespec-java) — Azure/autorest.java#3337

[yungshinlintw]

Additional Context — TypeSpec source and merged GA SDK

TypeSpec @@clientName directives (public GA spec, merged):

• C# renames: client.tsp L166-L174
• Java renames: client.tsp L207-L215

These @@clientName directives were respected by the previous emitter version and the GA SDK was published with ContentArrayField, ContentStringField, etc. as public types.

Merged GA SDK PR (proof that the older emitter correctly applied @@clientName):

• .NET GA SDK: Content Understanding GA SDK for DotNet — merged 2026-02-26
  • Example generated class: ContentArrayField.cs
  • Customization referencing these types: ContentUnderstandingClient.Customizations.cs

The new emitter (@azure-typespec/http-client-csharp v1.0.0-alpha.20260512.2) generates ArrayField.cs instead of ContentArrayField.cs for the exact same TypeSpec input.

[github-actions]

🎯 Issue Triage

The @@clientName`` decorator on discriminated union subtype models is being ignored by the C# HTTP client emitter, causing it to use original TypeSpec model names instead of the customized client names. This is a regression that breaks the published Azure.AI.ContentUnderstanding GA SDK.

Category: Bug
Area: emitter:client:csharp

📋 Reproduction code

`@discriminator`("type")
model ContentField { }

model ArrayField extends ContentField {
  type: ContentFieldType.array;
  valueArray?: ContentField[];
}

model StringField extends ContentField {
  type: ContentFieldType.string;
  valueString?: string;
}

// client.tsp
@`@clientName`(StringField, "ContentStringField", "csharp");
@`@clientName`(ArrayField, "ContentArrayField", "csharp");
@`@clientName`(DateField, "ContentDateTimeOffsetField", "csharp");

🔍 Analysis

• The @@clientName`` directive works for regular models but appears to be skipped for discriminated union subtypes.
• The emitter resolves the class name from the TypeSpec model name rather than honoring the override when processing discriminated union variants.
• This is a confirmed regression — an earlier emitter version correctly applied @@clientName`` for these types.
• The same bug also affects the Java emitter per the linked autorest.java issue, suggesting the root cause may be in shared TCGC logic that handles discriminated unions.

📝 Suggested next steps

• Investigate whether TCGC (typespec-client-generator-core) strips @@clientName`` for discriminated union subtypes before passing to the emitter.
• Add a regression test covering @@clientName`` on discriminated union subtypes for the C# emitter.
• Verify fix also resolves the Java emitter regression.

Generated by Agentic Triage

[jorgerangel-msft]

@yungshinlintw can you please provide a link to the generated library that shows this behavior? From testing, I was not able to repro this with the latest version of the .net emitter.