[Bug]: MergePatchUpdate keeps allOf on the unpatched base, so an empty merge patch is rejected

[SnowboardTechie]

Describe the bug

For a model that uses extends, MergePatchUpdate transforms the model's own properties but emits allOf pointing at the unpatched base schema. The inherited properties never go through the transform, so they stay required and non-nullable.

The generated patch schema therefore demands every inherited required property on every request, including on the empty patch. The algorithm in RFC 7396, which this template's doc comment says the transform follows, iterates over the pairs present in the patch document, so {} makes no changes and is always a valid patch. A schema that rejects {} is not describing a merge-patch body.

Versions: @typespec/compiler 1.14.0, @typespec/http 1.14.0, @typespec/openapi3 1.14.0.

Reproduction

import "@typespec/http";

using Http;

@service(#{ title: "Extends repro" })
namespace ExtendsRepro;

model Base {
  id: string;
}

model Widget extends Base {
  label?: string;
}

model WidgetPatch is MergePatchUpdate<Widget>;

@route("/widgets/{id}")
@patch
op update(@path id: string, @body body: WidgetPatch): Widget;

Emitted schemas:

Base:
  type: object
  required: [id]        # <- never transformed
  properties:
    id: { type: string }

WidgetPatch:
  type: object
  properties:
    label:              # <- own property: transformed correctly
      anyOf:
        - type: string
        - type: 'null'
  allOf:
    - $ref: '#/components/schemas/Base'   # <- unpatched base

Validating patch bodies against WidgetPatch:

INVALID  {}                        <- must have required property 'id'
INVALID  {"label":"x"}             <- must have required property 'id'
INVALID  {"label":null}            <- must have required property 'id'
VALID    {"id":"keep","label":"x"}

Expected: inherited properties go through the same transform as the model's own, so id becomes optional and nullable in the patch shape and {} validates.

The same happens when the extending model is used as a nested property rather than as the patch root; the generated *MergePatchUpdateOrCreate schema carries the same allOf to the unpatched base.

[SnowboardTechie]

Related: #11533, the other MergePatchUpdate conformance gap.

[github-actions]

🎯 Issue Triage

When a model uses extends, MergePatchUpdate correctly transforms the model's own properties but emits an allOf reference pointing to the unpatched base schema, leaving inherited required properties unchanged. This causes validators to reject valid merge-patch bodies (including {}), violating RFC 7396 semantics.

Category: Bug
Area: lib:http

🔗 No playground link provided.

📋 Reproduction code

import "`@typespec/http`";

using Http;

`@service`(#{ title: "Extends repro" })
namespace ExtendsRepro;

model Base {
  id: string;
}

model Widget extends Base {
  label?: string;
}

model WidgetPatch is MergePatchUpdate<Widget>;

`@route`("/widgets/{id}")
`@patch`
op update(`@path` id: string, `@body` body: WidgetPatch): Widget;

🔍 Analysis

• Affected component: MergePatchUpdate template in @typespec/http
• Root cause: The transform that makes properties optional and nullable is only applied to a model's own properties. When a model uses extends, the base model is referenced via allOf in the emitted OpenAPI schema, but the referenced base schema is the original unpatched one — its required properties are never transformed.
• Impact: Any MergePatchUpdate applied to an extends-based model produces an incorrect patch schema. An empty patch {} is invalid against the schema, contradicting RFC 7396.
• Also affects: MergePatchUpdateOrCreate when the extending model appears as a nested property.
• Related: [Bug]: MergePatchUpdate omits the null branch on Record values, so per-key deletion fails validation #11533 (another MergePatchUpdate conformance gap, per the issue author)

📝 Suggested next steps

• Investigate the MergePatchUpdate template implementation in @typespec/http to handle inherited properties from extends chains.
• Fix should flatten or recursively transform base model properties rather than emitting a bare allOf reference to the unpatched schema.
• Add test cases covering extends-based models with required base properties.

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