error decorator should be added to error response schema when importing from OpenAPI

[baywet]

Importing the following OpenAPI description

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths:
  /:
    get:
      operationId: myOp
      parameters: []
      responses:
        '200':
          description: The request has succeeded.
        4XX:
          description: Client error
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/MyError'
components:
  schemas:
    MyError:
      type: object

Results in the following TypeSpec definition

import "@typespec/http";

using TypeSpec.Http;

model MyError {

}

model MyResponse {

}

op myOp (): MyResponse | {
  @maxValue(499)
  @minValue(400)
  @statusCode
  statusCode: int32;

  @body
  error: MyError;
};

Note the absence of @error decorator on the MyError model.

This is also a problem without ranges

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths:
  /:
    get:
      operationId: myOp
      parameters: []
      responses:
        '200':
          description: The request has succeeded.
        '401':
          description: Access is unauthorized.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/MyError'
components:
  schemas:
    MyError:
      type: object

The import tool should add the error decorator when a model is used as the response body for an error response (specific code between 400 and 599 included, 400 to 499 or 500 to 599 ranges, etc...)

🎯 Issue Triage

When importing an OpenAPI description with error responses (4XX range or specific 4xx/5xx codes), the OpenAPI-to-TypeSpec converter omits the @error decorator on the referenced schema model.

Category: Bug
Area: openapi3:converter

⚠️ No reproduction provided. A minimal repro would help diagnose this issue.

🔍 Analysis

• The converter correctly handles the response status code mapping (using @statusCode, @minValue, @maxValue) but does not recognize that a model referenced in an error response body should be decorated with @error.
• According to TypeSpec semantics, models used as error response bodies should carry @error so tooling (e.g., client emitters) treats them as error types.
• The fix should detect when a schema is used exclusively (or primarily) as the body of responses with status codes in the 400–599 range — including wildcard ranges like 4XX/5XX — and apply @error to the generated model.
• Affected component: packages/openapi3 converter logic for response handling.

📝 Suggested next steps

• In the converter, when processing response entries, track which schemas are referenced only as error response bodies (4xx/5xx).
• Apply @error decorator to those generated models.
• Cover both exact status codes (401, 403, 500, ...) and wildcard ranges (4XX, 5XX).
• Add unit tests for both the range and specific-code cases provided in the issue.

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