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OpenApi v3.0 and later can describe multiple response "contents" for a single status code differentiated by the response content-type. Each "contents" definition can have its own schema. A common case may be:

responses:
  '200':
    description: OK
    content:
      application/json:
        schema:
          type: object
          properties:
            id:
              type: integer
              format: int32
            title:
              type: string
            content:
              type: string
      text/html:
        schema:
          type: string

Currently this response description can't be created just using Produces because each produces can specify only one response type (schema), so two Produces are needed for the case above, but the second Produces for a given status code overrides the information of the first, even if it is for a distinct content type.

Implementation Plan For Copilot

We need to add support for multiple response types with the same status code in ApiExplorer. This limitation currently exists because our ApiResponseTypeProvider only maintains a list of types based on status code. When two attributes specify the same code, the second entry overrides the first. This happens in ApiResponseTypeProvider.ReadResponseMetadata. We need to update the code to support deduplicating based on status-code and content-type.

The relevant files are:

We should solve this by completing the following steps:

1. Add a compound key record

   private readonly record struct ResponseKey(
       int    StatusCode,
       Type?  DeclaredType,
       string? ContentType);

2. Replace the status-code dictionary in ApiResponseTypeProvider

   - var results = new Dictionary<int, ApiResponseType>();
   + var results = new Dictionary<ResponseKey, ApiResponseType>();

3. Build a key for every discovered response

   var key = new ResponseKey(
       apiResponseType.StatusCode,
       apiResponseType.Type,                         // may be null
       apiResponseType.ContentTypes.FirstOrDefault() // may be null
   );
   results.TryAdd(key, apiResponseType);            // preserves duplicates

   Optional: if true duplicates arise (same code + type + content type) you may merge their ContentTypes collections instead of discarding either.

4. Populate SupportedResponseTypes

   action.SupportedResponseTypes = results.Values.ToList();

5. (Nice-to-have) deterministic ordering

   action.SupportedResponseTypes = results.Values
       .OrderBy(r => r.StatusCode)
       .ThenBy(r => r.Type?.Name)
       .ThenBy(r => r.ContentTypes.FirstOrDefault())
       .ToList();

6. Unit tests

   • Two [ProducesResponseType] attributes with identical status code but distinct content-types types ➜ assert SupportedResponseTypes.Count == 2.
   • Verify ordering and ContentTypes merging logic if implemented.
   • Verify that multiple Produces calls on minimal API with identical status codes and different content-types produces multiple responses.
   • Verify that the Microsoft.AspNetCore.OpenApi generator produces the correct OpenAPI document for each scenario.