Q: OpenAPI for streamed JSON request body & asking about IAsyncEnumerable<T> input support

[ryanelian]

Hi ASP.NET Core team,

Scenario:

I need to accept very large JSON arrays. To avoid buffering the whole payload I read HttpRequest.Body directly and stream‑deserialize with JsonSerializer.DeserializeAsyncEnumerable<Data>().

That (hopefully) keeps memory usage low, but because the action has no [FromBody] parameter, the OpenAPI shows an empty request body.

    [HttpPost]
    [EndpointSummary("...")]
    [EndpointDescription("...")]
    [ProducesResponseType(StatusCodes.Status200OK, Type = typeof(int))]
    [ProducesResponseType(StatusCodes.Status401Unauthorized, Type = typeof(string))]
    [ProducesResponseType(StatusCodes.Status400BadRequest, Type = typeof(string))]
    public async Task<ActionResult<int>> Post(CancellationToken cancellationToken)
    {
        var processedCount = 0;
        var jsonOptions = new JsonSerializerOptions
        {
            PropertyNameCaseInsensitive = true,
        };
        try
        {
            // The array can be a VERY LARGE array.
            await foreach (var data in JsonSerializer.DeserializeAsyncEnumerable<Data>(Request.Body, jsonOptions, cancellationToken))
            {
                if (data == null)
                {
                    continue;
                }

               // send data to SQS / background worker here
                processedCount++;
            }
            return processedCount;
        }
        catch
        {
            return BadRequest("Invalid request body");
        }
    }

Questions

1. How do I decorate the action so that OpenAPI documentation shows “array of Data in the request body” even though I’m reading Request.Body myself? (Is there an side-effect free attribute that you recommend? As far as my understanding if I use [FromBody] it will buffer and deserialize the content of the JSON into memory)

2. Is manual streaming still the right approach, or can MVC now bind directly to IAsyncEnumerable?

public async Task<ActionResult<int>> Post([FromBody] IAsyncEnumerable<Data> items, CancellationToken ct)

If this is (or will soon be) supported?

3. Any gotchas when using the manual Request.Body approach?

Related issues:

• Make SystemTextJsonInputFormatter IAsyncEnumerable aware #43489
• Support binding request body as Stream in Controller Actions #41540
• https://learn.microsoft.com/en-us/dotnet/core/compatibility/aspnet-core/6.0/iasyncenumerable-not-buffered-by-mvc (but for output, not input)

Any guidance or pointers to docs would be greatly appreciated.

Thanks for your time and for all the great work on ASP.NET Core!