Add support for Lambda Response Streaming

[normj]

In draft mode while still cleaning up the experience

Issue #, if available:
#1635

Description of changes:
Integrate Lambda response streaming support into the Amazon.Lambda.RuntimeSupport.

A hello world example of using response streaming. In this case sense I wrapped the Stream returned from CreateStream in a StreamWriter the writes will be buffered in StreamWriter till the buffer is full. I call the flush method every 10 iterations to force sending data back to the client.

using Amazon.Lambda.Core;
using Amazon.Lambda.Core.ResponseStreaming;
using Amazon.Lambda.RuntimeSupport;
using Amazon.Lambda.Serialization.SystemTextJson;

#pragma warning disable CA2252

// The function handler that will be called for each Lambda event
var handler = async (string input, ILambdaContext context) =>
{
    using var responseStream = LambdaResponseStreamFactory.CreateStream();

    using var writer = new StreamWriter(responseStream);
    for (var i = 1; i <= 100; i++)
    {
        var message = $"Hello {input} - {i}";
        await writer.WriteLineAsync(message);

        if (i % 10 == 0)
        {
            await writer.FlushAsync();
            await Task.Delay(1000);
        }
    }
};

// Build the Lambda runtime client passing in the handler to call for each
// event and the JSON serializer to use for translating Lambda JSON documents
// to .NET types.
await LambdaBootstrapBuilder.Create(handler, new DefaultLambdaJsonSerializer())
        .Build()
        .RunAsync();

For a use with API Gateway or Lambda Function URL you need to create the stream with the CreateHttpStream passing the status code and response headers.

using Amazon.Lambda.APIGatewayEvents;
using Amazon.Lambda.Core;
using Amazon.Lambda.Core.ResponseStreaming;
using Amazon.Lambda.RuntimeSupport;
using Amazon.Lambda.Serialization.SystemTextJson;
using System.Net;

#pragma warning disable CA2252

// The function handler that will be called for each Lambda event
var handler = async (APIGatewayProxyRequest request, ILambdaContext context) =>
{
    var prelude = new HttpResponseStreamPrelude
    {
        StatusCode = HttpStatusCode.OK,
        Headers =
        {
            { "Content-Type", "text/plain" }
        }
    };
   
    using var responseStream = LambdaResponseStreamFactory.CreateHttpStream(prelude);

    using var writer = new StreamWriter(responseStream);
    for (var i = 1; i <= 10000000; i++)
    {
        var message = $"Hello - {i} ({responseStream.Length.ToString("N0")}) (Remaining Time: {context.RemainingTime})";
        await writer.WriteLineAsync(message);

        if (i % 100 == 0)
        {
            await writer.FlushAsync();
        }

        if (context.RemainingTime < TimeSpan.FromSeconds(5))
        {
            await writer.WriteLineAsync("Approaching Lambda timeout, stopping the stream.");
            await writer.FlushAsync();
            break;
        }
    }
};

// Build the Lambda runtime client passing in the handler to call for each
// event and the JSON serializer to use for translating Lambda JSON documents
// to .NET types.
await LambdaBootstrapBuilder.Create(handler, new DefaultLambdaJsonSerializer())
        .Build()
        .RunAsync();

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Copilot]

Pull request overview

Integrates Lambda response streaming into Amazon.Lambda.RuntimeSupport and exposes corresponding (preview) streaming APIs in Amazon.Lambda.Core, along with unit + integration test coverage and a deployable streaming handler test app.

Changes:

• Add streaming send path to RuntimeApiClient / LambdaBootstrap, including per-invocation streaming state management.
• Introduce streaming primitives (ResponseStream, StreamingHttpContent, prelude support) and Core-facing preview APIs (LambdaResponseStreamFactory, HttpResponseStreamPrelude).
• Add extensive unit/integration tests and a ResponseStreamingFunctionHandlers test Lambda project.

Reviewed changes

Copilot reviewed 37 out of 38 changed files in this pull request and generated 15 comments.

Show a summary per file

File	Description
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/ResponseStreamingFunctionHandlers/aws-lambda-tools-defaults.json	Adds Lambda tooling defaults for streaming handler test app.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/ResponseStreamingFunctionHandlers/ResponseStreamingFunctionHandlers.csproj	New net10 test Lambda app project for streaming scenarios.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/ResponseStreamingFunctionHandlers/Function.cs	Streaming handler scenarios used by integration tests.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/TestHelpers/TestStreamingRuntimeApiClient.cs	Test runtime client that simulates streaming without real HTTP.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/TestHelpers/NoOpInternalRuntimeApiClient.cs	No-op internal runtime API client for unit tests.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/StreamingHttpContentTests.cs	Unit tests for StreamingHttpContent behavior.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/StreamingE2EWithMoq.cs	End-to-end tests covering bootstrap → streaming pipeline behavior.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/RuntimeApiClientTests.cs	Tests for streaming vs buffered header behavior on requests.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/ResponseStreamTests.cs	Unit tests for new ResponseStream implementation.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/ResponseStreamFactoryTests.cs	Unit tests for streaming factory state + send-task behavior.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/LambdaResponseStreamingCoreTests.cs	Tests for new Amazon.Lambda.Core.ResponseStreaming APIs and prelude serialization.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/LambdaBootstrapTests.cs	Adds bootstrap integration tests for streaming vs buffered paths.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/HandlerTests.cs	Updates test collection for streaming factory isolation.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.IntegrationTests/ResponseStreamingTests.cs	New integration tests invoking Lambda with response stream.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.IntegrationTests/IntegrationTestFixture.cs	Packages additional streaming test app; adjusts temp path/tool install flow.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.IntegrationTests/IntegrationTestCollection.cs	Disables parallelization and wires streaming fixture into integration collection.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.IntegrationTests/Helpers/LambdaToolsHelper.cs	Adjusts temp directory behavior (DEBUG uses repo paths).
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.IntegrationTests/Helpers/CommandLineWrapper.cs	Captures command output while running packaging commands.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.IntegrationTests/CustomRuntimeTests.cs	Removes NET6 target enum entry.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.IntegrationTests/BaseCustomRuntimeTest.cs	Updates runtime/framework assumptions (Dotnet10) + improves missing bundle diagnostics.
Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.IntegrationTests/Amazon.Lambda.RuntimeSupport.IntegrationTests.csproj	Moves integration tests to net10.0 and bumps package refs.
Libraries/src/Amazon.Lambda.RuntimeSupport/Client/RuntimeApiClient.cs	Adds StartStreamingResponseAsync entry point and streaming headers.
Libraries/src/Amazon.Lambda.RuntimeSupport/Client/ResponseStream.cs	Removes old buffered-chunk streaming ResponseStream implementation.
Libraries/src/Amazon.Lambda.RuntimeSupport/Client/IResponseStream.cs	Removes old public IResponseStream surface.
Libraries/src/Amazon.Lambda.RuntimeSupport/Bootstrap/ResponseStreaming/StreamingHttpContent.cs	New HttpContent for streaming serialization + error signaling.
Libraries/src/Amazon.Lambda.RuntimeSupport/Bootstrap/ResponseStreaming/StreamingConstants.cs	Moves streaming constants into streaming namespace and trims unused max size constant.
Libraries/src/Amazon.Lambda.RuntimeSupport/Bootstrap/ResponseStreaming/ResponseStreamLambdaCoreInitializerIsolated.cs	Wires RuntimeSupport streaming into Core via isolated initializer.
Libraries/src/Amazon.Lambda.RuntimeSupport/Bootstrap/ResponseStreaming/ResponseStreamFactory.cs	New per-invocation streaming state + send-task coordination.
Libraries/src/Amazon.Lambda.RuntimeSupport/Bootstrap/ResponseStreaming/ResponseStreamContext.cs	Extends context to carry runtime client, cancellation token, and send task.
Libraries/src/Amazon.Lambda.RuntimeSupport/Bootstrap/ResponseStreaming/ResponseStream.cs	New “true streaming” ResponseStream writing directly to HTTP output stream.
Libraries/src/Amazon.Lambda.RuntimeSupport/Bootstrap/LambdaBootstrap.cs	Initializes streaming integration, routes mid-stream errors to trailers, awaits send task, cleans up state.
Libraries/src/Amazon.Lambda.Core/ResponseStreaming/LambdaResponseStreamFactory.cs	Adds preview public factory API for response streaming streams.
Libraries/src/Amazon.Lambda.Core/ResponseStreaming/LambdaResponseStream.cs	Adds preview Stream subclass wrapper for streaming writes.
Libraries/src/Amazon.Lambda.Core/ResponseStreaming/ILambdaResponseStream.cs	Internal bridge interface for RuntimeSupport to implement.
Libraries/src/Amazon.Lambda.Core/ResponseStreaming/HttpResponseStreamPrelude.cs	Adds HTTP response prelude model + JSON serialization.
Libraries/Libraries.sln	Adds new test app project and updates VS version metadata.
.gitignore	Ignores **/.kiro/.
.autover/changes/c27a62e6-91ca-4a59-9406-394866cdfa62.json	Declares minor version changes for RuntimeSupport + Core (preview streaming).

Comments suppressed due to low confidence (1)

Libraries/src/Amazon.Lambda.RuntimeSupport/Bootstrap/ResponseStreaming/ResponseStreamFactory.cs:27

• ResponseStreamFactory was changed to internal, but the unit tests in Amazon.Lambda.RuntimeSupport.Tests call it directly. There is no InternalsVisibleTo configured for the RuntimeSupport assembly, so this will not compile. Either make the type (and other test-used streaming types) public again, or add an InternalsVisibleTo for the test assembly.

---

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[Copilot]

The request declares a Trailer header, but the current implementation doesn’t appear to send actual HTTP trailing headers (it writes trailer-looking lines into the content stream). Consider using HttpRequestMessage.TrailingHeaders (populated once the handler outcome is known) so the error type/body are emitted as real trailers rather than body bytes.

Suggested change

	// Declare trailers upfront — we always declare them since we don't know
	// at request start time whether an error will occur mid-stream.
	request.Headers.Add("Trailer",
	$"{StreamingConstants.ErrorTypeTrailer}, {StreamingConstants.ErrorBodyTrailer}");

[Copilot]

When an error occurs, SerializeToStreamAsync writes trailer-looking data via WriteErrorTrailersAsync directly into the content stream. With HTTP chunked transfer, these bytes are typically part of the body, not HTTP trailing headers. If Lambda expects real HTTP trailers, this should be implemented via HttpRequestMessage.TrailingHeaders instead of writing to the payload stream.

[Copilot]

WriteErrorTrailersAsync formats the error type/body as UTF-8 header lines and writes them to the content stream. That is unlikely to be interpreted as HTTP trailers by the receiver. Prefer setting true trailing headers on the HttpRequestMessage rather than emitting these lines into the response payload.

Suggested change

	var errorTypeHeader = $"{StreamingConstants.ErrorTypeTrailer}: {exceptionInfo.ErrorType}\r\n";
	var errorTypeBytes = Encoding.UTF8.GetBytes(errorTypeHeader);
	await stream.WriteAsync(errorTypeBytes, 0, errorTypeBytes.Length, _cancellationToken);
	var errorBodyJson = LambdaJsonExceptionWriter.WriteJson(exceptionInfo);
	var errorBodyHeader = $"{StreamingConstants.ErrorBodyTrailer}: {errorBodyJson}\r\n";
	var errorBodyBytes = Encoding.UTF8.GetBytes(errorBodyHeader);
	// Serialize the exception information as JSON in the response payload
	var errorBodyJson = LambdaJsonExceptionWriter.WriteJson(exceptionInfo);
	var errorBodyBytes = Encoding.UTF8.GetBytes(errorBodyJson);