Streaming

Streaming is available through all production client transports:

• gRPC native server streaming;
• HTTP+JSON SSE;
• JSON-RPC response envelopes carried in SSE data: fields.

Servers should advertise capabilities.streaming: true in their Agent Card.

Client APIs

• A2AClient::SendStreamingMessage(request, observer, options)
• A2AClient::SubscribeTask(request, observer, options)

Implement a2a::client::StreamObserver:

• OnEvent(const StreamResponse&) for each decoded event.
• OnError(const core::Error&) once for transport or protocol failure.
• OnCompleted() once after a clean remote close.

Both calls return a StreamHandle. Call Cancel() to request cancellation and IsActive() to check handle state. Destruction also cancels the request and joins its worker.

Threading contract

Observer callbacks run on transport-managed background threads. Keep observers alive until stream completion, cancellation, or handle destruction. Callback code should be thread-safe, fast, and non-blocking.

Build the streaming client example

The SDK requires gRPC and Protobuf at configure time. The default HTTP streaming transports also require libcurl.

Linux or macOS:

./scripts/run_examples.sh build-example streaming_client

Windows Git Bash:

./scripts/install_build_deps.sh
rm -rf build-example-streaming_client
./scripts/run_examples.sh build-example streaming_client
./build-example-streaming_client/RelWithDebInfo/a2a_example.exe --help

See Installation and Build and vcpkg for dependency and toolchain details.

Server side

Executors return std::unique_ptr<ServerStreamSession> from SendStreamingMessage and, optionally, SubscribeTask. A stream session publishes StreamResponse values until it returns an empty optional or an error.

Examples

• examples/apps/streaming_client/main.cpp
• examples/apps/streaming_server/main.cpp