Python: feat(a2a): use non-streaming transport and return_immediately for background ops#5963
Python: feat(a2a): use non-streaming transport and return_immediately for background ops#5963giles17 wants to merge 5 commits into
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…kground ops When stream=False, use a client configured with streaming=False so the SDK sends a single HTTP POST to message/send instead of opening an SSE connection via message/stream. This matches the A2A protocol's design: non-streaming calls use direct request/response, streaming calls use Server-Sent Events. Also sets return_immediately=background on SendMessageConfiguration so the server respects the caller's intent for background operations. Changes: - Create separate streaming and non-streaming internal clients (sharing the same httpx connection pool) to match protocol transport semantics - Select non-streaming client for run(stream=False) calls - Add SendMessageConfiguration with return_immediately=background - Fallback to streaming client when non-streaming unavailable (e.g. user provides their own client via constructor) - Add tests for client selection and return_immediately behavior Resolves microsoft#5936 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
Note
Copilot was unable to run its full agentic suite in this review.
This PR updates the Python A2A agent to use the correct transport for non-streaming calls and to signal background execution to the server via return_immediately, aligning behavior with A2A protocol intent and the .NET implementation.
Changes:
- Instantiate both streaming and non-streaming internal A2A clients and select between them per
run(stream=...). - Set
SendMessageConfiguration(return_immediately=background)on outgoingSendMessageRequest. - Add tests covering transport selection, fallback behavior, and
return_immediatelyconfiguration.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.
| File | Description |
|---|---|
| python/packages/a2a/agent_framework_a2a/_agent.py | Adds dual-client construction (streaming + non-streaming) and per-call client selection; attaches return_immediately to send requests. |
| python/packages/a2a/tests/test_a2a_agent.py | Adds tests for client selection/fallback and background return_immediately; captures last request in the mock client. |
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Automated Code Review
Reviewers: 4 | Confidence: 81%
✓ Correctness
The PR correctly implements non-streaming transport selection and return_immediately configuration. The logic in active_client selection, constructor initialization paths, and the continuation_token/subscribe code path are all sound. The two issues flaged in the prior review thread (mock last_request not initialized in init, and using truthiness instead of
is not Nonefor client check) remain unresolved but no new correctness issues were found.
✓ Security Reliability
The changes are well-structured from a security and reliability perspective. Both streaming and non-streaming clients share the same httpx.AsyncClient connection pool, which is properly cleaned up in aexit. The _non_streaming_client attribute is correctly initialized on all successful construction paths (set to None when a user-provided client is used, or set via factory on the normal path). The existing unresolved review comments already cover the relevant defensive-coding concerns (explicit is-not-None check, last_request initialization, and conditional configuration). No new security or reliability issues found beyond those.
✓ Test Coverage
The new tests cover the key behavioral aspects well: client selection logic (streaming vs non-streaming), fallback behavior, and
return_immediatelyconfiguration. However, the constructor path that creates dual clients viaClientFactorywithstreaming=True/Falseconfigs is entirely untested — all tests use theclient=parameter which bypasses that code. The existingtest_a2a_agent_initialization_with_timeout_parameterpatchesClientFactoryat class level, so it doesn't verify the two separate streaming configs are created correctly.
✗ Design Approach
The main design issue is in A2Agent initialization: the new constructor now makes creation of the optional non-streaming client part of the same required negotiation step as the streaming client. That means a server/configuration that can be used successfully via streaming can still fail agent construction entirely if only the non-streaming client cannot be created, even though the updated run path already treats a missing
_non_streaming_clientas a valid fallback.
Automated review by giles17's agents
- Initialize last_request in MockA2AClient.__init__ for explicit state - Use 'is not None' instead of truthiness for _non_streaming_client check - Assert return_immediately propagates through non-streaming client path Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Only attach SendMessageConfiguration to the request when background=True, keeping requests minimal and preserving server-side defaults for normal (foreground) operations. This follows the framework pattern of only setting optional fields when they have meaningful values. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Per the A2A spec, return_immediately only applies to message/send (non-streaming). It has no effect on streaming operations. Only set the configuration field when both background=True and stream=False. Adds test verifying streaming+background does not set return_immediately. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Motivation and Context
Resolves #5936
The Python A2A agent was always using SSE streaming transport (
message/stream) regardless of whether the caller requested streaming. This means even a simplerun('Hello')call opens an SSE connection instead of making a single HTTP POST tomessage/send. Additionally, whenbackground=True, the server had no way to know it should return immediately.Changes
Non-streaming transport selection
httpx.AsyncClientconnection pool) to match A2A protocol transport semanticsstream=False, select the non-streaming client which usesmessage/send(single request/response)stream=True, use the streaming client which usesmessage/stream(SSE)return_immediately configuration
SendMessageConfiguration(return_immediately=background)on outgoingSendMessageRequestbackground=True, the server is informed it should return the Task immediately without blockingWhy two clients?
The Python A2A SDK (
a2a-sdk) bakes the transport choice (streaming vs non-streaming) intoClientConfigat client construction time — there's no per-call control. The .NET SDK exposes separateSendMessageAsyncandSendStreamingMessageAsyncmethods on a single client. To achieve equivalent behavior in Python, we create two lightweight wrapper clients that share the same underlying HTTP connection pool.Tests
stream=Falsestream=Truereturn_immediatelybeing set correctlyComparison with .NET
This matches the .NET implementation in
A2AAgent.cs:RunCoreAsync→SendMessageAsync(non-streaming) withReturnImmediatelyRunCoreStreamingAsync→SendStreamingMessageAsync(streaming)