fix: fix REST error handling

[ishymko]

Do one iteration to catch exceptions occurred beforehand to return an error instead of sending headers for SSE. Error handling during the execution is not defined in the spec: a2aproject/A2A#1262.

[gemini-code-assist]

Summary of Changes

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This pull request enhances the robustness of REST API streaming endpoints by addressing a critical error handling flaw. Previously, errors occurring early in a streaming request could lead to a broken client connection rather than a structured error response. The changes ensure that such errors are now properly intercepted and returned as standard JSON error messages, significantly improving the client experience and API reliability for streaming operations.

Highlights

• Improved REST Streaming Error Handling: Modified the _handle_streaming_request function to eagerly fetch the first item from an asynchronous stream. This ensures that any errors occurring during the initial processing (e.g., validation, parsing) are caught before Server-Sent Events (SSE) headers are sent, allowing for a proper JSON error response instead of a broken stream.
• New Integration Test for Streaming Errors: Added a comprehensive integration test, test_client_handles_a2a_errors_streaming, to verify that A2A errors raised by streaming handlers are correctly propagated to the client. This test covers various error types and streaming methods, ensuring robust error handling in streaming scenarios.

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[gemini-code-assist]

Code Review

This pull request effectively addresses a key issue in REST streaming error handling by eagerly fetching the first event. This ensures that exceptions occurring before the stream begins are properly caught and returned as JSON errors instead of breaking the SSE connection. The implementation is clean and the addition of the test_client_handles_a2a_errors_streaming integration test provides excellent coverage for this new behavior. I've added one suggestion to further improve the robustness of the error handling for the first event.

[github-actions]

🧪 Code Coverage (vs 1.0-dev)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/apps/rest/rest_adapter.py	82.28%	83.53%	🟢 +1.25%
Total	91.38%	91.39%	🟢 +0.01%

Generated by coverage-comment.yml