fix: return UnsupportedOperationError when subscribing to terminal tasks#784
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kabir wants to merge 2 commits intoa2aproject:mainfrom
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fix: return UnsupportedOperationError when subscribing to terminal tasks#784kabir wants to merge 2 commits intoa2aproject:mainfrom
kabir wants to merge 2 commits intoa2aproject:mainfrom
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Per A2A spec, attempting to subscribe to a task in a terminal state (completed, failed, canceled, rejected) must return UnsupportedOperationError. The issue had two parts: 1. DefaultRequestHandler checked terminal state AFTER queue operations began, causing errors to be delivered via SSE stream instead of immediately. 2. For streaming endpoints (subscribeToTask), immediate errors must be wrapped in SSE format so the client's SSE parser can process them and deliver to the error handler callback. Previously they were sent as plain JSON/HTTP responses which the SSE parser couldn't handle. Changes: - Move terminal state validation BEFORE queue operations in DefaultRequestHandler - Wrap all immediate errors in SSE format for JSON-RPC routing layer - Wrap all immediate errors in SSE format for REST transport - Add integration test to verify fix across all three transports Applied to HTTP+JSON, JSON-RPC, and gRPC transports. Fixes a2aproject#767 Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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This pull request improves error handling for streaming requests by ensuring that attempts to subscribe to tasks in terminal states result in an UnsupportedOperationError, as per the A2A specification. The changes include updates to the routing and handler layers to correctly propagate these errors as SSE events or JSON-RPC error responses, along with a new test case to verify the behavior. A review comment suggests using String.format() for error messages in DefaultRequestHandler.java to improve code consistency.
server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java
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…ultRequestHandler.java Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
ehsavoie
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Apr 8, 2026
| } catch (A2AError e) { | ||
| return ZeroPublisher.fromItems(new SendStreamingMessageResponse(request.getId(), e)); | ||
| // Let routing layer catch TaskNotFoundError and UnsupportedOperationError to wrap in SSE format | ||
| throw e; |
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Maybe this is a bit agressive if there is an internal error or something else
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Per A2A spec, attempting to subscribe to a task in a terminal state (completed, failed, canceled, rejected) must return UnsupportedOperationError.
The issue had two parts:
DefaultRequestHandler checked terminal state AFTER queue operations began, causing errors to be delivered via SSE stream instead of immediately.
For streaming endpoints (subscribeToTask), immediate errors must be wrapped in SSE format so the client's SSE parser can process them and deliver to the error handler callback. Previously they were sent as plain JSON/HTTP responses which the SSE parser couldn't handle.
Changes:
Applied to HTTP+JSON, JSON-RPC, and gRPC transports.
Fixes #767