fix: return UnsupportedOperationError (-32004) when streaming is not supported

[ez-lbz]

What changed

1. Streaming-not-supported now returns UnsupportedOperationError (-32004) instead of InvalidRequestError (-32600)

Problem: When the agent card does not advertise streaming() capability, the JSON-RPC, gRPC, and REST handlers rejected streaming calls with InvalidRequestError (code -32600, "invalid request"), while other SDKs return UnsupportedOperationError (code -32004, "operation not supported"). -32600 is the wrong semantic for a valid-but-unavailable operation and diverges from the other SDKs.

Fix (transport/jsonrpc/src/main/java/org/a2aproject/sdk/transport/jsonrpc/handler/JSONRPCHandler.java):

• onMessageSendStream and onSubscribeToTask now emit new UnsupportedOperationError(null, "Streaming is not supported by the agent", null) (message preserved, code now -32004). Removed the now-unused InvalidRequestError import.

Fix (transport/grpc/src/main/java/org/a2aproject/sdk/transport/grpc/handler/GrpcHandler.java):

• sendStreamingMessage and subscribeToTask now call handleError(responseObserver, new UnsupportedOperationError()); the gRPC status therefore maps from INVALID_ARGUMENT to UNIMPLEMENTED. Updated the method javadoc accordingly.

Fix (transport/rest/src/main/java/org/a2aproject/sdk/transport/rest/handler/RestHandler.java):

• sendStreamingMessage and subscribeToTask now build the error response with new UnsupportedOperationError(null, "Streaming is not supported by the agent", null). HTTP status stays 400; the error reason changes from INVALID_REQUEST to UNSUPPORTED_OPERATION (via A2AErrorCodes.UNSUPPORTED_OPERATION).

Fix (tests updated in lockstep):

• JSONRPCHandlerTest (testStreamingNotSupportedErrorOnSendMessageStream, testStreamingNotSupportedErrorOnSubscribeToTask): assert UnsupportedOperationError instead of InvalidRequestError.
• GrpcHandlerTest (testStreamingNotSupportedError, testStreamingNotSupportedErrorOnSubscribeToTask): assert gRPC Status.Code.UNIMPLEMENTED instead of INVALID_ARGUMENT.
• RestHandlerTest (testSendStreamingMessageNotSupported): assert reason UNSUPPORTED_OPERATION instead of INVALID_REQUEST.

Behavior change: the JSON-RPC error code for streaming-not-supported changes from -32600 to -32004; the gRPC status changes from INVALID_ARGUMENT to UNIMPLEMENTED; the REST error reason changes from INVALID_REQUEST to UNSUPPORTED_OPERATION (HTTP status remains 400). The error message is unchanged.

Note: the compat-0.3 module (legacy 0.3-spec compatibility shim) has its own copies of the handlers with InvalidRequestError for this path; it was intentionally left unchanged as it is outside the scope of the current transports.

Testing

• mvn -pl transport/jsonrpc,transport/grpc,transport/rest test — 124 tests run, 0 failures, 1 skipped (BUILD SUCCESS): JSON-RPC 48 (1 skipped), gRPC 40, REST 36.

[kabir]

Hi @ez-lbz thanks! A few small issues.

Also, there is a pre-existing spec mismatch on gRPC status code. The A2A spec table (section 5.4) maps UnsupportedOperationError to gRPC FAILED_PRECONDITION, but A2AErrorCodes.UNSUPPORTED_OPERATION in this SDK maps it to UNIMPLEMENTED.

The PR's test correctly asserts UNIMPLEMENTED based on the SDK's current mapping, so this is consistent internally — but the SDK itself diverges from the spec on this particular gRPC status. This is not introduced by this PR, just something Claude found.

I've opened #1051 to address this latter part.

So if you just add the strings to the error constructor calls we should be good :-)

[kabir]

Maybe here pass in "Streaming is not supported by the agent" like for the other transports?

[kabir]

Maybe here pass in "Streaming is not supported by the agent" like for the other transports?

[ez-lbz]

Thanks! Both gRPC error constructors now pass the same "Streaming is not supported by the agent" message as the JSON-RPC and REST transports, and the gRPC transport tests pass (40 tests, 0 failures). Appreciate the heads-up on the pre-existing gRPC status mapping divergence (now tracked in #1051) — agreed it's out of scope here.