[v0ropaev]

Description

The v0.3 REST client in the compat package did not recognize the error bodies that this SDK's own v0.3 REST routes send. CompatRestTransport._handle_http_error only mapped a body shaped like {"type": "TaskNotFoundError", "message": ...}. The v0.3 routes mounted by create_rest_routes(..., enable_v0_3_compat=True) go through the same rest_error_handler as v1.0 and return a google.rpc.Status with an ErrorInfo reason, for example

404 {"error":{"code":404,"status":"NOT_FOUND","message":"Task not found","details":[{"@type":"type.googleapis.com/google.rpc.ErrorInfo","reason":"TASK_NOT_FOUND","domain":"a2a-protocol.org","metadata":{}}]}}

So against one server with both v0.3 bindings enabled, the two clients disagreed.

CompatJsonRpcTransport   get_task(missing)        -> TaskNotFoundError: Task not found
CompatJsonRpcTransport   cancel_task(missing)     -> TaskNotFoundError: Task not found
CompatJsonRpcTransport   cancel_task(completed)   -> TaskNotCancelableError: Task cannot be canceled
CompatRestTransport      get_task(missing)        -> MethodNotFoundError: Resource not found: http://testserver/v1/tasks/nope
CompatRestTransport      cancel_task(missing)     -> MethodNotFoundError: Resource not found: http://testserver/v1/tasks/nope:cancel
CompatRestTransport      cancel_task(completed)   -> A2AClientError: HTTP Error 400: Client error '400 Bad Request' ...

After the change all three REST rows raise TaskNotFoundError, TaskNotFoundError and TaskNotCancelableError, matching JSON-RPC.

The fix passes the parsed body to _parse_rest_error from a2a.client.transports.rest, which is what the v1.0 RestTransport already uses, and raises the mapped error when there is one. That covers every reason in A2A_REASON_TO_ERROR, not only the two task errors above. Against the same app, send_message with an unsupported content type now raises ContentTypeNotSupportedError and create_task_push_notification_config raises PushNotificationNotSupportedError when push is not supported, where both used to be a bare A2AClientError: HTTP Error 400.

The existing type lookup and the 404 and generic status fallback stay in place after it. I could not find any server in this repo or in a2a-sdk 0.3.26 that sends the type shape, but it is cheap to keep and an existing test covers it. A real 0.3.x Python server answers with {"message": ...} and no reason, so against that server a missing task still comes back as MethodNotFoundError. That is unchanged here and would need its own fix.

Tests added in tests/compat/v0_3/test_rest_transport.py. One is a parametrized unit test with the exact bodies the server produces for TASK_NOT_FOUND (404) and TASK_NOT_CANCELABLE (400). The other runs CompatRestTransport against a Starlette app built with create_rest_routes(enable_v0_3_compat=True) over httpx.ASGITransport, parametrized over get on a missing task, cancel on a missing task and cancel on a completed one.

With the source change reverted, uv run pytest tests/compat/v0_3/test_rest_transport.py gives 5 failed, 32 passed (the new cases get MethodNotFoundError and A2AClientError: HTTP Error 400). With it, 37 passed. Full uv run pytest is 2496 passed, 203 skipped, 4 xfailed, 1 xpassed. ./scripts/lint.sh passes, and --cov=src puts compat/v0_3/rest_transport.py at 96% with the new lines covered.

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[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/compat/v0_3/rest_transport.py	95.63%	95.76%	🟢 +0.13%
Total	93.40%	93.40%	⚪️ 0.00%

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[1AbdullahButt]

Looks correct, but it needs a rebase. git merge-tree against main conflicts in tests/compat/v0_3/test_rest_transport.py, which six compat fixes have changed since the branch point (#1420 through #1458). The source file merges cleanly.

What I checked:

• _parse_rest_error returns None for every shape the old code handled. It returns None unless the body has an error dict with a details list holding an ErrorInfo whose reason is in A2A_REASON_TO_ERROR (client/transports/rest.py:62-91). A {"type": ..., "message": ...} body, a bare {"message": ...} from a 0.3.x server, or {} from a non-JSON body all fall through to the existing type lookup and the 404/status fallback. So the existing tests for those shapes still apply.
• The raised error isn't swallowed by the outer except. The new raise mapped from e sits inside the try that catches (json.JSONDecodeError, ValueError) (rest_transport.py:385 on the branch). Every class in A2A_REASON_TO_ERROR derives from A2AError(Exception) (utils/errors.py:18), not ValueError, so the mapped error gets out. The existing type branch relies on the same thing.
• The v0.3 server sends google.rpc.Status. compat/v0_3/rest_adapter.py:71/81 uses the shared rest_error_handler / rest_stream_error_handler, and those build their body with _create_error_response. That's the body the new unit test copies.
• Streaming is covered too. _send_stream_request passes self._handle_http_error to send_http_stream_request. The v0.3 adapter fetches the first item before it opens the SSE response (rest_adapter.py:98-109), so pre-stream errors come back as HTTP errors and take this path. The adapter never sends an event: error frame, so the compat client doesn't need the sse_error_handler that the v1.0 RestTransport passes.
• It matches the v1.0 client: RestTransport._handle_http_error (client/transports/rest.py:373-387) calls _parse_rest_error with the same arguments.

I didn't run the tests.

Reviewed with help from Claude Code.