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[Bug]: Response Stream Array-Wrapping Crash (Transport Layer Mismatch)

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#1,089 6 comments 0 reactions 1 assignee View on GitHub

@ishymko is already working on this.

Since Jun 30, 2026.

Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Quiet
Domain
api, backend

Research direction

Look at the transport layer mismatch in a2a/compat/v0_3/rest_transport.py (CompatRestTransport) and a2a/client/transports/rest.py. The issue is that the compat transport does not append ?alt=sse to stream endpoint URLs, causing Google to wrap responses in a JSON array. The standard transport's Parse expects a single JSON object. Start by examining how stream URLs are constructed and where the alt=sse parameter should be added. Check the parsing logic for SSE events. 'Done' means streaming responses work without the client-side workaround.

Written by the indexing model from the issue text.

Description

component: client
What happened?

This is continuation from #1074 as splitting into two issues are suggested by @ishymko

Copied the issue description below for context

Response Stream Array-Wrapping Crash (Transport Layer Mismatch)

By default, Google wraps streaming RPC responses (Flow<StreamResponse>) inside a JSON array [ {"task": ...} ] to frame stream chunk boundaries on the wire.

  • The Mismatch: The SDK's legacy compatibility transport (CompatRestTransport inside a2a/compat/v0_3/rest_transport.py) fails to request standard SSE transcoding from Google because it does not append the standard ?alt=sse query parameter to the stream endpoint URL. This forces Google to fall back to default plain JSON array-wrapped streams.
  • The Crash: Standard transports in a2a/client/transports/rest.py parse stream events using standard Parse (from google.protobuf.json_format): 
    event: StreamResponse = Parse(sse_data, StreamResponse())
    Because standard Parse strictly expects a single JSON object {...}, the leading and trailing array brackets ([ and ]) cause json_format.Parse to crash immediately on the very first chunk: 
    google.protobuf.json_format.ParseError: Failed to parse {'task': {...}} field: unhashable type: 'dict'.

Thanks for pointing this issue is not specific to v0.3 but v1 as well. Regarding the workaround mentioned in https://github.com/a2aproject/a2a-python/issues/1074#issuecomment-4555290758 , yes it will work with the workaround but this mean all the customer/developer who integrated with Google A2A APIs will require this workaround introduces developer friction. Is there any way we could make this smoother so developer can use SDK with Google A2A API out of the box? Thanks!

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Python
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