fix(client): add extra_params option to RestTransport for provider-specific query params (Fixes #1089)

[isheng-eqi]

Summary

Add an extra_params option to RestTransport and CompatRestTransport that allows injecting query parameters (such as {'alt': 'sse'} for Google A2A API) into every streaming request URL. This resolves issue #1089 where Google's transcoding layer wraps SSE responses inside JSON arrays unless ?alt=sse is appended.

Problem

When connecting to Google A2A API endpoints, the transcoding layer wraps streaming RPC responses inside JSON arrays [{...}] by default. The SDK's RestTransport parses these using json_format.Parse, which expects a single JSON object {...}, causing a crash:

google.protobuf.json_format.ParseError: Failed to parse {'task': {...}} field: unhashable type: 'dict'.

Adding ?alt=sse tells Google to use standard SSE formatting.

Changes

• src/a2a/client/transports/rest.py — Accept optional extra_params dict in __init__, pass to send_http_stream_request via params kwarg
• src/a2a/compat/v0_3/rest_transport.py — Same extra_params support for v0.3 compat transport
• src/a2a/client/client.py — Add extra_params to ClientConfig
• src/a2a/client/client_factory.py — Thread extra_params from config to transport constructors

Usage

# One-liner opt-in for Google A2A API:
config = ClientConfig(extra_params={'alt': 'sse'})
factory = ClientFactory(config)
client = await factory.create_from_url('https://agent.example.com')

# Or via convenience function:
client = await create_client('https://agent.example.com', ClientConfig(extra_params={'alt': 'sse'}))

Test Plan

• 3 new tests for RestTransport (params passed, None safe, empty dict safe)
• 1 new test for CompatRestTransport (params passed)
• Existing test updated (CompatRestTransport now includes params=None in call args)
• ruff check + ruff format + ty check pass
• Compat transport module: 100% coverage
• Rest transport module: 80% coverage (new paths covered)

Closes #1089

[gemini-code-assist]

Code Review

This pull request introduces an extra_params configuration option to ClientConfig to allow appending optional query parameters to request URLs, passing it down to RestTransport and CompatRestTransport for streaming requests. Feedback suggests extending this support to non-streaming requests to align with the documentation, and copying the extra_params dictionary during initialization to prevent external mutations.

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[gemini-code-assist]

The extra_params are currently only appended to streaming requests via _send_stream_request. However, the documentation for ClientConfig.extra_params and RestTransport states that these are 'Optional query parameters to append to every request URL.' Non-streaming requests (which go through _execute_request) do not currently receive these parameters.

To ensure consistency and correctness, _execute_request should also merge self._extra_params with any method-specific params before building the request.

[gemini-code-assist]

It is safer to create a copy of the passed extra_params dictionary to prevent any external mutations to the dictionary from unexpectedly affecting the transport's behavior.

Suggested change

	self._extra_params = extra_params or {}
	self._extra_params = dict(extra_params) if extra_params else {}

References

1. For consistency, prefer using conditional expressions (ternary operators) for simple assignments over if/else blocks.

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/client/client.py	96.88%	97.06%	🟢 +0.18%
src/a2a/client/transports/rest.py	89.89%	89.58%	🔴 -0.30%
src/a2a/compat/v0_3/rest_transport.py	100.00%	97.45%	🔴 -2.55%
Total	92.99%	92.94%	🔴 -0.05%

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