ClientFactory.create() mutates shared ClientConfig.extensions across calls

[matanweks]

Bug Description

ClientFactory is designed for reuse — create one factory, call create() multiple times for different agents. However, when create() is called with per-client extensions, the merged list is written back to self._config.extensions, permanently mutating the shared config. Extensions silently accumulate across calls.

Reproduction

config = ClientConfig(
    httpx_client=httpx.AsyncClient(),
    extensions=['base-ext'],
)
factory = ClientFactory(config)

factory.create(card, extensions=['ext-a'])
factory.create(card, extensions=['ext-b'])

# BUG: config.extensions is now ['base-ext', 'ext-a', 'ext-b']
# Expected: config.extensions == ['base-ext']

Root Cause

In src/a2a/client/client_factory.py, the create() method:

all_extensions = self._config.extensions.copy()
if extensions:
    all_extensions.extend(extensions)
    self._config.extensions = all_extensions  # <-- mutates shared config

Line 242 writes the merged list back to self._config.extensions.

Expected Behavior

create() should produce a per-call config (using dataclasses.replace) and never mutate the factory's shared ClientConfig.

Related

This is the same class of bug as #744 (mutable field defaults in ServerCallContext).

[matanweks]

Thanks for pointing me to the alpha0 release! I verified the code and can confirm the bug no longer applies — extensions moved from ClientConfig to a per-call ServiceParameters model (via ClientCallContext), and ServiceParametersFactory.create_from() copies before mutating, so there's no shared state to accumulate into.

Closing as resolved in v1.0.0-alpha.0.