build_agent_card(): agent_registry cards stay streaming:false after #6673, and the streaming= parameter becomes unreachable

[arunpshankar]

Splitting this out of #6672 as suggested there, so it does not block #6673.

Two things, one root cause. In _compat.build_agent_card():

default_capabilities = {"streaming": streaming, "push_notifications": False}
...
"capabilities": _as_dict(capabilities) or default_capabilities,

The streaming= parameter only reaches the card through default_capabilities, and that is consulted only when capabilities is falsy. The two call sites take different branches, and #6673 changes which branch each one takes.

1. Cards built through agent_registry stay streaming: false

src/google/adk/integrations/agent_registry/agent_registry.py:640 passes neither capabilities nor streaming:

agent_card = _compat.build_agent_card(
    name=name,
    description=description,
    version=version,
    url=url,
    protocol_binding=getattr(binding, "value", binding),
    protocol_version=protocol_version,
    skills=skills,
    default_input_modes=["text"],
    default_output_modes=["text"],
)

So it falls to default_capabilities, where streaming defaults to False. #6673 changes AgentCardBuilder's default, which this call site does not go through, so a card built here advertises streaming: false after that fix as well as before it.

2. streaming= becomes unreachable

src/google/adk/a2a/utils/agent_card_builder.py:85 passes capabilities=self._capabilities (line 94) and never passes streaming=. #6673 changes that field to:

self._capabilities = capabilities or AgentCapabilities(streaming=True)

which is always truthy, so _as_dict(capabilities) wins and default_capabilities is never evaluated from this caller. Together with (1), no call site in the repository can reach the streaming= parameter, and a caller who sets it gets no effect and no warning.

Reproduced

google-adk 2.6.1, a2a-sdk 1.1.2, Python 3.13.5, clean venv. build_agent_card is byte-identical between 2.6.1 and current main, so this applies to main.

from google.adk.a2a import _compat
from a2a.types import AgentCapabilities

binding = getattr(_compat.TP_HTTP_JSON, "value", _compat.TP_HTTP_JSON)
common = dict(name="x", description="d", version="1", url="http://h/a",
              skills=[], default_input_modes=["text"],
              default_output_modes=["text"])

# the agent_registry call shape
card = _compat.build_agent_card(protocol_binding=binding, **common)
print(card.capabilities.streaming)                    # False

# streaming= is honoured, but only when capabilities is absent
card = _compat.build_agent_card(protocol_binding=binding, streaming=True, **common)
print(card.capabilities.streaming)                    # True

# once capabilities is passed, which is what #6673 makes the default, it is ignored
card = _compat.build_agent_card(protocol_binding=binding,
                                capabilities=AgentCapabilities(streaming=True),
                                streaming=False, **common)
print(card.capabilities.streaming)                    # True, streaming= had no effect

Possible shapes

1. Thread capabilities through the agent_registry call site the way AgentCardBuilder does, and drop streaming= from build_agent_card once nothing reaches it.
2. Keep streaming= and make it compose rather than fall back, so it applies on top of a passed capabilities. This changes the meaning of an existing parameter for anyone already passing both.
3. Keep streaming= and document it as the no-capabilities convenience path only.

I have not opened a pull request because (2) changes the behaviour of an existing parameter and (1) removes public surface, so the choice looks like yours rather than mine. Happy to send whichever you prefer.

[itimantasha]

I'd like to work on this issue. I will submit a PR shortly to fix the capability composition fallback logic in build_agent_card.

[lllakshit]

Opened a fix in #6781 — please review.

This takes the non-breaking path from the issue:

1. Keep streaming= as the no-capabilities convenience path. It is not dropped, and it is not composed onto a passed capabilities object (that would change the meaning of an existing parameter).
2. Thread streaming=True through the agent_registry call site so constructed cards no longer advertise streaming: false.

AgentCardBuilder's default is still tracked separately on #6672 (#6673 is closed and has not landed on GitHub main).

Verified on current main vs this PR with a2a-sdk 1.1.2: the registry-shaped build_agent_card(...) call is False on main and True after the call-site change.

@itimantasha I saw your note. There was no open PR yet, so I sent this conservative call-site fix rather than changing composition. Happy to defer to maintainers if they want a different shape.

[arunpshankar]

Worth noting for review: capabilities.streaming appears in none of the 676 source files in this repository, only in tests. RemoteA2aAgent never reads it, and the card built on the agent_registry fallback path is held internally to resolve the RPC URL rather than served to a peer, so the value is inert in both directions today.

That makes this a descriptor-accuracy change rather than a behavioural one. The docstring half stands on its own, and the call-site change gives streaming= a live caller again, so it is no longer unreachable.

One thing I cannot see from here: that scope is this repository only. If a registry-built card is ever handed to a client outside ADK, the value stops being inert, and the fallback path has no capability information to draw on. Is there a consumer you know of that reads it?