to_a2a() serves an agent card with capabilities.streaming=false while the mounted handler implements message/stream

[arunpshankar]

🔴 Required Information

Describe the Bug:

An agent published with to_a2a() serves an agent card whose
capabilities.streaming is false, while the handler it mounts
implements message/stream. Peers that read the card to decide how to
call the agent are told streaming is unavailable when it is served, so
they fall back to unary calls and lose incremental output for no reason.

I am filing this as a question rather than a PR because there is evidence
the false may be deliberate, and I would rather not presume:
_compat.build_agent_card() takes an explicit streaming: bool = False
parameter that AgentCardBuilder never passes, and make_client_config()
separately documents ADK applies its defaults streaming=False/polling=False
for the client side. If the conservative card default is intentional, this
is a documentation request instead — the mismatch is not obvious from
either side alone.

Steps to Reproduce:

1. Build an app with to_a2a(agent) and fetch the served agent card.
2. Read capabilities.streaming — it is false.
3. Confirm the mounted handler supports the streaming method:

$ python -c "
from a2a.server.request_handlers import DefaultRequestHandler as H
print([m for m in dir(H) if 'stream' in m.lower()])"
['on_message_send_stream']

The path in source, at bd239ce:

• a2a/utils/agent_to_a2a.py — AgentCardBuilder(agent=..., rpc_url=...)
  is constructed with no capabilities argument.
• a2a/utils/agent_card_builder.py — self._capabilities = capabilities or AgentCapabilities(),
  and AgentCapabilities() defaults streaming=False.
• a2a/_compat.py — build_agent_card(...) is called with
  capabilities=self._capabilities and without streaming=, so the
  card carries streaming: false either through the passed capabilities
  or through default_capabilities = {"streaming": streaming, ...}.
• a2a/_compat.py — attach_a2a_routes_to_app() mounts
  DefaultRequestHandler, which implements on_message_send_stream.

Expected Behavior:

The card reflects what the served handler supports, so a peer can trust it
when choosing a transport — or, if the conservative default is intended,
that intent is documented next to the streaming parameter so the
mismatch is discoverable without tracing both paths.

Observed Behavior:

capabilities.streaming is false on every card built by the default
to_a2a() path, on a server that implements message/stream.

Environment Details:

• ADK Library Version: read from source at bd239ce (current main);
  not reproduced against a pinned release
• Desktop OS: macOS
• Python Version: 3.13.5
• a2a-sdk: 1.1.2 (used to confirm the handler surface)

Model Information:

• Are you using LiteLLM: N/A — this is transport metadata, model-independent
• Which model is being used: N/A

---

🟡 Optional Information

Regression: Not investigated — I have not checked whether an earlier
version advertised streaming: true.

Possible fix, if the default is simply an oversight:

   card_builder = AgentCardBuilder(
       agent=agent,
       rpc_url=rpc_url,
+      capabilities=AgentCapabilities(streaming=True),
   )

or by threading the existing streaming= parameter of
_compat.build_agent_card() through AgentCardBuilder, which looks like
what that parameter was added for. Happy to send whichever you prefer.

[chelsealong]

Picking this one up now — opening a PR shortly. Flagging it here so nobody duplicates the work; if someone is already on it, say so and I will drop mine.

[surajksharma07]

@arunpshankar Confirmed this on main (v2.6.3) — not intentional, just capabilities never gets passed through in AgentCardBuilder/to_a2a().
Passing capabilities=AgentCapabilities(streaming=True) explicitly when building the card fixed it on our end.

Could you try that on your setup and confirm it works for you too before anyone puts up a PR?
#6673 already has a fix along these lines up for review, so no need for a separate one if that checks out!

[arunpshankar]

Confirmed on my setup too — thanks for picking this up.

google-adk 2.6.3, a2a-sdk 1.1.2, Python 3.13, clean venv:

default (no capabilities passed)      streaming = False
capabilities=AgentCapabilities(True)  streaming = True

So the explicit workaround works here exactly as you described.

I also applied #6673's one-line change to the installed copy and re-ran
the default path, since that is the case users actually hit:

with #6673, default path              streaming = True
handler implements message/stream     True

That looks right to me and matches what the mounted DefaultRequestHandler
supports, so I don't think a separate PR is needed — #6673 covers it.

One small thing while you are in there, in case it is useful: this is now
the second place a streaming default is set. _compat.build_agent_card()
already takes an explicit streaming: bool = False parameter that
AgentCardBuilder never passes, so after #6673 the builder's
AgentCapabilities(streaming=True) wins and that parameter stays dead.
Not a blocker for the fix — just flagging that a later caller passing
streaming= through build_agent_card() would silently disagree with the
builder default. Happy to open a separate issue if you would rather track
it on its own.