fix(server): warn when an AgentCard is missing required fields

[Patrick-SCH03]

Description

AgentCard marks eight fields REQUIRED, but nothing in the SDK checks them, so an empty AgentCard() is served as {} with HTTP 200. Following the "suggested first step" in #1267, this logs a warning instead of raising, so existing non-compliant cards keep working while the problem becomes visible.

Changes

• a2a.utils.proto_utils.warn_on_missing_required_fields(msg, source) -> bool: runs the existing required-field check (_validate_proto_required_fields_internal) and logs one WARNING listing the fields that are missing or empty, including nested paths such as skills[0].tags. It never raises and returns whether the message was complete. Objects that are not Protobuf messages are skipped: an existing test in test_default_request_handler.py passes a non-card object as agent_card, and user code does the same with test doubles, so a warning helper must not turn that into an AttributeError.
• Called where a card enters or leaves the SDK:
  • create_agent_card_routes(agent_card=...)
  • DefaultRequestHandler.__init__ and DefaultRequestHandlerV2.__init__ for agent_card and, when given, extended_agent_card
  • agent_card_to_dict, which serializes every served card (including cards from card_modifier / extended_card_modifier), so this one logs per request; it can be silenced through the a2a.utils.proto_utils logger
  • signing._canonicalize_agent_card, used when signing and verifying a card

For an empty card the log reads:

agent_card passed to create_agent_card_routes: AgentCard is not spec-compliant - REQUIRED fields missing or empty: name, description, supported_interfaces, version, capabilities, default_input_modes, default_output_modes, skills. This is allowed and does not raise, but it may not verify or interoperate across SDKs and could be rejected in a future major release. See https://a2a-protocol.org/latest/specification/#57-field-presence-and-optionality

Enforcement (raising) is left for a major version, as the issue suggests.

Tests (uv run pytest)

• tests/utils/test_proto_utils.py: complete message returns True with no log; empty message returns False and logs the exact message instead of raising; an empty repeated field is reported; nested paths are reported.
• tests/server/routes/test_agent_card_routes.py: an incomplete card logs a warning and is still served with 200; a complete card logs nothing.
• tests/server/request_handlers/test_default_request_handler.py and ..._v2.py: incomplete agent_card and extended_agent_card each log one warning; complete cards log nothing.
• tests/server/request_handlers/test_response_helpers.py: agent_card_to_dict warns for an incomplete card and stays silent for a complete one.
• tests/utils/test_signing.py: _canonicalize_agent_card warns for an incomplete card.
• The warning-asserting tests fail on main and pass with this change.
• bash scripts/lint.sh (ruff check, ruff format, ty) passes.
• uv run pytest --ignore=tests/integration: 1829 passed, 169 skipped, 3 xfailed.
• On Windows, some tests/integration tests that spawn server subprocesses fail identically on main, so I relied on CI for those.

---

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• Appropriate docs were updated (if necessary)

Fixes #1267 🦕

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/cluster/database_event_stream.py	92.86%	96.94%	🟢 +4.08%
src/a2a/server/request_handlers/default_request_handler.py	97.95%	97.96%	🟢 +0.01%
src/a2a/server/request_handlers/default_request_handler_v2.py	87.50%	87.46%	🔴 -0.04%
src/a2a/utils/proto_utils.py	91.20%	95.65%	🟢 +4.45%
Total	93.00%	93.14%	🟢 +0.14%

Generated by coverage-comment.yml

[Iwaniukooo11]

Thanks @Patrick-SCH03

You deliberately skipped agent_card_to_dict, but it should warn there too (the per-request noise is fine and silenceable via the a2a.utils.proto_utils logger).

Same for signing - please add a warn_on_missing_required_fields(...) in signing._canonicalize_agent_card, since the parent issue's scope has changed to cover it.

I left request changes - Please review them. Then we are free to merge:)

Once again, thanks for you contribution

[Patrick-SCH03]

Thanks for the review, @Iwaniukooo11! Addressed in 8fe8a93:

• The warning now says the card is not spec-compliant, lists the REQUIRED fields that are missing or empty, explains that this is allowed and does not raise but may not verify or interoperate across SDKs and could be enforced in a future major release, and links to spec section 5.7.
• agent_card_to_dict and signing._canonicalize_agent_card now warn as well, with tests for both.

I also rebased onto the latest main to resolve the conflict. The PR description is updated to match.