feat: add A2A Card to Agent builder

[Lancetnik]

Why are these changes needed?

Introduces a from_card classmethod to A2aRemoteAgent in autogen/a2a/client.py, allowing users to create remote agent instances directly from an existing AgentCard (e.g., from discovery services or external sources). This bypasses redundant card fetching. The agent's name is derived from the card's name field.

• Improves efficiency in scenarios like agent discovery or when cards are pre-provided.
• Enhances interoperability with A2A ecosystems by leveraging validated card data.
• Maintains full compatibility with existing ConversableAgent workflows.

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/latest/docs/contributor-guide/documentation/ to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[joggrbot]

📝 Documentation Analysis

All docs are up to date! 🎉

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✅ Latest commit analyzed: 40bead7 | Powered by Joggr

[claude]

Pull Request Review: Add A2A Card to Agent Builder

Overview

This PR adds a convenient from_card() class method to create A2aRemoteAgent instances from an AgentCard object, and changes the agent card attribute visibility from private (__agent_card) to protected (_agent_card).

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✅ Strengths

1. Good API Design: The from_card() factory method provides a clean and intuitive way to construct agents from cards
2. Consistent Pattern: Uses @classmethod pattern commonly found in Python for alternative constructors
3. Test Coverage: Includes a unit test that validates the basic functionality
4. Type Safety: Uses Self return type (from typing_extensions), which is the correct approach for factory methods
5. Code Quality: The implementation is clean and straightforward

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🔍 Potential Issues & Suggestions

1. Missing PR Description (Critical)

The PR template asks for: Why are these changes needed? and Related issue number. These sections are empty. Please add the motivation/use case for this feature and any related issue numbers.

Location: PR description

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2. Incomplete Test Coverage (Important)

The current test only validates basic construction. The main benefit of from_card() is avoiding the card fetch in a_generate_remote_reply:122. Consider adding an async test that verifies _get_agent_card is NOT called when using an agent created with from_card().

Location: test/a2a/test_client.py:115-131

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3. Attribute Visibility Change Lacks Justification (Minor)

The change from __agent_card (private) to _agent_card (protected) affects the public API. Name mangling is removed. Is this intentional for subclassing support? Should this be documented? Consider either keeping it private with a public property or documenting why it's protected.

Location: autogen/a2a/client.py:70

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4. Missing Docstring (Minor)

The from_card() method lacks a docstring explaining its purpose, parameters, and return value. Please add documentation following the pattern used in the __init__ method.

Location: autogen/a2a/client.py:81-102

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🔒 Security & Performance

• ✅ No security concerns identified
• ✅ No performance regressions (actually improves performance by allowing card reuse)
• ✅ No breaking changes to existing API

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🎯 Recommendation

Approve with minor revisions. This is a useful addition, but please address:

1. Must fix: Add PR description (motivation & related issues)
2. Should fix: Add test for card fetch optimization
3. Consider: Document the _agent_card visibility change
4. Nice to have: Add docstring to from_card()

Great work on this feature!

[claude]

Code Review for PR #2165: Add A2A Card to Agent builder

Overview

This PR adds a new from_card() classmethod to A2aRemoteAgent that allows instantiating agents directly from an existing AgentCard, avoiding redundant card fetching. The implementation is clean and well-documented.

Strengths

1. Good Design Pattern: The classmethod factory pattern is appropriate and follows Python conventions
2. Clear Documentation: Both docstring and user guide documentation are comprehensive
3. Backward Compatibility: The changes maintain backward compatibility
4. Refactoring: Moving from __agent_card to _agent_card improves consistency

Issues & Suggestions

1. Potential Bug: Card Resolver with UNKNOWN URL (BLOCKING)

Location: autogen/a2a/client.py:62-65, :117

When using from_card(), the URL is set to UNKNOWN, but the card resolver is still initialized with this invalid URL. If _get_agent_card() is called later, it will try to fetch from an invalid UNKNOWN base URL, causing errors.

Recommendation: Use card.url when creating instance in from_card(), or defer card resolver initialization, or add validation to ensure _get_agent_card() is never called when created via from_card().

2. Test Coverage Gaps

Location: test/a2a/test_client.py:115-131

The test only validates basic initialization but does not test:

• That the agent works without fetching the card again
• That _get_agent_card() is never called when card is already set
• Error handling scenarios
• Integration with actual message sending

3. Documentation Clarity

Location: autogen/a2a/client.py:102

The docstring mentions registryURL but there is no registryURL attribute—only url.

4. Error Handling Edge Case

Location: autogen/a2a/client.py:145-146

If _agent_card is None when created via from_card(), the code will call _get_agent_card() with the invalid UNKNOWN URL, causing cryptic errors. Add defensive validation.

Security & Performance

• No significant security concerns
• Positive performance impact: avoids redundant HTTP requests

Test Coverage Summary

• Basic initialization test added ✓
• No integration test with actual message flow ✗
• No test verifying card fetching is skipped ✗

Recommendations Summary

Must Fix (Blocking): Fix the card resolver URL issue when using from_card() with UNKNOWN URL

Should Fix: Add integration tests, add error handling for edge case where _agent_card is None

Nice to Have: Update docstring clarity, document typing_extensions requirement

Overall Assessment

Well-structured PR with useful functionality. Main concern is the potential bug with UNKNOWN URL being passed to card resolver. Once addressed with better test coverage, this will be ready to merge.

[marklysze]

Thanks @Lancetnik!

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/a2a/client.py	62.50%	9 Missing ⚠️

Files with missing lines	Coverage Δ
autogen/a2a/client.py	60.52% <62.50%> (+0.25%)	⬆️

... and 20 files with indirect coverage changes

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