feat(graph): Add AgentBase support for A2AAgent compatibility

[mkmeral]

Description

Graph currently only accepts Agent | MultiAgentBase as node executors, preventing A2AAgent and other AgentBase implementations from being used in graph workflows. Since A2AAgent implements the AgentBase protocol (not the concrete Agent class), users cannot build graphs that orchestrate remote A2A agents alongside local agents.

This change updates Graph to accept AgentBase | MultiAgentBase, enabling any protocol-compliant agent implementation to participate in graph-based multi-agent workflows.

Related Issues

Resolves: #907

Documentation PR

N/A - no new public API, just expanded type acceptance

Type of Change

New feature

Public API Changes

GraphBuilder.add_node() and GraphNode.executor now accept AgentBase implementations:

# Before: A2AAgent couldn't be used in graphs
from strands.agent.a2a_agent import A2AAgent
from strands.multiagent.graph import GraphBuilder

a2a_agent = A2AAgent(endpoint="http://remote-agent:9000")
builder = GraphBuilder()
builder.add_node(a2a_agent, "remote")  # TypeError or unsupported

# After: A2AAgent works as a graph node
builder = GraphBuilder()
builder.add_node(a2a_agent, "remote")
builder.add_node(local_agent, "local")
builder.add_edge("remote", "local")
graph = builder.build()
result = await graph.invoke_async("Process this request")

Additionally, Agent now explicitly extends AgentBase for compile-time protocol verification.

Testing

• Unit test: test_graph_with_agentbase_implementation - verifies custom AgentBase implementations work with Graph

• Integration test: test_graph_with_a2a_agent_and_regular_agent - verifies real A2AAgent + Agent in Graph workflow

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/multiagent/graph.py	91.66%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[mkmeral]

/strands review

[mkmeral]

/strands review

[pgrayy]

Nit: we could now remove the MultiAgentBase instance check directly above.