Migrate A2A runtime integration to a2a-sdk v1.x

[jariy17]

Summary

Migrate the A2A runtime integration (src/bedrock_agentcore/runtime/a2a.py) to support a2a-sdk v1.x. The SDK currently only works with a2a-sdk 0.3.x; #438 reported that v1.0+ breaks at import time, and PR (cap fix) pins a2a-sdk[http-server]>=0.3,<1.0 as a stopgap. This issue tracks lifting that cap by porting to the new v1.x server API.

Background

a2a-sdk v1.0.0 (March 2026) redesigned the server API. Verified against a2a-sdk 1.1.0:

What a2a.py uses today (0.3.x)	Status in v1.x
a2a.server.apps (module)	❌ removed — replaced by a2a.server.routes
a2a.server.apps.A2AStarletteApplication	❌ removed — app model replaced (create_jsonrpc_routes, add_a2a_routes_to_fastapi)
a2a.server.apps.CallContextBuilder	❌ removed — replaced by a2a.server.routes.ServerCallContextBuilder / DefaultServerCallContextBuilder
a2a.types.DataPart	❌ moved — now under a2a.compat.v0_3.types
a2a.server.request_handlers.DefaultRequestHandler	✅ still present
a2a.server.tasks.InMemoryTaskStore	✅ still present
a2a.server.context.ServerCallContext	✅ still present
a2a.types.AgentCard / AgentCapabilities / AgentSkill	✅ still present

Scope of work

• Rewrite build_a2a_app() to construct the app via the v1.x a2a.server.routes API instead of A2AStarletteApplication.add_routes_to_app().
• Replace BedrockCallContextBuilder's base (CallContextBuilder) with the v1.x ServerCallContextBuilder interface, preserving the Bedrock header/baggage extraction behavior.
• Audit all a2a.* imports in a2a.py against v1.x and update moved symbols.
• Decide on the support window: either move the floor to >=1.0 (drop 0.3.x), or support both 0.3.x and 1.x behind a version check. Recommendation to be settled in the PR.
• Update the a2a extra pin in pyproject.toml accordingly and refresh uv.lock.
• Update src/bedrock_agentcore/runtime/a2a.py docstrings/README A2A examples.
• Add/adjust tests to cover the v1.x code path.

Acceptance criteria

• pip install "bedrock-agentcore[a2a]" with a2a-sdk v1.x installed starts the server via serve_a2a without ModuleNotFoundError.
• A2A server starts on port 9000 and serves the agent card + /ping.
• Tests pass against the supported a2a-sdk version(s).

Related

• Bug report: Bedrock AgentCore >=1.6.3 + A2A v1.0 is throwing error No module named 'a2a.server.apps' #438
• Stopgap cap PR: (fix(a2a): cap a2a-sdk below 1.0 to restore A2A server startup #510)