Foundry-to-Foundry A2A: child agent writes a SendMessage payload as text instead of calling its configured agent

I'm testing a small native hierarchy of prompt agents in Microsoft Foundry: a coordinator calls two managers, and one manager should delegate to another agent through A2A.

The coordinator, using gpt-4.1-mini, successfully invoked both managers. The manager using gpt-5-mini then returned text saying "Now sending", followed by a SendMessage payload for its configured downstream agent. It did not return the requested work.

What the saved configuration and traces show: • The manager has an A2A 1.0 tool targeting the downstream Foundry agent, with an authenticated project connection configured. • Its runtime input includes its saved instructions to delegate through that connection and not treat a written handoff as sent. • Its recorded model output contains only assistant text, with finish_reason "stop". • Its invocation span lists generic metadata: [{"type":"remote_tool","name":"remote_tool"}]. I can't see the expanded downstream SendMessage schema. • Correlated traces show the coordinator's successful calls to both managers, but no downstream tool-execution or agent-invocation span in the returned set. No 401/403, timeout or tool-depth error was recorded. • Attempts to retrieve child response history returned 404 through both project and agent-specific Responses endpoints, although the child invocation/model spans exist.

There was also a separate handoff problem: the coordinator shortened the original request and omitted constraints, then described unfinished work as pending after its root response had completed. I don't consider that evidence of active background work.

My questions:

1. Is this native Foundry-to-Foundry multi-hop A2A pattern supported?
2. How can I inspect the expanded tool schema and tool_choice passed to the child model, rather than generic remote_tool metadata?
3. Is there a read-only way to distinguish discovery or authorization failure before schema exposure from the model choosing text despite receiving a usable tool?
4. How should child response history be retrieved when traces contain response IDs but both Responses read routes return 404?

I'm not claiming that nesting is unsupported or that permissions caused this. I'd like to understand the calling interface from existing evidence before running another paid reproduction or changing the configuration.