[SergeyMenshykh]

We’re postponing task support in A2A hosting until we can reliably support task resumption, including deployments with multiple hosts. Returning a task is only part of the experience: clients also need to be able to continue it later, even when the follow-up request reaches a different host. That behavior isn’t currently supported, so we’re leaving tasks out of the release rather than offering an incomplete experience.

As we prepare for release, we’re narrowing the built-in hosting support to message-based responses. The hosted agent completes its run and returns a message, without creating a task for the client to manage.

This keeps the release focused on behavior we can support reliably today. We can revisit task support when there’s a reliable way to resume tasks across hosts.

[Copilot]

🟢 Approval recommended

No active review issues are reported for the change narrowing built-in .NET A2A hosting to message-based responses.

No open issues 🎉

What changed in this PR

This PR narrows built-in .NET A2A hosting to message-based responses, postponing task support until reliable resumption across hosts is available.

Changes:

• Removes the registration-level AgentRunMode option and makes default registrations use ReturnMessage.
• Expands tests to verify aggregated messages across registration methods, streaming modes, and ReturnImmediately settings.
• Updates handler documentation and configuration tests to reflect the reduced hosting scope.

File	Description
dotnet/​tests/​Microsoft.Agents.AI.Hosting.A2A.UnitTests/​A2AServerServiceCollectionExtensionsTests.cs	Tests aggregated message responses across eight configuration combinations.
dotnet/​tests/​Microsoft.Agents.AI.Hosting.A2A.UnitTests/​A2AEndpointRouteBuilderExtensionsTests.cs	Updates the custom-options test to configure streaming instead of task mode.
dotnet/​src/​Microsoft.Agents.AI.Hosting.A2A/​A2AServerServiceCollectionExtensions.cs	Constructs the default handler with ReturnMessage.
dotnet/​src/​Microsoft.Agents.AI.Hosting.A2A/​A2AServerRegistrationOptions.cs	Removes the public AgentRunMode registration property.
dotnet/​src/​Microsoft.Agents.AI.Hosting.A2A/​A2AAgentHandler.cs	Clarifies constructor-controlled run modes and default registration behavior in documentation.

_{Review effort: Balanced}

[github-actions]

MAF Automated Review — Iteration 1

Result: Findings reported
Scope: full PR (1 commit(s)): 481223ac53c1
Model: gpt-5.6-sol

Overview

The PR removes registration-level task-mode configuration, hard-codes the built-in server path to aggregate new runs into messages, and covers both registration forms, endpoint modes, and ReturnImmediately values with a focused test matrix. Existing cancellation propagation, session persistence, isolation wrapping, and the custom-handler extension point remain intact. The residual risks are process-wide memory exhaustion from unbounded streaming aggregation and continued acceptance of persisted task continuations by the supposedly message-only built-in path.

Reviewed the supplied pull-request change set across correctness, security/reliability, architecture, and failure behavior.
2 verified findings remained after source verification (1 high, 1 medium) across 1 file. Details are attached to the affected lines below.

Affected areas: dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AServerServiceCollectionExtensions.cs