[Marco Minerva (marcominerva)]

Motivation & Context

The OpenAI Responses endpoints are bound to a single agent at mapping time, so applications that need to choose the agent per request (for example from a route value, header or claim) have to map one set of endpoints per agent.

This PR is the OpenAI Responses part of #9382, which was split per hosting protocol to make review easier. It is independent from the other PRs of the split.

Description & Review Guide

• What are the major changes?
  • New MapOpenAIResponses(responsesPath, Func<HttpContext, AIAgent?> agentSelector, mapOptions) overload. The selector applies to every operation (create, get, cancel, delete, list input items); a null selection returns 404 before any response, session or conversation state is loaded.
  • Stored responses are bound to the selected agent's Name (InMemoryResponsesService.ForAgent), so a response can only be retrieved, canceled or deleted through requests that select the same agent. The agent name also keys the session store and partitions persisted sessions across new agent instances.
  • The internal IResponsesService/ResponsesHttpHandler accept the executor selected for the request; after execution the stored response releases the executor so the agent graph is not kept alive by stored responses.
  • Shared source helper src/Shared/HostedAgents/HostedAgentResolution.cs (identical in each PR of this split, so they merge cleanly in any order).
• What is the impact of these changes? Additive public API only; the changed service types are internal. Existing single-agent mappings behave as before.

Related Issue

Part of #7733 (split from #9382).

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change.

[Copilot]

🟡 Changes recommended

Selected agents and session stores use different scopes, and disconnected streams can continue using disposed request-scoped dependencies.

2 open issues

• Separate scopes break shared request-scoped agent state · New
• Streaming execution ignores disconnect cancellation for scoped agents · New

What changed in this PR

Adds request-time agent selection to .NET OpenAI Responses hosting as part of #7733.

Changes:

• Adds a selector overload covering all response operations.
• Binds stored responses and session identity to the selected agent’s name.
• Releases completed executors and adds selection and persistence regression tests.

File	Description
dotnet/​tests/​Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/​ResponseSessionRegressionTests.cs	Tests selection, isolation, and session continuity.
dotnet/​tests/​Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/​InMemoryResponsesServiceTests.cs	Tests executor release after completion.
dotnet/​src/​Shared/​HostedAgents/​HostedAgentResolution.cs	Adds shared agent identity validation.
dotnet/​src/​Microsoft.Agents.AI.Hosting.OpenAI/​Responses/​ResponsesHttpHandler.cs	Selects agents for every operation.
dotnet/​src/​Microsoft.Agents.AI.Hosting.OpenAI/​Responses/​IResponsesService.cs	Adds request-selected executor overloads.
dotnet/​src/​Microsoft.Agents.AI.Hosting.OpenAI/​Responses/​InMemoryResponsesService.cs	Partitions responses and releases executors.
dotnet/​src/​Microsoft.Agents.AI.Hosting.OpenAI/​Responses/​AIAgentResponseExecutor.cs	Supports explicit session storage identity.
dotnet/​src/​Microsoft.Agents.AI.Hosting.OpenAI/​Microsoft.Agents.AI.Hosting.OpenAI.csproj	Includes shared hosting helpers.
dotnet/​src/​Microsoft.Agents.AI.Hosting.OpenAI/​EndpointRouteBuilderExtensions.Responses.cs	Adds selector mapping and endpoint wiring.
dotnet/​eng/​MSBuild/​Shared.props	Enables shared hosting source inclusion.

_{Review effort: Balanced}