.NET: Explain DevUI session persistence across requests

[Naveen Chatlapalli (1aifanatic)]

Motivation & Context

A custom ChatHistoryProvider does not persist the enclosing AgentSession between DevUI HTTP requests. The setup guide currently mentions session stores only for function approval, leaving users with stateful history providers to discover the same requirement themselves.

Description & Review Guide

• What are the major changes? Add an in-memory session store to the usage registration and explain conversation IDs, agent session state, custom history providers, protocol conversation storage, and persistent stores.
• What is the impact of these changes? The development setup now preserves session state across turns. Documentation only; guidance was checked against the hosted Responses executor's session-store branch and existing session-store APIs. The relative hosting-guide link resolves.
• What do you want reviewers to focus on? The distinction between history storage and agent session storage for the configuration reported in the issue.

Related Issue

Fixes #7435

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 is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and the title prefix in sync automatically.

[Copilot]

Copilot review overview

🟡 Changes recommended

The updated sample enables strict isolation without configuring the required isolation-key provider, causing session requests to fail.

Review effort: Balanced
Findings: 1

Open (1)

• Configure agent isolation for the in-memory session sample · New

What changed in this PR

Documents how DevUI persists agent sessions across HTTP requests and clarifies session storage responsibilities.

Changes:

• Adds an in-memory session store to the setup example.
• Distinguishes agent sessions, chat history, and protocol conversation storage.
• Links production persistence and isolation guidance.

File	Description
dotnet/​src/​Microsoft.Agents.AI.DevUI/​README.md	Adds DevUI session-persistence setup and guidance.

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