[MCP] Respond to stdio initialize before metadata inference

[Souvik Ghosh (souvikghosh04)]

Why make this change?

Fixes #3430.

MCP stdio currently performs database schema introspection before entering the JSON-RPC read loop. For remote databases with many entities, clients can time out waiting for the initialize response even though the process and database are healthy.

Related: #3783 and #3784. This change also ensures stdio initializes metadata providers without starting the HTTP host.

What is this change?

• Enter the MCP stdio protocol loop before database metadata inference and tool registration.
• Defer that work until the first tools/list or valid tools/call request.
• Cache the initialization task so repeated requests cannot run inference more than once.
• Await initialization before reading or executing tools, including metadata-enriched custom tool schemas.
• Add a deterministic test that blocks metadata inference and verifies initialize responds while tools/list remains pending.

How was this tested?

• Integration Tests
• Unit Tests

Focused MCP stdio, protocol, and registry suite: 76 passed, 0 failed.

dotnet format --verify-no-changes and git diff --check also pass.

Sample Request(s)

Send initialize, notifications/initialized, then tools/list over stdin. The initialize response is emitted immediately; tools/list waits until schema inference and tool registration complete.

{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"probe","version":"1"}}}
{"jsonrpc":"2.0","method":"notifications/initialized","params":{}}
{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}

[Copilot]

🔵 Needs a closer look

Approval assessments are split, and the lazy DI tool-registration path lacks direct coverage.

Pull request overview

Defers MCP stdio metadata inference and tool registration so initialize can respond immediately.

Changes:

• Lazily caches metadata and tool initialization.
• Awaits initialization for tool listing and execution.
• Adds deterministic initialization-order tests.

File summaries

File	Description
src/Service/Utilities/McpStdioHelper.cs	Removes eager tool registration.
src/Service.Tests/UnitTests/McpStdioServerRunAsyncTests.cs	Tests response timing and cached initialization.
src/Service.Tests/UnitTests/McpStdioServerProtocolTests.cs	Provides no-op metadata initialization for protocol tests.
src/Azure.DataApiBuilder.Mcp/Core/McpStdioServer.cs	Adds lazy metadata and tool initialization; direct coverage of DI-based tool registration remains requested as a nit.

Review details

Suppressed comments (1)

src/Azure.DataApiBuilder.Mcp/Core/McpStdioServer.cs:331

• Please add coverage for the new lazy registration path. The blocking test registers no IMcpTool, while the protocol tests pre-populate McpToolRegistry, so all tests still pass if these registration lines are removed. Register a tool through DI and assert that it appears in tools/list (ideally also verifying a custom tool's metadata is enriched only after inference completes).

            IEnumerable<IMcpTool> tools = _serviceProvider.GetServices<IMcpTool>();
            McpToolRegistry.InitializeAndRegisterTools(tools, _toolRegistry, _serviceProvider);

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Balanced

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[Souvik Ghosh (souvikghosh04)]

/azp run

[azure-pipelines]

Azure Pipelines:
Successfully started running 6 pipeline(s).