mcp: order initialized notification before the first request

[mahsaforati]

Fixes #336841

McpHTTPHandle.send() only serializes through _requestSequencer while the transport mode is HttpMode.Unknown. Once initialize returns and the mode becomes HttpMode.Http, later sends take the unserialized path — so notifications/initialized and the first real request go out as two concurrent HTTP POSTs with no ordering guarantee.

When the request wins that race, servers that track session state reject it as uninitialized. The client treats the resulting 400 as a stale session, retries with a new session id, and can lose the race again, so the connection appears permanently broken while succeeding on other attempts. It reproduces more often the higher the latency.

This keeps sends serialized until the initialized notification has actually been delivered, then restores the existing unserialized path. The flag is set only after that POST resolves, so a request entering send() while it is still in flight queues behind it.

Scoped to McpHTTPHandle — no change to IMcpMessageTransport or the extension host protocol, and no added serialization for normal traffic after the handshake.

🤖 Generated with Claude Code

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

TylerLeonhardt

Matched files:

• src/vs/workbench/api/common/extHostMcp.ts

[Copilot]

🟡 Changes recommended

The sequencer can deadlock when an initialize SSE response remains open, and the timing behavior lacks regression coverage.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Orders MCP initialization traffic to prevent stateful HTTP servers from receiving requests too early.

Changes:

• Serializes sends until notifications/initialized completes.
• Detects initialized notifications from JSON-RPC messages.

File summaries

File	Description
src/vs/workbench/api/common/extHostMcp.ts	Adds initialization tracking and startup request sequencing.

Review details

Suppressed comments (1)

src/vs/workbench/api/common/extHostMcp.ts:394

• Queued startup sends can stall indefinitely when the initialize response is text/event-stream: _sendStreamableHttp dispatches the JSON-RPC response but awaits _doSSE() until the entire body closes (lines 550–551 and 712–734). If that stream remains open, notifications/initialized is queued behind the still-pending initialize send and is never POSTed, so every subsequent request also blocks. Start the initialized POST once the mode is known, retain its promise, and make only later requests wait for that promise rather than for the initialize response body to close.

			if (this._mode.value === HttpMode.Unknown || !this._didSendInitialized) {
				await this._requestSequencer.queue(async () => {
					await this._send(message);

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Balanced

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[mahsaforati]

Hi, @microsoft-github-policy-service agree Best regards, Mahsa Forati

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