feat(server): add keepAliveInterval for standalone GET SSE stream

[rcdailey]

Summary

Adds an opt-in keepAliveInterval option to WebStandardStreamableHTTPServerTransportOptions. When set, the transport sends periodic SSE comments (: keepalive\n\n) on the standalone GET SSE stream to reset proxy idle timers and prevent silent disconnections.

SSE comments are ignored by spec-compliant clients but keep the connection alive through reverse proxies (nginx, Envoy, HAProxy, cloud load balancers) that enforce idle timeouts.

Usage

const transport = new WebStandardStreamableHTTPServerTransport({
    sessionIdGenerator: () => crypto.randomUUID(),
    keepAliveInterval: 15_000 // send keepalive every 15 seconds
});

Omitting the option preserves existing behavior (no keepalive, fully backwards compatible).

What changed

• keepAliveInterval?: number added to the options interface (milliseconds, disabled by default)
• handleGetRequest starts a setInterval that enqueues : keepalive\n\n to the stream controller
• Timer is cleared on client disconnect, close(), and closeStandaloneSSEStream()
• Controller enqueue is guarded against closed/errored state
• The NodeStreamableHTTPServerTransport wrapper picks this up automatically (it type-aliases and passes through the options)
• Three new tests covering: comments sent when enabled, no comments when disabled, cleanup on close

Related issues

Ref #28 (server-side ping automation, P1)
Ref #876 (SSE connections drop after ~5 min idle behind proxies)

This is complementary to protocol-level ping approaches (like PR #1717). SSE comments operate at the transport layer and don't require JSON-RPC round-trips, making them a lightweight way to keep the connection alive independently of protocol-level health checks.

[changeset-bot]

🦋 Changeset detected

Latest commit: 941226e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@modelcontextprotocol/server	Minor
@modelcontextprotocol/express	Major
@modelcontextprotocol/hono	Major
@modelcontextprotocol/node	Major

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