fix(mcp): use HTTP/1.1 for the streamable-HTTP transport (drop allowH2)

[waleedlatif1]

Summary

• The MCP transport was the only place in the codebase opting undici into HTTP/2 (allowH2: true in createPinnedMcpFetch). Everything else — including the OAuth legs that now work — uses h1.1.
• undici's HTTP/2 support is experimental and has a documented cluster of stalls where response headers arrive but the body DATA frames never do, on POST bodies over reused/coalesced sessions (undici #2311, #3433, #4143). Behind a shared egress IP fronted by a CDN, that is exactly what hung the streamable-HTTP initialize after OAuth succeeded: 200 + Mcp-Session-Id, then an empty body until the SDK's 30s timeout — reproducible only from the deployed egress, never from a fresh IP or in isolation (the same authenticated initialize returns the full result in ~400-600ms over h2 or h1.1 from a dev machine).
• Fix: drop allowH2 → the transport runs HTTP/1.1.

Why this is the correct, best-practice config (verified against the reference clients)

• Official TypeScript SDKStreamableHTTPClientTransport: issues requests via (this._fetch ?? fetch)(...) — global fetch = undici on HTTP/1.1 — and never sets allowH2 / imports node:http2.
• Official Python SDK: builds its httpx.AsyncClient with no http2=True → HTTP/1.1.
• mcp-remote and SDK-based clients (Continue, LibreChat, …) consume that transport as-is (h1.1).
• h2's only real win is multiplexing concurrent requests; the MCP transport is one POST per message plus a single long-lived SSE stream, so it gains nothing. CDN fronts serve h1.1 regardless.

What does NOT change

• SSRF IP-pinning (connect.lookup) is orthogonal to ALPN and is untouched.
• Agent teardown on disconnect (close()) still tears down pooled sockets (incl. the SSE connection).

Type of Change

• Bug fix

Testing

• Full lib/mcp suite green (372); tsc clean; biome clean.
• Added a regression test asserting the transport is built withoutallowH2 (stays on h1.1) and still tears down on close().
• Honest caveat: the stall reproduces only from the deployed shared egress, so the definitive confirmation is re-testing the Gauge connect on staging once this deploys.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Skipped		Jul 21, 2026 4:07am

[cursor]

PR Summary

Medium Risk
Changes how all long-lived MCP streamable-HTTP connections negotiate HTTP (h2 → h1.1); behavior is intentional and aligned with official MCP clients, but it affects production MCP connectivity paths.

Overview
Fixes streamable-HTTP MCP connections that returned 200 and session headers but never delivered the initialize body (until the SDK timeout), especially from shared egress behind CDNs.

createPinnedMcpFetch no longer passes allowH2: true to the pinned undici Agent, so the long-lived transport stays on HTTP/1.1 instead of undici’s experimental HTTP/2 path (known POST/reuse stalls with headers but no body). SSRF IP pinning and close() Agent teardown are unchanged. Docs/comments in pinned-fetch.ts and probe.ts are tightened to match; a createPinnedMcpFetch regression test asserts no allowH2 and that close() still destroys the dispatcher.

Removes optional helpText under the Claude Platform API key field in token service-account descriptors (copy-only).

^{Reviewed by Cursor Bugbot for commit 71a7259. Configure here.}

[greptile-apps]

Greptile Summary

This PR moves the streamable HTTP transport back to HTTP/1.1. The main changes are:

• Removes the MCP transport's explicit undici HTTP/2 opt-in.
• Preserves SSRF IP pinning and dispatcher teardown.
• Adds a test for HTTP/1.1 configuration and socket cleanup.
• Removes the Claude Platform credential hint.
• Shortens comments in the MCP fetch and OAuth probe code.

Confidence Score: 5/5

This looks safe to merge.

• The pinned agent defaults to HTTP/1.1 when allowH2 is omitted.
• IP pinning and dispatcher destruction remain unchanged.
• The new test covers the protocol option and cleanup behavior.
• No blocking issue remains in the updated code.

Important Files Changed

Filename	Overview
apps/sim/lib/mcp/pinned-fetch.ts	Uses the pinned agent's HTTP/1.1 default while preserving IP pinning and socket teardown.
apps/sim/lib/mcp/pinned-fetch.test.ts	Adds coverage for omitting allowH2 and destroying the dispatcher on close.
apps/sim/lib/mcp/oauth/probe.ts	Shortens lifecycle comments without changing probe behavior.
apps/sim/lib/credentials/token-service-accounts/descriptors.ts	Removes the optional help text from the Claude Platform credential descriptor.

_{Reviews (3): Last reviewed commit: "chore(credentials): drop help text on th..." | Re-trigger Greptile}

[waleedlatif1]

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[waleedlatif1]

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^{Reviewed by Cursor Bugbot for commit c1f8195. Configure here.}

[waleedlatif1]

@greptile

[waleedlatif1]

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✅ Bugbot reviewed your changes and found no new issues!

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^{Reviewed by Cursor Bugbot for commit 71a7259. Configure here.}