From 980a6fe84e6fd2c2386ae365096e6753ec4fb50f Mon Sep 17 00:00:00 2001
From: Waleed Latif <walif6@gmail.com>
Date: Tue, 21 Jul 2026 14:11:35 -0700
Subject: [PATCH 1/2] diag(mcp): per-request phase logging on the
 streamable-HTTP transport
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Mirrors the OAuth guarded-fetch logging that isolated the /oauth/start body
stall. Logs each transport request (host+method), the response headers with
TTFB, and failures — so a first-connect stall shows whether it hangs before
response headers (connect/request) or after (the SDK's stream read), which
isolates the client-side first-connect stall we haven't yet root-caused.
---
 apps/sim/lib/mcp/pinned-fetch.ts | 38 +++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/apps/sim/lib/mcp/pinned-fetch.ts b/apps/sim/lib/mcp/pinned-fetch.ts
index 4de133fe81b..87942dca075 100644
--- a/apps/sim/lib/mcp/pinned-fetch.ts
+++ b/apps/sim/lib/mcp/pinned-fetch.ts
@@ -6,6 +6,7 @@ import { validateMcpServerSsrf } from '@/lib/mcp/domain-check'
 import { McpError } from '@/lib/mcp/types'
 
 const logger = createLogger('McpOauthFetch')
+const transportLogger = createLogger('McpTransportFetch')
 
 /** Pinned fetch for the live MCP transport, plus a handle to release its sockets. */
 export interface PinnedMcpFetch {
@@ -25,7 +26,42 @@ export interface PinnedMcpFetch {
  */
 export function createPinnedMcpFetch(resolvedIP: string): PinnedMcpFetch {
   const { fetch: pinnedFetch, dispatcher } = createPinnedFetchWithDispatcher(resolvedIP)
-  return { fetch: pinnedFetch, close: () => dispatcher.destroy() }
+  // Per-request phase logging: a stalled transport request (e.g. a first `initialize` that hangs
+  // to the client timeout) shows whether it stalls BEFORE response headers ("request" with no
+  // "response headers" = connect/request stall) or AFTER ("response headers" then the SDK's
+  // stream read stalls). Isolates the client-side first-connect stall.
+  const instrumentedFetch: typeof fetch = async (input, init) => {
+    const method = init?.method ?? (input instanceof Request ? input.method : 'GET')
+    const target =
+      typeof input === 'string'
+        ? input
+        : input instanceof URL
+          ? input.href
+          : input instanceof Request
+            ? input.url
+            : String(input)
+    const host = URL.canParse(target) ? new URL(target).host : target
+    const startedAt = Date.now()
+    transportLogger.info('MCP transport request', { host, method })
+    try {
+      const response = await pinnedFetch(input, init)
+      transportLogger.info('MCP transport response headers', {
+        host,
+        method,
+        status: response.status,
+        ttfbMs: Date.now() - startedAt,
+      })
+      return response
+    } catch (error) {
+      transportLogger.warn('MCP transport request failed', {
+        host,
+        method,
+        ms: Date.now() - startedAt,
+      })
+      throw error
+    }
+  }
+  return { fetch: instrumentedFetch, close: () => dispatcher.destroy() }
 }
 
 /**

From a12bac543161ed542e8b92d36bc4a401e807d753 Mon Sep 17 00:00:00 2001
From: Waleed Latif <walif6@gmail.com>
Date: Tue, 21 Jul 2026 14:48:59 -0700
Subject: [PATCH 2/2] diag(mcp): include error name/code in transport failure
 log

Distinguishes abort vs connect-refused vs TLS vs timeout in the failure-phase
data.
---
 apps/sim/lib/mcp/pinned-fetch.ts | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/apps/sim/lib/mcp/pinned-fetch.ts b/apps/sim/lib/mcp/pinned-fetch.ts
index 87942dca075..162f568c2af 100644
--- a/apps/sim/lib/mcp/pinned-fetch.ts
+++ b/apps/sim/lib/mcp/pinned-fetch.ts
@@ -53,10 +53,14 @@ export function createPinnedMcpFetch(resolvedIP: string): PinnedMcpFetch {
       })
       return response
     } catch (error) {
+      const e = error as { name?: string; code?: string; cause?: { name?: string; code?: string } }
       transportLogger.warn('MCP transport request failed', {
         host,
         method,
         ms: Date.now() - startedAt,
+        errorName: e?.name,
+        errorCode: e?.code ?? e?.cause?.code,
+        causeName: e?.cause?.name,
       })
       throw error
     }