fix(core): stop MCP SSE error reconnect loops (#39671)

Co-authored-by: Aiden Cline <rekram1-node@users.noreply.github.com>

packages/core/test/mcp.test.ts

@@ -1,6 +1,7 @@
 import path from "node:path"
 import { describe, expect, test } from "bun:test"
 import { Client } from "@modelcontextprotocol/sdk/client/index.js"
+import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js"
 import { InMemoryTransport } from "@modelcontextprotocol/sdk/inMemory.js"
 import { Server } from "@modelcontextprotocol/sdk/server/index.js"
 import { WebStandardStreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/webStandardStreamableHttp.js"
@@ -559,6 +560,42 @@ test("lists, reads, and reports MCP resource changes", async () => {
   )
 })
 
+test("does not reconnect an SSE stream after a JSON-RPC error response", async () => {
+  let requests = 0
+  const transport = new StreamableHTTPClientTransport(new URL("http://mcp.invalid"), {
+    fetch: async () => {
+      requests += 1
+      return new Response(
+        new ReadableStream({
+          start(controller) {
+            controller.enqueue(new TextEncoder().encode("id: prime\nretry: 1\ndata:\n\n"))
+            controller.enqueue(
+              new TextEncoder().encode(
+                'id: error\ndata: {"jsonrpc":"2.0","error":{"code":-32601,"message":"Method not found"},"id":1}\n\n',
+              ),
+            )
+            controller.close()
+          },
+        }),
+        { status: 200, headers: { "content-type": "text/event-stream" } },
+      )
+    },
+    reconnectionOptions: {
+      initialReconnectionDelay: 1,
+      maxReconnectionDelay: 1,
+      reconnectionDelayGrowFactor: 1,
+      maxRetries: 2,
+    },
+  })
+
+  await transport.start()
+  await transport.send({ jsonrpc: "2.0", method: "resources/list", id: 1 })
+  await Bun.sleep(25)
+  await transport.close()
+
+  expect(requests).toBe(1)
+})
+
 test("skips MCP resource requests when the capability is absent", async () => {
   await Effect.runPromise(
     Effect.scoped(