From 58ca0a16ba2a05a11ba81153dab674f2844c5133 Mon Sep 17 00:00:00 2001
From: bitloi <89318445+bitloi@users.noreply.github.com>
Date: Mon, 11 May 2026 08:55:11 -0300
Subject: [PATCH] :bug: Fix MCP SSE sessions leaking on zombie connections
 (#9432) (#9464)

SSE sessions were never included in the periodic inactivity timeout
checker, so a stale connection whose TCP close event never fired would
retain its SSEServerTransport and McpServer indefinitely.

Changes:
- Add lastActiveTime: number to the sseTransports entry type
- Initialise lastActiveTime at SSE session creation (GET /sse)
- Refresh lastActiveTime on every incoming message (POST /messages)
- Extend startSessionTimeoutChecker() to sweep and forcibly close SSE
  sessions idle for more than SESSION_TIMEOUT_MINUTES, mirroring the
  existing Streamable HTTP logic
- Update the checker log to count both transport maps

The existing res.on('close') cleanup path is preserved unchanged:
it remains the primary cleanup for normal disconnections; the timer
is a safety net for zombie sessions only.

Closes #9432

Signed-off-by: Andrey Antukh <niwi@niwi.nz>
Co-authored-by: Andrey Antukh <niwi@niwi.nz>
---
 CHANGES.md                                 |  1 +
 mcp/packages/server/src/PenpotMcpServer.ts | 24 +++++++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/CHANGES.md b/CHANGES.md
index 41a05857fa..7994247dc1 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -13,6 +13,7 @@
 
 ### :bug: Bugs fixed
 
+- Fix MCP SSE sessions leaking memory on zombie connections by adding inactivity timeout parity with Streamable HTTP sessions (by @bitloi) [Github #9432](https://github.com/penpot/penpot/issues/9432)
 - Fix missing `labels.open` translation (by @MilosM348) [Github #9320](https://github.com/penpot/penpot/pull/9320)
 - Harden Nginx responses with standard security headers and hide upstream `X-Powered-By` headers
 - Fix plugin API `shape.fills` and `shape.strokes` arrays being read-only [Github #8357](https://github.com/penpot/penpot/issues/8357)
diff --git a/mcp/packages/server/src/PenpotMcpServer.ts b/mcp/packages/server/src/PenpotMcpServer.ts
index 47a3ee3355..3d1e23c7cf 100644
--- a/mcp/packages/server/src/PenpotMcpServer.ts
+++ b/mcp/packages/server/src/PenpotMcpServer.ts
@@ -45,7 +45,7 @@ class ToolInfo {
 
 export class PenpotMcpServer {
     /**
-     * Timeout, in minutes, for idle Streamable HTTP sessions before they are automatically closed and removed.
+     * Timeout, in minutes, for idle sessions (Streamable HTTP and SSE) before they are automatically closed and removed.
      */
     private static readonly SESSION_TIMEOUT_MINUTES = 60;
 
@@ -65,7 +65,10 @@ export class PenpotMcpServer {
     private readonly sessionContext = new AsyncLocalStorage<SessionContext>();
 
     private readonly streamableTransports: Record<string, StreamableSession> = {};
-    private readonly sseTransports: Record<string, { transport: SSEServerTransport; userToken?: string }> = {};
+    private readonly sseTransports: Record<
+        string,
+        { transport: SSEServerTransport; userToken?: string; lastActiveTime: number }
+    > = {};
 
     public readonly host: string;
     public readonly port: number;
@@ -179,7 +182,7 @@ export class PenpotMcpServer {
     }
 
     /**
-     * Starts a periodic timer that closes and removes Streamable HTTP sessions that have been
+     * Starts a periodic timer that closes and removes Streamable HTTP and SSE sessions that have been
      * idle for longer than {@link SESSION_TIMEOUT_MINUTES}.
      */
     private startSessionTimeoutChecker(): void {
@@ -195,8 +198,18 @@ export class PenpotMcpServer {
                     removed++;
                 }
             }
+            for (const [id, session] of Object.entries(this.sseTransports)) {
+                if (now - session.lastActiveTime > timeoutMs) {
+                    this.logger.info(`Closing stale SSE session ${id}`);
+                    session.transport.close();
+                    delete this.sseTransports[id];
+                    removed++;
+                }
+            }
             this.logger.info(
-                `Removed ${removed} stale session(s); total sessions remaining: ${Object.keys(this.streamableTransports).length}`
+                `Removed ${removed} stale session(s); total sessions remaining: ${
+                    Object.keys(this.streamableTransports).length + Object.keys(this.sseTransports).length
+                }`
             );
         }, checkIntervalMs);
     }
@@ -262,7 +275,7 @@ export class PenpotMcpServer {
 
             await this.sessionContext.run({ userToken }, async () => {
                 const transport = new SSEServerTransport("/messages", res);
-                this.sseTransports[transport.sessionId] = { transport, userToken };
+                this.sseTransports[transport.sessionId] = { transport, userToken, lastActiveTime: Date.now() };
 
                 const server = this.createMcpServer();
                 await server.connect(transport);
@@ -281,6 +294,7 @@ export class PenpotMcpServer {
             const session = this.sseTransports[sessionId];
 
             if (session) {
+                session.lastActiveTime = Date.now();
                 await this.sessionContext.run({ userToken: session.userToken }, async () => {
                     await session.transport.handlePostMessage(req, res, req.body);
                 });