diff --git a/src/everything/resources/subscriptions.ts b/src/everything/resources/subscriptions.ts
index 854a8633a2..7e7982a9b2 100644
--- a/src/everything/resources/subscriptions.ts
+++ b/src/everything/resources/subscriptions.ts
@@ -166,3 +166,22 @@ export const stopSimulatedResourceUpdates = (sessionId?: string) => {
     subsUpdateIntervals.delete(sessionId);
   }
 };
+
+/**
+ * Removes a disconnected session from all subscription Sets.
+ *
+ * Called from the server factory's `cleanup()` when a transport closes so
+ * that a client that subscribed and then disconnects without unsubscribing
+ * does not leave its sessionId in every Set it joined for the life of the
+ * process. Drops empty entries to avoid unbounded map growth.
+ *
+ * @param sessionId - The session ID to remove (can be undefined for stdio).
+ */
+export const removeSubscriber = (sessionId?: string) => {
+  for (const [uri, subscribers] of subscriptions.entries()) {
+    subscribers.delete(sessionId);
+    if (subscribers.size === 0) {
+      subscriptions.delete(uri);
+    }
+  }
+};
diff --git a/src/everything/server/index.ts b/src/everything/server/index.ts
index f1459cc812..da130ce391 100644
--- a/src/everything/server/index.ts
+++ b/src/everything/server/index.ts
@@ -6,6 +6,7 @@ import {
 import {
   setSubscriptionHandlers,
   stopSimulatedResourceUpdates,
+  removeSubscriber,
 } from "../resources/subscriptions.js";
 import { registerConditionalTools, registerTools } from "../tools/index.js";
 import { registerResources, readInstructions } from "../resources/index.js";
@@ -110,6 +111,7 @@ export const createServer: () => ServerFactoryResponse = () => {
       // Stop any simulated logging or resource updates that may have been initiated.
       stopSimulatedLogging(sessionId);
       stopSimulatedResourceUpdates(sessionId);
+      removeSubscriber(sessionId);
       // Clean up task store timers
       taskStore.cleanup();
       if (initializeTimeout) clearTimeout(initializeTimeout);