diff --git a/src/ToolSocket.js b/src/ToolSocket.js
index 3cd64d5..c68bee1 100644
--- a/src/ToolSocket.js
+++ b/src/ToolSocket.js
@@ -823,6 +823,22 @@ class ToolSocket {
         // infoName() keeps the children in sync if the parent is named later
         if (!toolsocket.parallelSockets) toolsocket.parallelSockets = [];
         toolsocket.parallelSockets.push(parallel);
+        // Stop tracking a parallel once it closes. Most parallels are one-shot (the
+        // proxy closes them after a single request), so a list that only ever grew
+        // pinned a ToolSocket, its WebSocket, Sender, Receiver and ping Timeout per
+        // request for the life of the process. A parallel that reconnects re-registers
+        // itself on 'open', mirroring how the NB layer gates its own teardown.
+        parallel.addEventListener('close', () => {
+            const index = toolsocket.parallelSockets.indexOf(parallel);
+            if (index > -1) {
+                toolsocket.parallelSockets.splice(index, 1);
+            }
+        });
+        parallel.addEventListener('open', () => {
+            if (!toolsocket.parallelSockets.includes(parallel)) {
+                toolsocket.parallelSockets.push(parallel);
+            }
+        });
         if (toolsocket.remoteInfoName) {
             parallel.infoName(toolsocket.remoteInfoName + ' · data');
         }
diff --git a/src/integration.test.js b/src/integration.test.js
index 842404d..962c3ba 100644
--- a/src/integration.test.js
+++ b/src/integration.test.js
@@ -160,4 +160,29 @@ describe('ToolSocket', () => {
 
         toolSocket.message('test/binary', { message: 'Binary data' }, null, [binaryData1, binaryData2]);
     });
+
+    test('a closed parallel socket stops being tracked by its parent', (done) => {
+        const parallel = ToolSocket.makeParallelSocket(toolSocket);
+
+        parallel.addEventListener('open', () => {
+            try {
+                expect(toolSocket.parallelSockets).toContain(parallel);
+            } catch (error) {
+                done(error);
+                return;
+            }
+            parallel.close();
+        });
+
+        parallel.addEventListener('close', () => {
+            try {
+                // Parallels are one-shot, so a list that only ever grew pinned the
+                // whole socket (and its ping timer) once per proxied request
+                expect(toolSocket.parallelSockets).not.toContain(parallel);
+                done();
+            } catch (error) {
+                done(error);
+            }
+        });
+    });
 });