diff --git a/packages/server/src/service.ts b/packages/server/src/service.ts
index 8bd0a20..9ba683b 100644
--- a/packages/server/src/service.ts
+++ b/packages/server/src/service.ts
@@ -6,7 +6,8 @@
 // a running simulation stops, what it left written back), end a person's
 // grant to a project or all their sessions (whoever is connected, to the
 // document or the room, is let go), and list who is signed in to what (the
-// dashboard checks it against its shares). FILIUS files (F6.1,
+// dashboard checks it against its shares, and ends a teacher's share for one
+// session when no tab of theirs has the project open). FILIUS files (F6.1,
 // transfer.ts): make a project from one of FILIUS' examples or from a .fls
 // the dashboard sends, copy a project (as it is now, also while people work
 // in it), and give a project as a .fls (with what it could not hold, as
@@ -97,6 +98,23 @@ function uuid(value: unknown, code: string): string {
 
 const contextOf = (connection: { context: unknown }) => connection.context as ConnectionContext;
 
+/**
+ * Who has which project open right now — connected to its document, not only
+ * granted: a grant lasts the session's school day, a connection ends with the
+ * tab. The dashboard ends a teacher's write share for one session by it.
+ * Once per person and project, sorted.
+ */
+function openDocuments(sync: Sync): { readonly user: string; readonly project: string }[] {
+  const seen = new Map<string, { readonly user: string; readonly project: string }>();
+  for (const [project, document] of sync.hocuspocus.documents) {
+    for (const connection of document.getConnections()) {
+      const user = contextOf(connection)?.person?.user;
+      if (typeof user === 'string') seen.set(`${user} ${project}`, { user, project });
+    }
+  }
+  return [...seen.entries()].sort(([left], [right]) => left.localeCompare(right)).map(([, open]) => open);
+}
+
 export async function handleService(request: IncomingMessage, response: ServerResponse, path: string, options: ServiceOptions): Promise<void> {
   const send = (status: number, value: object) => {
     response.writeHead(status, { 'content-type': 'application/json', 'cache-control': 'no-store' });
@@ -234,6 +252,6 @@ async function route(request: IncomingMessage, path: string, { store, sessions,
     log.info('svc.revoked', { user, ...(only === undefined ? {} : { project: only }), revoked, disconnected });
     return [200, { revoked, disconnected }];
   }
-  if (request.method === 'GET' && path === '/svc/sessions') return [200, await sessions.active()];
+  if (request.method === 'GET' && path === '/svc/sessions') return [200, { ...(await sessions.active()), open: openDocuments(sync) }];
   throw new HttpError(404, 'notFound');
 }
diff --git a/packages/server/test/access.test.ts b/packages/server/test/access.test.ts
index a0c5319..6e31d45 100644
--- a/packages/server/test/access.test.ts
+++ b/packages/server/test/access.test.ts
@@ -412,7 +412,25 @@ describe("the dashboard's service", () => {
       { user: mia, project, level: 'write' },
       { user: ben, project: other, level: 'read' },
     ].sort((x, y) => (x.user + x.project).localeCompare(y.user + y.project));
-    expect(await answer(await service(server, 'GET', '/svc/sessions'))).toEqual([200, { users: [mia, ben].sort(), grants }]);
+    expect(await answer(await service(server, 'GET', '/svc/sessions'))).toEqual([200, { users: [mia, ben].sort(), grants, open: [] }]);
+    await server.close();
+  });
+
+  it('lists who has which project open: connected, once per person and project; a closed tab is gone', async () => {
+    await sql.query('DELETE FROM filius_session');
+    const server = await start();
+    const [project, other] = [await newProject(), await newProject()];
+    const [mia, ben] = [randomUUID(), randomUUID()].sort() as [string, string];
+    const miaCookie = await signIn(server, project, { user: mia });
+    const tab = await join(server, project, { cookie: miaCookie });
+    const second = await join(server, project, { cookie: miaCookie });
+    await signIn(server, other, { user: ben });
+    const open = async () => ((await (await service(server, 'GET', '/svc/sessions')).json()) as { open: unknown }).open;
+    expect(await open()).toEqual([{ user: mia, project }]);
+    tab.close();
+    expect(await open()).toEqual([{ user: mia, project }]);
+    second.close();
+    await until(async () => ((await open()) as unknown[]).length === 0);
     await server.close();
   });