diff --git a/extensions/matrix/src/matrix/monitor/direct.test.ts b/extensions/matrix/src/matrix/monitor/direct.test.ts
index 9e8fcb2a030..905771a4b85 100644
--- a/extensions/matrix/src/matrix/monitor/direct.test.ts
+++ b/extensions/matrix/src/matrix/monitor/direct.test.ts
@@ -8,14 +8,14 @@ function createMockClient(params: {
   selfDirect?: boolean;
   members?: string[];
 }) {
-  const members = params.members ?? ["@alice:example.org", "@bot:example.org"];
+  let members = params.members ?? ["@alice:example.org", "@bot:example.org"];
   return {
     dms: {
       update: vi.fn().mockResolvedValue(undefined),
       isDm: vi.fn().mockReturnValue(params.isDm === true),
     },
     getUserId: vi.fn().mockResolvedValue("@bot:example.org"),
-    getJoinedRoomMembers: vi.fn().mockResolvedValue(members),
+    getJoinedRoomMembers: vi.fn().mockImplementation(async () => members),
     getRoomStateEvent: vi
       .fn()
       .mockImplementation(async (_roomId: string, eventType: string, stateKey: string) => {
@@ -27,6 +27,9 @@ function createMockClient(params: {
         }
         return {};
       }),
+    __setMembers(next: string[]) {
+      members = next;
+    },
   } as unknown as MatrixClient;
 }
 
@@ -98,6 +101,33 @@ describe("createDirectRoomTracker", () => {
     ).resolves.toBe(false);
   });
 
+  it("re-checks room membership after invalidation when a DM gains extra members", async () => {
+    const client = createMockClient({ isDm: true });
+    const tracker = createDirectRoomTracker(client);
+
+    await expect(
+      tracker.isDirectMessage({
+        roomId: "!room:example.org",
+        senderId: "@alice:example.org",
+      }),
+    ).resolves.toBe(true);
+
+    (client as MatrixClient & { __setMembers: (members: string[]) => void }).__setMembers([
+      "@alice:example.org",
+      "@bot:example.org",
+      "@mallory:example.org",
+    ]);
+
+    tracker.invalidateRoom("!room:example.org");
+
+    await expect(
+      tracker.isDirectMessage({
+        roomId: "!room:example.org",
+        senderId: "@alice:example.org",
+      }),
+    ).resolves.toBe(false);
+  });
+
   it("still recognizes exact 2-member rooms when member state also claims is_direct", async () => {
     const tracker = createDirectRoomTracker(createMockClient({ senderDirect: true }));
     await expect(
diff --git a/extensions/matrix/src/matrix/monitor/direct.ts b/extensions/matrix/src/matrix/monitor/direct.ts
index 38518673655..e58580f04f0 100644
--- a/extensions/matrix/src/matrix/monitor/direct.ts
+++ b/extensions/matrix/src/matrix/monitor/direct.ts
@@ -64,6 +64,11 @@ export function createDirectRoomTracker(client: MatrixClient, opts: DirectRoomTr
   };
 
   return {
+    invalidateRoom: (roomId: string): void => {
+      joinedMembersCache.delete(roomId);
+      lastDmUpdateMs = 0;
+      log(`matrix: invalidated dm cache room=${roomId}`);
+    },
     isDirectMessage: async (params: DirectMessageCheck): Promise<boolean> => {
       const { roomId, senderId } = params;
       await refreshDmCache();
diff --git a/extensions/matrix/src/matrix/monitor/events.test.ts b/extensions/matrix/src/matrix/monitor/events.test.ts
index af352953d43..fb7e28cde4e 100644
--- a/extensions/matrix/src/matrix/monitor/events.test.ts
+++ b/extensions/matrix/src/matrix/monitor/events.test.ts
@@ -37,6 +37,7 @@ function createHarness(params?: {
   const onRoomMessage = vi.fn(async () => {});
   const listVerifications = vi.fn(async () => params?.verifications ?? []);
   const sendMessage = vi.fn(async () => "$notice");
+  const invalidateRoom = vi.fn();
   const logger = { info: vi.fn(), warn: vi.fn(), error: vi.fn() };
   const formatNativeDependencyHint = vi.fn(() => "install hint");
   const client = {
@@ -66,6 +67,9 @@ function createHarness(params?: {
       accountId: params?.accountId ?? "default",
       encryption: params?.authEncryption ?? true,
     } as MatrixAuth,
+    directTracker: {
+      invalidateRoom,
+    },
     logVerboseMessage: vi.fn(),
     warnedEncryptedRooms: new Set<string>(),
     warnedCryptoMissingRooms: new Set<string>(),
@@ -82,6 +86,7 @@ function createHarness(params?: {
   return {
     onRoomMessage,
     sendMessage,
+    invalidateRoom,
     roomEventListener,
     listVerifications,
     logger,
@@ -117,6 +122,23 @@ describe("registerMatrixMonitorEvents verification routing", () => {
     expect(sendMessage).not.toHaveBeenCalled();
   });
 
+  it("invalidates direct-room membership cache on room member events", async () => {
+    const { invalidateRoom, roomEventListener } = createHarness();
+
+    roomEventListener("!room:example.org", {
+      event_id: "$member1",
+      sender: "@alice:example.org",
+      state_key: "@mallory:example.org",
+      type: EventType.RoomMember,
+      origin_server_ts: Date.now(),
+      content: {
+        membership: "join",
+      },
+    });
+
+    expect(invalidateRoom).toHaveBeenCalledWith("!room:example.org");
+  });
+
   it("posts verification request notices directly into the room", async () => {
     const { onRoomMessage, sendMessage, roomMessageListener } = createHarness();
     if (!roomMessageListener) {
diff --git a/extensions/matrix/src/matrix/monitor/events.ts b/extensions/matrix/src/matrix/monitor/events.ts
index 4020b5f7dcb..f4718efc59f 100644
--- a/extensions/matrix/src/matrix/monitor/events.ts
+++ b/extensions/matrix/src/matrix/monitor/events.ts
@@ -11,6 +11,9 @@ export function registerMatrixMonitorEvents(params: {
   cfg: CoreConfig;
   client: MatrixClient;
   auth: MatrixAuth;
+  directTracker?: {
+    invalidateRoom: (roomId: string) => void;
+  };
   logVerboseMessage: (message: string) => void;
   warnedEncryptedRooms: Set<string>;
   warnedCryptoMissingRooms: Set<string>;
@@ -22,6 +25,7 @@ export function registerMatrixMonitorEvents(params: {
     cfg,
     client,
     auth,
+    directTracker,
     logVerboseMessage,
     warnedEncryptedRooms,
     warnedCryptoMissingRooms,
@@ -68,6 +72,7 @@ export function registerMatrixMonitorEvents(params: {
   );
 
   client.on("room.invite", (roomId: string, event: MatrixRawEvent) => {
+    directTracker?.invalidateRoom(roomId);
     const eventId = event?.event_id ?? "unknown";
     const sender = event?.sender ?? "unknown";
     const isDirect = (event?.content as { is_direct?: boolean } | undefined)?.is_direct === true;
@@ -77,6 +82,7 @@ export function registerMatrixMonitorEvents(params: {
   });
 
   client.on("room.join", (roomId: string, event: MatrixRawEvent) => {
+    directTracker?.invalidateRoom(roomId);
     const eventId = event?.event_id ?? "unknown";
     logVerboseMessage(`matrix: join room=${roomId} id=${eventId}`);
   });
@@ -105,6 +111,7 @@ export function registerMatrixMonitorEvents(params: {
       return;
     }
     if (eventType === EventType.RoomMember) {
+      directTracker?.invalidateRoom(roomId);
       const membership = (event?.content as { membership?: string } | undefined)?.membership;
       const stateKey = (event as { state_key?: string }).state_key ?? "";
       logVerboseMessage(
diff --git a/extensions/matrix/src/matrix/monitor/index.ts b/extensions/matrix/src/matrix/monitor/index.ts
index 96314fc6aea..49ea6f99376 100644
--- a/extensions/matrix/src/matrix/monitor/index.ts
+++ b/extensions/matrix/src/matrix/monitor/index.ts
@@ -212,6 +212,7 @@ export async function monitorMatrixProvider(opts: MonitorMatrixOpts = {}): Promi
     cfg,
     client,
     auth,
+    directTracker,
     logVerboseMessage,
     warnedEncryptedRooms,
     warnedCryptoMissingRooms,