diff --git a/extensions/matrix/src/matrix/monitor/direct.test.ts b/extensions/matrix/src/matrix/monitor/direct.test.ts
index 905771a4b85..5ed4a67d687 100644
--- a/extensions/matrix/src/matrix/monitor/direct.test.ts
+++ b/extensions/matrix/src/matrix/monitor/direct.test.ts
@@ -152,4 +152,23 @@ describe("createDirectRoomTracker", () => {
       }),
     ).resolves.toBe(false);
   });
+
+  it("bounds joined-room membership cache size", async () => {
+    const client = createMockClient({ isDm: false });
+    const tracker = createDirectRoomTracker(client);
+
+    for (let i = 0; i <= 1024; i += 1) {
+      await tracker.isDirectMessage({
+        roomId: `!room-${i}:example.org`,
+        senderId: "@alice:example.org",
+      });
+    }
+
+    await tracker.isDirectMessage({
+      roomId: "!room-0:example.org",
+      senderId: "@alice:example.org",
+    });
+
+    expect(client.getJoinedRoomMembers).toHaveBeenCalledTimes(1026);
+  });
 });
diff --git a/extensions/matrix/src/matrix/monitor/direct.ts b/extensions/matrix/src/matrix/monitor/direct.ts
index e58580f04f0..adc4f74a85c 100644
--- a/extensions/matrix/src/matrix/monitor/direct.ts
+++ b/extensions/matrix/src/matrix/monitor/direct.ts
@@ -11,6 +11,17 @@ type DirectRoomTrackerOptions = {
 };
 
 const DM_CACHE_TTL_MS = 30_000;
+const MAX_TRACKED_DM_ROOMS = 1024;
+
+function rememberBounded<T>(map: Map<string, T>, key: string, value: T): void {
+  map.set(key, value);
+  if (map.size > MAX_TRACKED_DM_ROOMS) {
+    const oldest = map.keys().next().value;
+    if (typeof oldest === "string") {
+      map.delete(oldest);
+    }
+  }
+}
 
 export function createDirectRoomTracker(client: MatrixClient, opts: DirectRoomTrackerOptions = {}) {
   const log = opts.log ?? (() => {});
@@ -55,7 +66,7 @@ export function createDirectRoomTracker(client: MatrixClient, opts: DirectRoomTr
         .filter((entry): entry is string => typeof entry === "string")
         .map((entry) => entry.trim())
         .filter(Boolean);
-      joinedMembersCache.set(roomId, { members: normalized, ts: now });
+      rememberBounded(joinedMembersCache, roomId, { members: normalized, ts: now });
       return normalized;
     } catch (err) {
       log(`matrix: dm member lookup failed room=${roomId} (${String(err)})`);
diff --git a/extensions/matrix/src/matrix/monitor/room-info.test.ts b/extensions/matrix/src/matrix/monitor/room-info.test.ts
new file mode 100644
index 00000000000..ec27c6c4880
--- /dev/null
+++ b/extensions/matrix/src/matrix/monitor/room-info.test.ts
@@ -0,0 +1,62 @@
+import { describe, expect, it, vi } from "vitest";
+import type { MatrixClient } from "../sdk.js";
+import { createMatrixRoomInfoResolver } from "./room-info.js";
+
+function createClientStub() {
+  return {
+    getRoomStateEvent: vi.fn(
+      async (
+        roomId: string,
+        eventType: string,
+        stateKey: string,
+      ): Promise<Record<string, unknown>> => {
+        if (eventType === "m.room.name") {
+          return { name: `Room ${roomId}` };
+        }
+        if (eventType === "m.room.canonical_alias") {
+          return {
+            alias: `#alias-${roomId}:example.org`,
+            alt_aliases: [`#alt-${roomId}:example.org`],
+          };
+        }
+        if (eventType === "m.room.member") {
+          return { displayname: `Display ${roomId}:${stateKey}` };
+        }
+        return {};
+      },
+    ),
+  } as unknown as MatrixClient & {
+    getRoomStateEvent: ReturnType<typeof vi.fn>;
+  };
+}
+
+describe("createMatrixRoomInfoResolver", () => {
+  it("caches room info and member display names", async () => {
+    const client = createClientStub();
+    const resolver = createMatrixRoomInfoResolver(client);
+
+    await resolver.getRoomInfo("!room:example.org");
+    await resolver.getRoomInfo("!room:example.org");
+    await resolver.getMemberDisplayName("!room:example.org", "@alice:example.org");
+    await resolver.getMemberDisplayName("!room:example.org", "@alice:example.org");
+
+    expect(client.getRoomStateEvent).toHaveBeenCalledTimes(3);
+  });
+
+  it("bounds cached room and member entries", async () => {
+    const client = createClientStub();
+    const resolver = createMatrixRoomInfoResolver(client);
+
+    for (let i = 0; i <= 1024; i += 1) {
+      await resolver.getRoomInfo(`!room-${i}:example.org`);
+    }
+    await resolver.getRoomInfo("!room-0:example.org");
+
+    for (let i = 0; i <= 4096; i += 1) {
+      await resolver.getMemberDisplayName("!room:example.org", `@user-${i}:example.org`);
+    }
+    await resolver.getMemberDisplayName("!room:example.org", "@user-0:example.org");
+
+    expect(client.getRoomStateEvent).toHaveBeenCalledTimes(6150);
+  });
+});
diff --git a/extensions/matrix/src/matrix/monitor/room-info.ts b/extensions/matrix/src/matrix/monitor/room-info.ts
index 095f1dc307a..2b5a01454d0 100644
--- a/extensions/matrix/src/matrix/monitor/room-info.ts
+++ b/extensions/matrix/src/matrix/monitor/room-info.ts
@@ -6,8 +6,22 @@ export type MatrixRoomInfo = {
   altAliases: string[];
 };
 
+const MAX_TRACKED_ROOM_INFO = 1024;
+const MAX_TRACKED_MEMBER_DISPLAY_NAMES = 4096;
+
+function rememberBounded<T>(map: Map<string, T>, key: string, value: T, maxEntries: number): void {
+  map.set(key, value);
+  if (map.size > maxEntries) {
+    const oldest = map.keys().next().value;
+    if (typeof oldest === "string") {
+      map.delete(oldest);
+    }
+  }
+}
+
 export function createMatrixRoomInfoResolver(client: MatrixClient) {
   const roomInfoCache = new Map<string, MatrixRoomInfo>();
+  const memberDisplayNameCache = new Map<string, string>();
 
   const getRoomInfo = async (roomId: string): Promise<MatrixRoomInfo> => {
     const cached = roomInfoCache.get(roomId);
@@ -40,16 +54,27 @@ export function createMatrixRoomInfoResolver(client: MatrixClient) {
       // ignore
     }
     const info = { name, canonicalAlias, altAliases };
-    roomInfoCache.set(roomId, info);
+    rememberBounded(roomInfoCache, roomId, info, MAX_TRACKED_ROOM_INFO);
     return info;
   };
 
   const getMemberDisplayName = async (roomId: string, userId: string): Promise<string> => {
+    const cacheKey = `${roomId}:${userId}`;
+    const cached = memberDisplayNameCache.get(cacheKey);
+    if (cached) {
+      return cached;
+    }
     try {
       const memberState = await client
         .getRoomStateEvent(roomId, "m.room.member", userId)
         .catch(() => null);
       if (memberState && typeof memberState.displayname === "string") {
+        rememberBounded(
+          memberDisplayNameCache,
+          cacheKey,
+          memberState.displayname,
+          MAX_TRACKED_MEMBER_DISPLAY_NAMES,
+        );
         return memberState.displayname;
       }
       return userId;