diff --git a/bridgev2/matrix/intent.go b/bridgev2/matrix/intent.go
index 173f7c15b..74ff2f9c1 100644
--- a/bridgev2/matrix/intent.go
+++ b/bridgev2/matrix/intent.go
@@ -583,7 +583,10 @@ func (as *ASIntent) MarkAsDM(ctx context.Context, roomID id.RoomID, withUser id.
 	var directChats event.DirectChatsEventContent
 	err := as.Matrix.GetAccountData(ctx, event.AccountDataDirectChats.Type, &directChats)
 	if err != nil {
-		return err
+		if !errors.Is(err, mautrix.MNotFound) {
+			return err
+		}
+		directChats = make(event.DirectChatsEventContent)
 	}
 	as.directChatsCache = directChats
 	rooms := directChats[withUser]
diff --git a/bridgev2/matrix/intent_test.go b/bridgev2/matrix/intent_test.go
new file mode 100644
index 000000000..2da6c494a
--- /dev/null
+++ b/bridgev2/matrix/intent_test.go
@@ -0,0 +1,223 @@
+// Copyright (c) 2026 Tulir Asokan
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package matrix
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"maunium.net/go/mautrix/appservice"
+	"maunium.net/go/mautrix/bridgev2/bridgeconfig"
+	"maunium.net/go/mautrix/event"
+	"maunium.net/go/mautrix/id"
+)
+
+func newTestASIntent(t *testing.T, handler http.Handler) *ASIntent {
+	t.Helper()
+	ts := httptest.NewServer(handler)
+	t.Cleanup(ts.Close)
+	as := appservice.Create()
+	as.Registration = &appservice.Registration{SenderLocalpart: "bridge"}
+	as.HomeserverDomain = "example.com"
+	err := as.SetHomeserverURL(ts.URL)
+	require.NoError(t, err)
+	intent := as.Intent(id.NewUserID("bridge", "example.com"))
+	cfg := &bridgeconfig.Config{}
+	cfg.Matrix.SyncDirectChatList = true
+	return &ASIntent{
+		Matrix:    intent,
+		Connector: &Connector{Config: cfg, AS: as},
+	}
+}
+
+func TestASIntent_MarkAsDM_ExistingDirectChats(t *testing.T) {
+	existingUser := id.UserID("@alice:example.com")
+	existingRoom := id.RoomID("!old:example.com")
+	newUser := id.UserID("@bob:example.com")
+	newRoom := id.RoomID("!new:example.com")
+
+	var setCalled bool
+	var setBody event.DirectChatsEventContent
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.Method {
+		case http.MethodGet:
+			json.NewEncoder(w).Encode(event.DirectChatsEventContent{
+				existingUser: {existingRoom},
+			})
+		case http.MethodPut:
+			setCalled = true
+			json.NewDecoder(r.Body).Decode(&setBody)
+			w.WriteHeader(http.StatusOK)
+			w.Write([]byte(`{}`))
+		}
+	})
+	as := newTestASIntent(t, handler)
+
+	err := as.MarkAsDM(context.Background(), newRoom, newUser)
+	require.NoError(t, err)
+	assert.True(t, setCalled)
+	assert.Equal(t, []id.RoomID{existingRoom}, setBody[existingUser])
+	assert.Equal(t, []id.RoomID{newRoom}, setBody[newUser])
+}
+
+func TestASIntent_MarkAsDM_NotFoundCreatesNew(t *testing.T) {
+	userID := id.UserID("@bob:example.com")
+	roomID := id.RoomID("!dm:example.com")
+
+	var setCalled bool
+	var setBody event.DirectChatsEventContent
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.Method {
+		case http.MethodGet:
+			w.Header().Set("Content-Type", "application/json")
+			w.WriteHeader(http.StatusNotFound)
+			w.Write([]byte(`{"errcode":"M_NOT_FOUND","error":"Account data not found"}`))
+		case http.MethodPut:
+			setCalled = true
+			json.NewDecoder(r.Body).Decode(&setBody)
+			w.WriteHeader(http.StatusOK)
+			w.Write([]byte(`{}`))
+		}
+	})
+	as := newTestASIntent(t, handler)
+
+	err := as.MarkAsDM(context.Background(), roomID, userID)
+	require.NoError(t, err)
+	assert.True(t, setCalled)
+	assert.Equal(t, event.DirectChatsEventContent{
+		userID: {roomID},
+	}, setBody)
+}
+
+func TestASIntent_MarkAsDM_OtherErrorReturned(t *testing.T) {
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusInternalServerError)
+		w.Write([]byte(`{"errcode":"M_UNKNOWN","error":"Internal server error"}`))
+	})
+	as := newTestASIntent(t, handler)
+
+	err := as.MarkAsDM(context.Background(), "!room:example.com", "@user:example.com")
+	assert.Error(t, err)
+}
+
+func TestASIntent_MarkAsDM_AlreadyInList(t *testing.T) {
+	userID := id.UserID("@bob:example.com")
+	roomID := id.RoomID("!dm:example.com")
+
+	var setCalled bool
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.Method {
+		case http.MethodGet:
+			json.NewEncoder(w).Encode(event.DirectChatsEventContent{
+				userID: {roomID},
+			})
+		case http.MethodPut:
+			setCalled = true
+			w.WriteHeader(http.StatusOK)
+			w.Write([]byte(`{}`))
+		}
+	})
+	as := newTestASIntent(t, handler)
+
+	err := as.MarkAsDM(context.Background(), roomID, userID)
+	require.NoError(t, err)
+	assert.False(t, setCalled, "SetAccountData should not be called when room is already in list")
+}
+
+func TestASIntent_MarkAsDM_SyncDirectChatListDisabled(t *testing.T) {
+	var called bool
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		called = true
+	})
+	as := newTestASIntent(t, handler)
+	as.Connector.Config.Matrix.SyncDirectChatList = false
+
+	err := as.MarkAsDM(context.Background(), "!room:example.com", "@user:example.com")
+	require.NoError(t, err)
+	assert.False(t, called, "No HTTP calls should be made when SyncDirectChatList is false")
+}
+
+func TestASIntent_MarkAsDM_CacheHit(t *testing.T) {
+	userID := id.UserID("@bob:example.com")
+	roomID := id.RoomID("!dm:example.com")
+
+	var getCalled bool
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method == http.MethodGet {
+			getCalled = true
+		}
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{}`))
+	})
+	as := newTestASIntent(t, handler)
+	as.directChatsCache = event.DirectChatsEventContent{
+		userID: {roomID},
+	}
+
+	err := as.MarkAsDM(context.Background(), roomID, userID)
+	require.NoError(t, err)
+	assert.False(t, getCalled, "GetAccountData should not be called when cache has the entry")
+}
+
+func TestASIntent_MarkAsDM_SetFailureRollsBack(t *testing.T) {
+	userID := id.UserID("@bob:example.com")
+	roomID := id.RoomID("!dm:example.com")
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.Method {
+		case http.MethodGet:
+			json.NewEncoder(w).Encode(event.DirectChatsEventContent{})
+		case http.MethodPut:
+			w.Header().Set("Content-Type", "application/json")
+			w.WriteHeader(http.StatusInternalServerError)
+			w.Write([]byte(`{"errcode":"M_UNKNOWN","error":"failed"}`))
+		}
+	})
+	as := newTestASIntent(t, handler)
+
+	err := as.MarkAsDM(context.Background(), roomID, userID)
+	assert.Error(t, err)
+	// After a failed set, the cache should have rolled back (user entry removed since it was new)
+	as.dmUpdateLock.Lock()
+	_, exists := as.directChatsCache[userID]
+	as.dmUpdateLock.Unlock()
+	assert.False(t, exists, "Failed SetAccountData should roll back the change")
+}
+
+func TestASIntent_MarkAsDM_ConcurrentSafe(t *testing.T) {
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.Method {
+		case http.MethodGet:
+			json.NewEncoder(w).Encode(event.DirectChatsEventContent{})
+		case http.MethodPut:
+			w.WriteHeader(http.StatusOK)
+			w.Write([]byte(`{}`))
+		}
+	})
+	as := newTestASIntent(t, handler)
+
+	var wg sync.WaitGroup
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			userID := id.UserID("@user:example.com")
+			roomID := id.RoomID(id.RoomID("!room:example.com"))
+			err := as.MarkAsDM(context.Background(), roomID, userID)
+			assert.NoError(t, err)
+		}(i)
+	}
+	wg.Wait()
+}