diff --git a/internal/httpapi/payload_userid_test.go b/internal/httpapi/payload_userid_test.go
new file mode 100644
index 0000000..85d3258
--- /dev/null
+++ b/internal/httpapi/payload_userid_test.go
@@ -0,0 +1,81 @@
+package httpapi
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	"git.118im.com/im_team/im_workbench/internal/store"
+)
+
+func TestPayloadUserIDsAreStrings(t *testing.T) {
+	now := time.Date(2026, 9, 16, 9, 30, 0, 0, time.UTC)
+	userA, err := parseUserID("usrag7w9xJ6hto")
+	if err != nil {
+		t.Fatal(err)
+	}
+	userB, err := parseUserID("usr1RqTFgHSenM")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	payloads := []map[string]any{
+		organizationMemberPayload(store.OrganizationMember{UserID: userA}),
+		attendanceRecordPayload(&store.AttendanceRecord{RecordID: "att_1", UserID: userA, Date: now, CreatedAt: now, UpdatedAt: now}),
+		attendanceAppealPayload(&store.AttendanceAppeal{AppealID: "atta_1", UserID: userA, SubmittedAt: now, CreatedAt: now, UpdatedAt: now}),
+		meetingPayload(store.Meeting{
+			MeetingID:   "mtg_1",
+			OrganizerID: userA,
+			RecorderID:  userB,
+			StartAt:     now,
+			EndAt:       now.Add(time.Hour),
+			CreatedAt:   now,
+			UpdatedAt:   now,
+			Participants: []store.MeetingParticipant{
+				{UserID: userA, Response: "accepted"},
+				{UserID: userB, Response: "pending"},
+			},
+		}, userA),
+		meetingNoteFilePayload(store.MeetingNoteFile{NoteFileID: "mnote_1", UploaderID: userA, CreatedAt: now, UpdatedAt: now}, userA),
+		dailyReportPayload(store.DailyReport{ReportID: "dr_1", AuthorID: userA, ReportDate: now, CreatedAt: now, UpdatedAt: now}),
+		monthlyReportPayload(store.MonthlyReport{ReportID: "mr_1", AuthorID: userA, CreatedAt: now, UpdatedAt: now}),
+		mailboxSubmissionPayload(store.MailboxSubmission{SubmissionID: "mb_1", SenderID: userA, SubmittedAt: now}),
+	}
+
+	for _, payload := range payloads {
+		assertNoNumericUserIDs(t, payload)
+	}
+}
+
+func assertNoNumericUserIDs(t *testing.T, value any) {
+	t.Helper()
+	switch typed := value.(type) {
+	case map[string]any:
+		for key, item := range typed {
+			if isUserIDKey(key) {
+				if _, ok := item.(string); !ok {
+					t.Fatalf("%s is %T, want string", key, item)
+				}
+			}
+			assertNoNumericUserIDs(t, item)
+		}
+	case []map[string]any:
+		for _, item := range typed {
+			assertNoNumericUserIDs(t, item)
+		}
+	case []any:
+		for _, item := range typed {
+			assertNoNumericUserIDs(t, item)
+		}
+	}
+}
+
+func isUserIDKey(key string) bool {
+	return key == "user_id" ||
+		key == "organizer_id" ||
+		key == "recorder_user_id" ||
+		key == "uploader_user_id" ||
+		key == "author_id" ||
+		key == "sender_id" ||
+		strings.HasSuffix(key, "_user_id")
+}
diff --git a/internal/httpapi/uid.go b/internal/httpapi/uid.go
index e88e67d..878e524 100644
--- a/internal/httpapi/uid.go
+++ b/internal/httpapi/uid.go
@@ -27,8 +27,6 @@ type optionalUserID struct {
 
 func (id *optionalUserID) UnmarshalJSON(raw []byte) error {
 	if strings.TrimSpace(string(raw)) == "null" {
-		id.set = true
-		id.value = 0
 		return nil
 	}
 	value, err := parseUserIDJSON(raw)
@@ -52,11 +50,7 @@ func parseUserIDJSON(raw []byte) (int64, error) {
 	if err := json.Unmarshal(raw, &text); err == nil {
 		return parseUserID(text)
 	}
-	var number int64
-	if err := json.Unmarshal(raw, &number); err == nil {
-		return number, nil
-	}
-	return 0, fmt.Errorf("invalid user id")
+	return 0, fmt.Errorf("user id must be a string")
 }
 
 func parseUserID(value string) (int64, error) {
diff --git a/internal/httpapi/uid_test.go b/internal/httpapi/uid_test.go
index 91be278..b61b5e8 100644
--- a/internal/httpapi/uid_test.go
+++ b/internal/httpapi/uid_test.go
@@ -22,12 +22,9 @@ func TestUserIDJSONAcceptsIMUserID(t *testing.T) {
 	}
 }
 
-func TestUserIDJSONStillAcceptsNumericID(t *testing.T) {
+func TestUserIDJSONRejectsNumericID(t *testing.T) {
 	var id userID
-	if err := json.Unmarshal([]byte(`1573862062872858624`), &id); err != nil {
-		t.Fatalf("unmarshal numeric: %v", err)
-	}
-	if id != 1573862062872858624 {
-		t.Fatalf("id=%d", id)
+	if err := json.Unmarshal([]byte(`1573862062872858624`), &id); err == nil {
+		t.Fatal("expected numeric user id to be rejected")
 	}
 }