diff --git a/backend/internal/handler/auth_handler.go b/backend/internal/handler/auth_handler.go
index 76ca153d..9801b3b3 100644
--- a/backend/internal/handler/auth_handler.go
+++ b/backend/internal/handler/auth_handler.go
@@ -1,6 +1,7 @@
 package handler
 
 import (
+	"context"
 	"log/slog"
 	"strings"
 
@@ -105,6 +106,34 @@ func (h *AuthHandler) respondWithTokenPair(c *gin.Context, user *service.User) {
 	})
 }
 
+func (h *AuthHandler) ensureBackendModeAllowsUser(ctx context.Context, user *service.User) error {
+	if user == nil {
+		return infraerrors.Unauthorized("INVALID_USER", "user not found")
+	}
+	if h == nil || !h.isBackendModeEnabled(ctx) || user.IsAdmin() {
+		return nil
+	}
+	return infraerrors.Forbidden("BACKEND_MODE_ADMIN_ONLY", "Backend mode is active. Only admin login is allowed.")
+}
+
+func (h *AuthHandler) ensureBackendModeAllowsNewUserLogin(ctx context.Context) error {
+	if h == nil || !h.isBackendModeEnabled(ctx) {
+		return nil
+	}
+	return infraerrors.Forbidden("BACKEND_MODE_ADMIN_ONLY", "Backend mode is active. Only admin login is allowed.")
+}
+
+func (h *AuthHandler) isBackendModeEnabled(ctx context.Context) bool {
+	if h == nil || h.settingSvc == nil {
+		return false
+	}
+	settings, err := h.settingSvc.GetPublicSettings(ctx)
+	if err == nil && settings != nil {
+		return settings.BackendModeEnabled
+	}
+	return h.settingSvc.IsBackendModeEnabled(ctx)
+}
+
 // Register handles user registration
 // POST /api/v1/auth/register
 func (h *AuthHandler) Register(c *gin.Context) {
@@ -178,6 +207,11 @@ func (h *AuthHandler) Login(c *gin.Context) {
 	}
 	_ = token // token 由 authService.Login 返回但此处由 respondWithTokenPair 重新生成
 
+	if err := h.ensureBackendModeAllowsUser(c.Request.Context(), user); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
 	// Check if TOTP 2FA is enabled for this user
 	if h.totpService != nil && h.settingSvc.IsTotpEnabled(c.Request.Context()) && user.TotpEnabled {
 		// Create a temporary login session for 2FA
@@ -195,11 +229,7 @@ func (h *AuthHandler) Login(c *gin.Context) {
 		return
 	}
 
-	// Backend mode: only admin can login
-	if h.settingSvc.IsBackendModeEnabled(c.Request.Context()) && !user.IsAdmin() {
-		response.Forbidden(c, "Backend mode is active. Only admin login is allowed.")
-		return
-	}
+	h.authService.RecordSuccessfulLogin(c.Request.Context(), user.ID)
 
 	h.respondWithTokenPair(c, user)
 }
@@ -264,9 +294,8 @@ func (h *AuthHandler) Login2FA(c *gin.Context) {
 		return
 	}
 
-	// Backend mode: only admin can login (check BEFORE deleting session)
-	if h.settingSvc.IsBackendModeEnabled(c.Request.Context()) && !user.IsAdmin() {
-		response.Forbidden(c, "Backend mode is active. Only admin login is allowed.")
+	if err := h.ensureBackendModeAllowsUser(c.Request.Context(), user); err != nil {
+		response.ErrorFrom(c, err)
 		return
 	}
 
@@ -330,6 +359,10 @@ func (h *AuthHandler) Login2FA(c *gin.Context) {
 	// Delete the login session (only after all checks pass)
 	_ = h.totpService.DeleteLoginSession(c.Request.Context(), req.TempToken)
 
+	if session.PendingOAuthBind == nil {
+		h.authService.RecordSuccessfulLogin(c.Request.Context(), user.ID)
+	}
+
 	h.respondWithTokenPair(c, user)
 }
 
diff --git a/backend/internal/handler/auth_linuxdo_oauth.go b/backend/internal/handler/auth_linuxdo_oauth.go
index 175b1e1f..c3a9041b 100644
--- a/backend/internal/handler/auth_linuxdo_oauth.go
+++ b/backend/internal/handler/auth_linuxdo_oauth.go
@@ -474,6 +474,14 @@ func (h *AuthHandler) CompleteLinuxDoOAuthRegistration(c *gin.Context) {
 		response.ErrorFrom(c, err)
 		return
 	}
+	if err := ensurePendingOAuthCompleteRegistrationSession(session); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+	if err := h.ensureBackendModeAllowsNewUserLogin(c.Request.Context()); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
 
 	email := strings.TrimSpace(session.ResolvedEmail)
 	username := pendingSessionStringValue(session.UpstreamIdentityClaims, "username")
@@ -499,6 +507,7 @@ func (h *AuthHandler) CompleteLinuxDoOAuthRegistration(c *gin.Context) {
 		response.ErrorFrom(c, infraerrors.InternalServer("PENDING_AUTH_ADOPTION_APPLY_FAILED", "failed to apply oauth profile adoption").WithCause(err))
 		return
 	}
+	h.authService.RecordSuccessfulLogin(c.Request.Context(), user.ID)
 	if _, err := pendingSvc.ConsumeBrowserSession(c.Request.Context(), sessionToken, browserSessionKey); err != nil {
 		clearOAuthPendingSessionCookie(c, secureCookie)
 		clearOAuthPendingBrowserCookie(c, secureCookie)
diff --git a/backend/internal/handler/auth_linuxdo_oauth_test.go b/backend/internal/handler/auth_linuxdo_oauth_test.go
index fb57e570..7938f3e7 100644
--- a/backend/internal/handler/auth_linuxdo_oauth_test.go
+++ b/backend/internal/handler/auth_linuxdo_oauth_test.go
@@ -591,6 +591,58 @@ func TestCompleteLinuxDoOAuthRegistrationAppliesPendingAdoptionDecision(t *testi
 	require.NotNil(t, consumed.ConsumedAt)
 }
 
+func TestCompleteLinuxDoOAuthRegistrationRejectsAdoptExistingUserSession(t *testing.T) {
+	handler, client := newOAuthPendingFlowTestHandler(t, false)
+	ctx := context.Background()
+
+	existingUser, err := client.User.Create().
+		SetEmail("owner@example.com").
+		SetUsername("owner-user").
+		SetPasswordHash("hash").
+		SetRole(service.RoleUser).
+		SetStatus(service.StatusActive).
+		Save(ctx)
+	require.NoError(t, err)
+
+	session, err := client.PendingAuthSession.Create().
+		SetSessionToken("linuxdo-complete-invalid-session").
+		SetIntent("adopt_existing_user_by_email").
+		SetProviderType("linuxdo").
+		SetProviderKey("linuxdo").
+		SetProviderSubject("linuxdo-invalid-subject-1").
+		SetTargetUserID(existingUser.ID).
+		SetResolvedEmail(existingUser.Email).
+		SetBrowserSessionKey("linuxdo-invalid-browser").
+		SetUpstreamIdentityClaims(map[string]any{
+			"username": "linuxdo_user",
+		}).
+		SetLocalFlowState(map[string]any{
+			oauthCompletionResponseKey: map[string]any{
+				"step": "bind_login_required",
+			},
+		}).
+		SetExpiresAt(time.Now().UTC().Add(10 * time.Minute)).
+		Save(ctx)
+	require.NoError(t, err)
+
+	body := bytes.NewBufferString(`{"invitation_code":"invite-1"}`)
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/auth/oauth/linuxdo/complete-registration", body)
+	req.Header.Set("Content-Type", "application/json")
+	req.AddCookie(&http.Cookie{Name: oauthPendingSessionCookieName, Value: encodeCookieValue(session.SessionToken)})
+	req.AddCookie(&http.Cookie{Name: oauthPendingBrowserCookieName, Value: encodeCookieValue("linuxdo-invalid-browser")})
+	c.Request = req
+
+	handler.CompleteLinuxDoOAuthRegistration(c)
+
+	require.Equal(t, http.StatusBadRequest, recorder.Code)
+
+	storedSession, err := client.PendingAuthSession.Get(ctx, session.ID)
+	require.NoError(t, err)
+	require.Nil(t, storedSession.ConsumedAt)
+}
+
 func newLinuxDoOAuthTestHandler(t *testing.T, invitationEnabled bool, oauthCfg config.LinuxDoConnectConfig) *AuthHandler {
 	t.Helper()
 	handler, _ := newLinuxDoOAuthHandlerAndClient(t, invitationEnabled, oauthCfg)
diff --git a/backend/internal/handler/auth_oauth_pending_flow.go b/backend/internal/handler/auth_oauth_pending_flow.go
index 461810f1..6041e5dd 100644
--- a/backend/internal/handler/auth_oauth_pending_flow.go
+++ b/backend/internal/handler/auth_oauth_pending_flow.go
@@ -253,6 +253,35 @@ func pendingSessionWantsInvitation(payload map[string]any) bool {
 	return strings.EqualFold(strings.TrimSpace(pendingSessionStringValue(payload, "error")), "invitation_required")
 }
 
+func pendingOAuthCompletionIncludesTokenPayload(payload map[string]any) bool {
+	if len(payload) == 0 {
+		return false
+	}
+	for _, key := range []string{"access_token", "refresh_token"} {
+		if value := pendingSessionStringValue(payload, key); value != "" {
+			return true
+		}
+	}
+	return false
+}
+
+func ensurePendingOAuthCompleteRegistrationSession(session *dbent.PendingAuthSession) error {
+	if session == nil {
+		return infraerrors.BadRequest("PENDING_AUTH_SESSION_INVALID", "pending auth registration context is invalid")
+	}
+	if strings.TrimSpace(session.Intent) != oauthIntentLogin {
+		return infraerrors.BadRequest("PENDING_AUTH_SESSION_INVALID", "pending auth registration context is invalid")
+	}
+	if session.TargetUserID != nil && *session.TargetUserID > 0 {
+		return infraerrors.BadRequest("PENDING_AUTH_SESSION_INVALID", "pending auth registration context is invalid")
+	}
+	payload, _ := readCompletionResponse(session.LocalFlowState)
+	if strings.EqualFold(strings.TrimSpace(pendingSessionStringValue(payload, "step")), "bind_login_required") {
+		return infraerrors.BadRequest("PENDING_AUTH_SESSION_INVALID", "pending auth registration context is invalid")
+	}
+	return nil
+}
+
 func (r oauthAdoptionDecisionRequest) hasDecision() bool {
 	return r.AdoptDisplayName != nil || r.AdoptAvatar != nil
 }
@@ -1090,6 +1119,10 @@ func (h *AuthHandler) bindPendingOAuthLogin(c *gin.Context, provider string) {
 		response.ErrorFrom(c, infraerrors.Conflict("PENDING_AUTH_TARGET_USER_MISMATCH", "pending oauth session must be completed by the targeted user"))
 		return
 	}
+	if err := h.ensureBackendModeAllowsUser(c.Request.Context(), user); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
 
 	decision, err := h.ensurePendingOAuthAdoptionDecision(c, session.ID, req.adoptionDecision())
 	if err != nil {
@@ -1192,6 +1225,10 @@ func (h *AuthHandler) createPendingOAuthAccount(c *gin.Context, provider string)
 		c.JSON(http.StatusOK, buildPendingOAuthSessionStatusPayload(session))
 		return
 	}
+	if err := h.ensureBackendModeAllowsNewUserLogin(c.Request.Context()); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
 
 	tokenPair, user, err := h.authService.RegisterOAuthEmailAccount(
 		c.Request.Context(),
@@ -1215,6 +1252,7 @@ func (h *AuthHandler) createPendingOAuthAccount(c *gin.Context, provider string)
 		response.ErrorFrom(c, infraerrors.InternalServer("PENDING_AUTH_BIND_APPLY_FAILED", "failed to bind pending oauth identity").WithCause(err))
 		return
 	}
+	h.authService.RecordSuccessfulLogin(c.Request.Context(), user.ID)
 
 	if _, err := pendingSvc.ConsumeBrowserSession(c.Request.Context(), session.SessionToken, session.BrowserSessionKey); err != nil {
 		clearCookies()
@@ -1279,6 +1317,25 @@ func (h *AuthHandler) ExchangePendingOAuthCompletion(c *gin.Context) {
 		}
 	}
 	applySuggestedProfileToCompletionResponse(payload, session.UpstreamIdentityClaims)
+	if pendingOAuthCompletionIncludesTokenPayload(payload) {
+		if session.TargetUserID == nil || *session.TargetUserID <= 0 {
+			clearCookies()
+			response.ErrorFrom(c, infraerrors.InternalServer("PENDING_AUTH_COMPLETION_INVALID", "pending auth completion payload is invalid"))
+			return
+		}
+		user, err := h.userService.GetByID(c.Request.Context(), *session.TargetUserID)
+		if err != nil {
+			clearCookies()
+			response.ErrorFrom(c, err)
+			return
+		}
+		if err := h.ensureBackendModeAllowsUser(c.Request.Context(), user); err != nil {
+			clearCookies()
+			response.ErrorFrom(c, err)
+			return
+		}
+		h.authService.RecordSuccessfulLogin(c.Request.Context(), user.ID)
+	}
 
 	if pendingSessionWantsInvitation(payload) {
 		if adoptionDecision.hasDecision() {
diff --git a/backend/internal/handler/auth_oauth_pending_flow_test.go b/backend/internal/handler/auth_oauth_pending_flow_test.go
index d29e4b88..c2b83c73 100644
--- a/backend/internal/handler/auth_oauth_pending_flow_test.go
+++ b/backend/internal/handler/auth_oauth_pending_flow_test.go
@@ -523,6 +523,60 @@ func TestExchangePendingOAuthCompletionLoginFalseFalseBindsIdentityWithoutAdopti
 	require.NotNil(t, storedSession.ConsumedAt)
 }
 
+func TestExchangePendingOAuthCompletionBlocksBackendModeBeforeReturningTokenPayload(t *testing.T) {
+	handler, client := newOAuthPendingFlowTestHandlerWithDependencies(t, oauthPendingFlowTestHandlerOptions{
+		settingValues: map[string]string{
+			service.SettingKeyBackendModeEnabled: "true",
+		},
+	})
+	ctx := context.Background()
+
+	userEntity, err := client.User.Create().
+		SetEmail("blocked@example.com").
+		SetUsername("blocked-user").
+		SetPasswordHash("hash").
+		SetRole(service.RoleUser).
+		SetStatus(service.StatusActive).
+		Save(ctx)
+	require.NoError(t, err)
+
+	session, err := client.PendingAuthSession.Create().
+		SetSessionToken("blocked-backend-mode-session-token").
+		SetIntent("login").
+		SetProviderType("linuxdo").
+		SetProviderKey("linuxdo").
+		SetProviderSubject("blocked-subject-123").
+		SetTargetUserID(userEntity.ID).
+		SetResolvedEmail(userEntity.Email).
+		SetBrowserSessionKey("blocked-backend-mode-browser-session-key").
+		SetLocalFlowState(map[string]any{
+			oauthCompletionResponseKey: map[string]any{
+				"access_token":  "access-token",
+				"refresh_token": "refresh-token",
+				"expires_in":    float64(3600),
+				"token_type":    "Bearer",
+			},
+		}).
+		SetExpiresAt(time.Now().UTC().Add(10 * time.Minute)).
+		Save(ctx)
+	require.NoError(t, err)
+
+	recorder := httptest.NewRecorder()
+	ginCtx, _ := gin.CreateTestContext(recorder)
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/auth/oauth/pending/exchange", nil)
+	req.AddCookie(&http.Cookie{Name: oauthPendingSessionCookieName, Value: encodeCookieValue(session.SessionToken)})
+	req.AddCookie(&http.Cookie{Name: oauthPendingBrowserCookieName, Value: encodeCookieValue("blocked-backend-mode-browser-session-key")})
+	ginCtx.Request = req
+
+	handler.ExchangePendingOAuthCompletion(ginCtx)
+
+	require.Equal(t, http.StatusForbidden, recorder.Code)
+
+	storedSession, err := client.PendingAuthSession.Get(ctx, session.ID)
+	require.NoError(t, err)
+	require.Nil(t, storedSession.ConsumedAt)
+}
+
 func TestExchangePendingOAuthCompletionInvitationRequiredFalseFalsePersistsDecisionWithoutBinding(t *testing.T) {
 	handler, client := newOAuthPendingFlowTestHandler(t, true)
 	ctx := context.Background()
@@ -773,6 +827,60 @@ func TestCreateOIDCOAuthAccountExistingEmailNormalizesLegacySpacingAndCase(t *te
 	require.Equal(t, "owner@example.com", storedSession.ResolvedEmail)
 }
 
+func TestCreateOIDCOAuthAccountBlocksBackendModeBeforeCreatingUser(t *testing.T) {
+	handler, client := newOAuthPendingFlowTestHandlerWithDependencies(t, oauthPendingFlowTestHandlerOptions{
+		emailVerifyEnabled: true,
+		emailCache: &oauthPendingFlowEmailCacheStub{
+			verificationCodes: map[string]*service.VerificationCodeData{
+				"fresh@example.com": {
+					Code:      "246810",
+					CreatedAt: time.Now().UTC(),
+					ExpiresAt: time.Now().UTC().Add(15 * time.Minute),
+				},
+			},
+		},
+		settingValues: map[string]string{
+			service.SettingKeyBackendModeEnabled: "true",
+		},
+	})
+	ctx := context.Background()
+
+	session, err := client.PendingAuthSession.Create().
+		SetSessionToken("create-account-backend-mode-session-token").
+		SetIntent("login").
+		SetProviderType("oidc").
+		SetProviderKey("https://issuer.example").
+		SetProviderSubject("oidc-create-backend-mode-123").
+		SetBrowserSessionKey("create-account-backend-mode-browser-session-key").
+		SetUpstreamIdentityClaims(map[string]any{
+			"username": "oidc_user",
+		}).
+		SetExpiresAt(time.Now().UTC().Add(10 * time.Minute)).
+		Save(ctx)
+	require.NoError(t, err)
+
+	body := bytes.NewBufferString(`{"email":"fresh@example.com","verify_code":"246810","password":"secret-123"}`)
+	recorder := httptest.NewRecorder()
+	ginCtx, _ := gin.CreateTestContext(recorder)
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/auth/oauth/oidc/create-account", body)
+	req.Header.Set("Content-Type", "application/json")
+	req.AddCookie(&http.Cookie{Name: oauthPendingSessionCookieName, Value: encodeCookieValue(session.SessionToken)})
+	req.AddCookie(&http.Cookie{Name: oauthPendingBrowserCookieName, Value: encodeCookieValue("create-account-backend-mode-browser-session-key")})
+	ginCtx.Request = req
+
+	handler.CreateOIDCOAuthAccount(ginCtx)
+
+	require.Equal(t, http.StatusForbidden, recorder.Code)
+
+	userCount, err := client.User.Query().Where(dbuser.EmailEQ("fresh@example.com")).Count(ctx)
+	require.NoError(t, err)
+	require.Zero(t, userCount)
+
+	storedSession, err := client.PendingAuthSession.Get(ctx, session.ID)
+	require.NoError(t, err)
+	require.Nil(t, storedSession.ConsumedAt)
+}
+
 func TestBindOIDCOAuthLoginBindsExistingUserAndConsumesSession(t *testing.T) {
 	handler, client := newOAuthPendingFlowTestHandler(t, false)
 	ctx := context.Background()
@@ -842,6 +950,70 @@ func TestBindOIDCOAuthLoginBindsExistingUserAndConsumesSession(t *testing.T) {
 	require.NotNil(t, storedSession.ConsumedAt)
 }
 
+func TestBindOIDCOAuthLoginBlocksBackendModeBeforeTokenIssue(t *testing.T) {
+	handler, client := newOAuthPendingFlowTestHandlerWithDependencies(t, oauthPendingFlowTestHandlerOptions{
+		settingValues: map[string]string{
+			service.SettingKeyBackendModeEnabled: "true",
+		},
+	})
+	ctx := context.Background()
+
+	passwordHash, err := handler.authService.HashPassword("secret-123")
+	require.NoError(t, err)
+
+	existingUser, err := client.User.Create().
+		SetEmail("owner@example.com").
+		SetUsername("owner-user").
+		SetPasswordHash(passwordHash).
+		SetRole(service.RoleUser).
+		SetStatus(service.StatusActive).
+		Save(ctx)
+	require.NoError(t, err)
+
+	session, err := client.PendingAuthSession.Create().
+		SetSessionToken("bind-login-backend-mode-session-token").
+		SetIntent("adopt_existing_user_by_email").
+		SetProviderType("oidc").
+		SetProviderKey("https://issuer.example").
+		SetProviderSubject("oidc-bind-backend-mode-123").
+		SetTargetUserID(existingUser.ID).
+		SetResolvedEmail(existingUser.Email).
+		SetBrowserSessionKey("bind-login-backend-mode-browser-session-key").
+		SetUpstreamIdentityClaims(map[string]any{
+			"username": "oidc_user",
+		}).
+		SetExpiresAt(time.Now().UTC().Add(10 * time.Minute)).
+		Save(ctx)
+	require.NoError(t, err)
+
+	body := bytes.NewBufferString(`{"email":"owner@example.com","password":"secret-123"}`)
+	recorder := httptest.NewRecorder()
+	ginCtx, _ := gin.CreateTestContext(recorder)
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/auth/oauth/oidc/bind-login", body)
+	req.Header.Set("Content-Type", "application/json")
+	req.AddCookie(&http.Cookie{Name: oauthPendingSessionCookieName, Value: encodeCookieValue(session.SessionToken)})
+	req.AddCookie(&http.Cookie{Name: oauthPendingBrowserCookieName, Value: encodeCookieValue("bind-login-backend-mode-browser-session-key")})
+	ginCtx.Request = req
+
+	handler.BindOIDCOAuthLogin(ginCtx)
+
+	require.Equal(t, http.StatusForbidden, recorder.Code)
+
+	identityCount, err := client.AuthIdentity.Query().
+		Where(
+			authidentity.ProviderTypeEQ("oidc"),
+			authidentity.ProviderKeyEQ("https://issuer.example"),
+			authidentity.ProviderSubjectEQ("oidc-bind-backend-mode-123"),
+		).
+		Count(ctx)
+	require.NoError(t, err)
+	require.Zero(t, identityCount)
+
+	storedSession, err := client.PendingAuthSession.Get(ctx, session.ID)
+	require.NoError(t, err)
+	require.Nil(t, storedSession.ConsumedAt)
+}
+
 func TestBindOIDCOAuthLoginRejectsInvalidPasswordWithoutConsumingSession(t *testing.T) {
 	handler, client := newOAuthPendingFlowTestHandler(t, false)
 	ctx := context.Background()
diff --git a/backend/internal/handler/auth_oidc_oauth.go b/backend/internal/handler/auth_oidc_oauth.go
index 0f9f1895..5901a953 100644
--- a/backend/internal/handler/auth_oidc_oauth.go
+++ b/backend/internal/handler/auth_oidc_oauth.go
@@ -516,6 +516,14 @@ func (h *AuthHandler) CompleteOIDCOAuthRegistration(c *gin.Context) {
 		response.ErrorFrom(c, err)
 		return
 	}
+	if err := ensurePendingOAuthCompleteRegistrationSession(session); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+	if err := h.ensureBackendModeAllowsNewUserLogin(c.Request.Context()); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
 
 	email := strings.TrimSpace(session.ResolvedEmail)
 	username := pendingSessionStringValue(session.UpstreamIdentityClaims, "username")
@@ -541,6 +549,7 @@ func (h *AuthHandler) CompleteOIDCOAuthRegistration(c *gin.Context) {
 		response.ErrorFrom(c, infraerrors.InternalServer("PENDING_AUTH_ADOPTION_APPLY_FAILED", "failed to apply oauth profile adoption").WithCause(err))
 		return
 	}
+	h.authService.RecordSuccessfulLogin(c.Request.Context(), user.ID)
 	if _, err := pendingSvc.ConsumeBrowserSession(c.Request.Context(), sessionToken, browserSessionKey); err != nil {
 		clearOAuthPendingSessionCookie(c, secureCookie)
 		clearOAuthPendingBrowserCookie(c, secureCookie)
diff --git a/backend/internal/handler/auth_oidc_oauth_test.go b/backend/internal/handler/auth_oidc_oauth_test.go
index 07f5ef68..ba736db2 100644
--- a/backend/internal/handler/auth_oidc_oauth_test.go
+++ b/backend/internal/handler/auth_oidc_oauth_test.go
@@ -431,6 +431,58 @@ func TestCompleteOIDCOAuthRegistrationAppliesPendingAdoptionDecision(t *testing.
 	require.NotNil(t, consumed.ConsumedAt)
 }
 
+func TestCompleteOIDCOAuthRegistrationRejectsAdoptExistingUserSession(t *testing.T) {
+	handler, client := newOAuthPendingFlowTestHandler(t, false)
+	ctx := context.Background()
+
+	existingUser, err := client.User.Create().
+		SetEmail("owner@example.com").
+		SetUsername("owner-user").
+		SetPasswordHash("hash").
+		SetRole(service.RoleUser).
+		SetStatus(service.StatusActive).
+		Save(ctx)
+	require.NoError(t, err)
+
+	session, err := client.PendingAuthSession.Create().
+		SetSessionToken("oidc-complete-invalid-session").
+		SetIntent("adopt_existing_user_by_email").
+		SetProviderType("oidc").
+		SetProviderKey("https://issuer.example.com").
+		SetProviderSubject("oidc-invalid-subject-1").
+		SetTargetUserID(existingUser.ID).
+		SetResolvedEmail(existingUser.Email).
+		SetBrowserSessionKey("oidc-invalid-browser").
+		SetUpstreamIdentityClaims(map[string]any{
+			"username": "oidc_user",
+		}).
+		SetLocalFlowState(map[string]any{
+			oauthCompletionResponseKey: map[string]any{
+				"step": "bind_login_required",
+			},
+		}).
+		SetExpiresAt(time.Now().UTC().Add(10 * time.Minute)).
+		Save(ctx)
+	require.NoError(t, err)
+
+	body := bytes.NewBufferString(`{"invitation_code":"invite-1"}`)
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/auth/oauth/oidc/complete-registration", body)
+	req.Header.Set("Content-Type", "application/json")
+	req.AddCookie(&http.Cookie{Name: oauthPendingSessionCookieName, Value: encodeCookieValue(session.SessionToken)})
+	req.AddCookie(&http.Cookie{Name: oauthPendingBrowserCookieName, Value: encodeCookieValue("oidc-invalid-browser")})
+	c.Request = req
+
+	handler.CompleteOIDCOAuthRegistration(c)
+
+	require.Equal(t, http.StatusBadRequest, recorder.Code)
+
+	storedSession, err := client.PendingAuthSession.Get(ctx, session.ID)
+	require.NoError(t, err)
+	require.Nil(t, storedSession.ConsumedAt)
+}
+
 type oidcProviderFixture struct {
 	Subject           string
 	PreferredUsername string
diff --git a/backend/internal/handler/auth_wechat_oauth.go b/backend/internal/handler/auth_wechat_oauth.go
index 45de30a8..5e697fb5 100644
--- a/backend/internal/handler/auth_wechat_oauth.go
+++ b/backend/internal/handler/auth_wechat_oauth.go
@@ -506,6 +506,14 @@ func (h *AuthHandler) CompleteWeChatOAuthRegistration(c *gin.Context) {
 		response.ErrorFrom(c, err)
 		return
 	}
+	if err := ensurePendingOAuthCompleteRegistrationSession(session); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+	if err := h.ensureBackendModeAllowsNewUserLogin(c.Request.Context()); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
 
 	email := strings.TrimSpace(session.ResolvedEmail)
 	username := pendingSessionStringValue(session.UpstreamIdentityClaims, "username")
@@ -531,6 +539,7 @@ func (h *AuthHandler) CompleteWeChatOAuthRegistration(c *gin.Context) {
 		response.ErrorFrom(c, infraerrors.InternalServer("PENDING_AUTH_ADOPTION_APPLY_FAILED", "failed to apply oauth profile adoption").WithCause(err))
 		return
 	}
+	h.authService.RecordSuccessfulLogin(c.Request.Context(), user.ID)
 	if _, err := pendingSvc.ConsumeBrowserSession(c.Request.Context(), sessionToken, browserSessionKey); err != nil {
 		clearOAuthPendingSessionCookie(c, secureCookie)
 		clearOAuthPendingBrowserCookie(c, secureCookie)
diff --git a/backend/internal/handler/auth_wechat_oauth_test.go b/backend/internal/handler/auth_wechat_oauth_test.go
index c65f4cd1..cd34f52f 100644
--- a/backend/internal/handler/auth_wechat_oauth_test.go
+++ b/backend/internal/handler/auth_wechat_oauth_test.go
@@ -846,6 +846,59 @@ func TestWeChatOAuthCallbackRepairsLegacyOpenIDOnlyIdentity(t *testing.T) {
 	require.Equal(t, repairedIdentity.ID, channel.IdentityID)
 }
 
+func TestCompleteWeChatOAuthRegistrationRejectsAdoptExistingUserSession(t *testing.T) {
+	handler, client := newWeChatOAuthTestHandler(t, false)
+	defer client.Close()
+
+	ctx := context.Background()
+	existingUser, err := client.User.Create().
+		SetEmail("owner@example.com").
+		SetUsername("owner-user").
+		SetPasswordHash("hash").
+		SetRole(service.RoleUser).
+		SetStatus(service.StatusActive).
+		Save(ctx)
+	require.NoError(t, err)
+
+	session, err := client.PendingAuthSession.Create().
+		SetSessionToken("wechat-complete-invalid-session").
+		SetIntent("adopt_existing_user_by_email").
+		SetProviderType("wechat").
+		SetProviderKey("wechat-main").
+		SetProviderSubject("union-invalid-1").
+		SetTargetUserID(existingUser.ID).
+		SetResolvedEmail(existingUser.Email).
+		SetBrowserSessionKey("wechat-invalid-browser").
+		SetUpstreamIdentityClaims(map[string]any{
+			"username": "wechat_user",
+		}).
+		SetLocalFlowState(map[string]any{
+			oauthCompletionResponseKey: map[string]any{
+				"step": "bind_login_required",
+			},
+		}).
+		SetExpiresAt(time.Now().UTC().Add(10 * time.Minute)).
+		Save(ctx)
+	require.NoError(t, err)
+
+	body := bytes.NewBufferString(`{"invitation_code":"invite-1"}`)
+	recorder := httptest.NewRecorder()
+	completeCtx, _ := gin.CreateTestContext(recorder)
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/auth/oauth/wechat/complete-registration", body)
+	req.Header.Set("Content-Type", "application/json")
+	req.AddCookie(&http.Cookie{Name: oauthPendingSessionCookieName, Value: encodeCookieValue(session.SessionToken)})
+	req.AddCookie(&http.Cookie{Name: oauthPendingBrowserCookieName, Value: encodeCookieValue("wechat-invalid-browser")})
+	completeCtx.Request = req
+
+	handler.CompleteWeChatOAuthRegistration(completeCtx)
+
+	require.Equal(t, http.StatusBadRequest, recorder.Code)
+
+	storedSession, err := client.PendingAuthSession.Get(ctx, session.ID)
+	require.NoError(t, err)
+	require.Nil(t, storedSession.ConsumedAt)
+}
+
 func TestWeChatOAuthCallbackRepairsLegacyProviderKeyCanonicalIdentity(t *testing.T) {
 	t.Setenv("WECHAT_OAUTH_OPEN_APP_ID", "wx-open-app")
 	t.Setenv("WECHAT_OAUTH_OPEN_APP_SECRET", "wx-open-secret")
diff --git a/backend/internal/service/auth_oauth_email_flow.go b/backend/internal/service/auth_oauth_email_flow.go
index 4ab4e245..2e0107ae 100644
--- a/backend/internal/service/auth_oauth_email_flow.go
+++ b/backend/internal/service/auth_oauth_email_flow.go
@@ -104,7 +104,7 @@ func (s *AuthService) RegisterOAuthEmailAccount(
 		return nil, nil, ErrServiceUnavailable
 	}
 
-	s.postAuthUserBootstrap(ctx, user, signupSource, true)
+	s.postAuthUserBootstrap(ctx, user, signupSource, false)
 	s.assignSubscriptions(ctx, user.ID, grantPlan.Subscriptions, "auto assigned by signup defaults")
 
 	if invitationRedeemCode != nil {
diff --git a/backend/internal/service/auth_service.go b/backend/internal/service/auth_service.go
index 00fefd82..d63a8753 100644
--- a/backend/internal/service/auth_service.go
+++ b/backend/internal/service/auth_service.go
@@ -430,8 +430,6 @@ func (s *AuthService) Login(ctx context.Context, email, password string) (string
 	if !user.IsActive() {
 		return "", nil, ErrUserNotActive
 	}
-	s.backfillEmailIdentityOnSuccessfulLogin(ctx, user)
-	s.touchUserLogin(ctx, user.ID)
 
 	// 生成JWT token
 	token, err := s.GenerateToken(user)
@@ -507,7 +505,7 @@ func (s *AuthService) LoginOrRegisterOAuth(ctx context.Context, email, username
 				}
 			} else {
 				user = newUser
-				s.postAuthUserBootstrap(ctx, user, signupSource, true)
+				s.postAuthUserBootstrap(ctx, user, signupSource, false)
 				s.assignSubscriptions(ctx, user.ID, grantPlan.Subscriptions, "auto assigned by signup defaults")
 			}
 		} else {
@@ -527,8 +525,6 @@ func (s *AuthService) LoginOrRegisterOAuth(ctx context.Context, email, username
 			logger.LegacyPrintf("service.auth", "[Auth] Failed to update username after oauth login: %v", err)
 		}
 	}
-	s.touchUserLogin(ctx, user.ID)
-
 	token, err := s.GenerateToken(user)
 	if err != nil {
 		return "", nil, fmt.Errorf("generate token: %w", err)
@@ -634,7 +630,7 @@ func (s *AuthService) LoginOrRegisterOAuthWithTokenPair(ctx context.Context, ema
 						return nil, nil, ErrServiceUnavailable
 					}
 					user = newUser
-					s.postAuthUserBootstrap(ctx, user, signupSource, true)
+					s.postAuthUserBootstrap(ctx, user, signupSource, false)
 					s.assignSubscriptions(ctx, user.ID, grantPlan.Subscriptions, "auto assigned by signup defaults")
 				}
 			} else {
@@ -651,7 +647,7 @@ func (s *AuthService) LoginOrRegisterOAuthWithTokenPair(ctx context.Context, ema
 					}
 				} else {
 					user = newUser
-					s.postAuthUserBootstrap(ctx, user, signupSource, true)
+					s.postAuthUserBootstrap(ctx, user, signupSource, false)
 					s.assignSubscriptions(ctx, user.ID, grantPlan.Subscriptions, "auto assigned by signup defaults")
 					if invitationRedeemCode != nil {
 						if err := s.redeemRepo.Use(ctx, invitationRedeemCode.ID, user.ID); err != nil {
@@ -676,8 +672,6 @@ func (s *AuthService) LoginOrRegisterOAuthWithTokenPair(ctx context.Context, ema
 			logger.LegacyPrintf("service.auth", "[Auth] Failed to update username after oauth login: %v", err)
 		}
 	}
-	s.touchUserLogin(ctx, user.ID)
-
 	tokenPair, err := s.GenerateTokenPair(ctx, user, "")
 	if err != nil {
 		return nil, nil, fmt.Errorf("generate token pair: %w", err)
diff --git a/backend/internal/service/auth_service_identity_sync_test.go b/backend/internal/service/auth_service_identity_sync_test.go
index 95c9c933..85c13604 100644
--- a/backend/internal/service/auth_service_identity_sync_test.go
+++ b/backend/internal/service/auth_service_identity_sync_test.go
@@ -170,24 +170,26 @@ func TestAuthServiceRegisterDualWritesEmailIdentity(t *testing.T) {
 	require.NotNil(t, identity.VerifiedAt)
 }
 
-func TestAuthServiceLoginTouchesLastLoginAt(t *testing.T) {
-	svc, repo, client := newAuthServiceWithEnt(t, map[string]string{
+func TestAuthServiceLoginDefersLastLoginTouchUntilRecordSuccessfulLogin(t *testing.T) {
+	svc, _, client := newAuthServiceWithEnt(t, map[string]string{
 		service.SettingKeyRegistrationEnabled: "true",
 	}, nil)
 	ctx := context.Background()
 
-	user := &service.User{
-		Email:       "login@example.com",
-		Role:        service.RoleUser,
-		Status:      service.StatusActive,
-		Balance:     1,
-		Concurrency: 1,
-	}
-	require.NoError(t, user.SetPassword("password"))
-	require.NoError(t, repo.Create(ctx, user))
+	passwordHash, err := svc.HashPassword("password")
+	require.NoError(t, err)
+	user, err := client.User.Create().
+		SetEmail("login@example.com").
+		SetPasswordHash(passwordHash).
+		SetRole(service.RoleUser).
+		SetStatus(service.StatusActive).
+		SetBalance(1).
+		SetConcurrency(1).
+		Save(ctx)
+	require.NoError(t, err)
 
 	old := time.Now().Add(-2 * time.Hour).UTC().Round(time.Second)
-	_, err := client.User.UpdateOneID(user.ID).
+	_, err = client.User.UpdateOneID(user.ID).
 		SetLastLoginAt(old).
 		SetLastActiveAt(old).
 		Save(ctx)
@@ -202,8 +204,20 @@ func TestAuthServiceLoginTouchesLastLoginAt(t *testing.T) {
 	require.NoError(t, err)
 	require.NotNil(t, storedUser.LastLoginAt)
 	require.NotNil(t, storedUser.LastActiveAt)
-	require.True(t, storedUser.LastLoginAt.After(old))
-	require.True(t, storedUser.LastActiveAt.After(old))
+	require.True(t, storedUser.LastLoginAt.Equal(old))
+	require.True(t, storedUser.LastActiveAt.Equal(old))
+
+	identityCount, err := client.AuthIdentity.Query().
+		Where(
+			authidentity.ProviderTypeEQ("email"),
+			authidentity.ProviderKeyEQ("email"),
+			authidentity.ProviderSubjectEQ("login@example.com"),
+		).
+		Count(ctx)
+	require.NoError(t, err)
+	require.Zero(t, identityCount)
+
+	svc.RecordSuccessfulLogin(ctx, user.ID)
 
 	identity, err := client.AuthIdentity.Query().
 		Where(
@@ -273,6 +287,7 @@ func TestAuthServiceLogin_AppliesEmailFirstBindDefaultsOnlyWhenEmailIdentityIsNe
 	require.NoError(t, err)
 	require.NotEmpty(t, token)
 	require.NotNil(t, gotUser)
+	svc.RecordSuccessfulLogin(ctx, user.ID)
 
 	storedUser, err := client.User.Get(ctx, user.ID)
 	require.NoError(t, err)
@@ -343,6 +358,7 @@ func TestAuthServiceLogin_DoesNotApplyEmailFirstBindDefaultsWhenIdentityAlreadyE
 	require.NoError(t, err)
 	require.NotEmpty(t, token)
 	require.NotNil(t, gotUser)
+	svc.RecordSuccessfulLogin(ctx, user.ID)
 
 	storedUser, err := client.User.Get(ctx, user.ID)
 	require.NoError(t, err)
@@ -380,6 +396,7 @@ func TestAuthServiceLogin_RetriesEmailFirstBindDefaultsAfterPreviousFailure(t *t
 	require.NoError(t, err)
 	require.NotEmpty(t, token)
 	require.NotNil(t, gotUser)
+	svc.RecordSuccessfulLogin(ctx, user.ID)
 
 	storedUser, err := client.User.Get(ctx, user.ID)
 	require.NoError(t, err)
@@ -392,6 +409,7 @@ func TestAuthServiceLogin_RetriesEmailFirstBindDefaultsAfterPreviousFailure(t *t
 	require.NoError(t, err)
 	require.NotEmpty(t, token)
 	require.NotNil(t, gotUser)
+	svc.RecordSuccessfulLogin(ctx, user.ID)
 
 	storedUser, err = client.User.Get(ctx, user.ID)
 	require.NoError(t, err)