diff --git a/src/api/billingApi.ts b/src/api/billingApi.ts
index 69ebf34..7c78496 100644
--- a/src/api/billingApi.ts
+++ b/src/api/billingApi.ts
@@ -36,11 +36,16 @@ export interface BillingTransaction {
   description: string;
   referenceType?: ReferenceType;
   workflowId?: string;
+  featureInstanceId?: string;
   featureCode?: string;
   aicoreProvider?: string;
+  aicoreModel?: string;
+  createdByUserId?: string;
   createdAt?: string;
   mandateId?: string;
   mandateName?: string;
+  userId?: string;
+  userName?: string;
 }
 
 export interface BillingSettings {
@@ -70,6 +75,7 @@ export interface UsageReport {
   totalCost: number;
   transactionCount: number;
   costByProvider: Record<string, number>;
+  costByModel: Record<string, number>;
   costByFeature: Record<string, number>;
 }
 
@@ -260,6 +266,7 @@ export async function fetchTransactionsAdmin(
  */
 export interface MandateUserSummary {
   id: string;
+  username?: string;
   email?: string;
   firstName?: string;
   lastName?: string;
diff --git a/src/components/ProviderSelector/ProviderSelector.tsx b/src/components/ProviderSelector/ProviderSelector.tsx
index 55596a8..b9706d3 100644
--- a/src/components/ProviderSelector/ProviderSelector.tsx
+++ b/src/components/ProviderSelector/ProviderSelector.tsx
@@ -137,21 +137,19 @@ export const ProviderMultiSelect: React.FC<ProviderMultiSelectProps> = ({
     }
   }, [isExpanded, handleClickOutside]);
   
-  // Check if all providers are selected (or none selected = all used)
-  const isAllSelected = selectedProviders.length === 0 || 
-    (allowedProviders.length > 0 && selectedProviders.length === allowedProviders.length);
+  // Check if all providers are explicitly selected
+  const isAllSelected = allowedProviders.length > 0 && selectedProviders.length === allowedProviders.length;
   
-  // For checkbox display: if none selected, show all as checked (since all are used)
-  const effectiveSelection = selectedProviders.length === 0 ? allowedProviders : selectedProviders;
+  // Check if no providers are selected (= no restriction, all allowed by default)
+  const isNoneSelected = selectedProviders.length === 0;
   
   const handleToggle = (provider: string) => {
-    // Use effectiveSelection for toggle logic (handles empty = all case)
-    if (effectiveSelection.includes(provider)) {
-      // Deactivate: remove from effective selection
-      onChange(effectiveSelection.filter((p) => p !== provider));
+    if (selectedProviders.includes(provider)) {
+      // Deactivate: remove from selection
+      onChange(selectedProviders.filter((p) => p !== provider));
     } else {
-      // Activate: add to effective selection
-      onChange([...effectiveSelection, provider]);
+      // Activate: add to selection
+      onChange([...selectedProviders, provider]);
     }
   };
   
@@ -165,14 +163,11 @@ export const ProviderMultiSelect: React.FC<ProviderMultiSelectProps> = ({
   
   // Summary icon for button
   const summaryIcon = useMemo(() => {
-    if (selectedProviders.length === 0 || selectedProviders.length === allowedProviders.length) {
-      return '🤖';
-    }
     if (selectedProviders.length === 1) {
       return PROVIDER_ICONS[selectedProviders[0]] || '🔌';
     }
     return '🤖';
-  }, [selectedProviders, allowedProviders]);
+  }, [selectedProviders]);
   
   return (
     <div 
@@ -208,7 +203,7 @@ export const ProviderMultiSelect: React.FC<ProviderMultiSelectProps> = ({
               type="button" 
               onClick={handleSelectNone} 
               disabled={disabled}
-              className={styles.actionButton}
+              className={`${styles.actionButton} ${isNoneSelected ? styles.active : ''}`}
             >
               Keine
             </button>
@@ -225,7 +220,7 @@ export const ProviderMultiSelect: React.FC<ProviderMultiSelectProps> = ({
                 >
                   <input
                     type="checkbox"
-                    checked={effectiveSelection.includes(provider)}
+                    checked={selectedProviders.includes(provider)}
                     onChange={() => handleToggle(provider)}
                     disabled={disabled}
                   />
diff --git a/src/pages/billing/BillingAdmin.tsx b/src/pages/billing/BillingAdmin.tsx
index 8efb3cf..8a27eda 100644
--- a/src/pages/billing/BillingAdmin.tsx
+++ b/src/pages/billing/BillingAdmin.tsx
@@ -268,7 +268,7 @@ const CreditAdder: React.FC<CreditAdderProps> = ({ settings, accounts, users, on
                   const balanceInfo = account ? ` (${formatCurrency(account.balance)})` : ' (kein Konto)';
                   return (
                     <option key={user.id} value={user.id}>
-                      {user.displayName || user.email || user.id}{balanceInfo}
+                      {user.displayName || user.username || user.id}{balanceInfo}
                     </option>
                   );
                 })}
@@ -339,7 +339,7 @@ const AccountsOverview: React.FC<AccountsOverviewProps> = ({ accounts, users, lo
     for (const user of users) {
       const displayName = user.displayName
         || [user.firstName, user.lastName].filter(Boolean).join(' ')
-        || user.email
+        || user.username
         || user.id;
       map.set(user.id, displayName);
     }
diff --git a/src/pages/billing/BillingDashboard.tsx b/src/pages/billing/BillingDashboard.tsx
index 4e3f963..8e73efb 100644
--- a/src/pages/billing/BillingDashboard.tsx
+++ b/src/pages/billing/BillingDashboard.tsx
@@ -114,6 +114,28 @@ const StatisticsChart: React.FC<StatisticsChartProps> = ({ statistics, loading }
         )}
       </div>
       
+      <div className={styles.chartSection}>
+        <h4>Kosten nach Modell</h4>
+        {Object.entries(statistics.costByModel || {}).length === 0 ? (
+          <div className={styles.noData}>Keine Daten</div>
+        ) : (
+          <div className={styles.barChart}>
+            {Object.entries(statistics.costByModel || {}).map(([model, cost]) => (
+              <div key={model} className={styles.barRow}>
+                <span className={styles.barLabel}>{model}</span>
+                <div className={styles.barContainer}>
+                  <div 
+                    className={styles.bar}
+                    style={{ width: `${(cost / maxProviderCost) * 100}%` }}
+                  />
+                </div>
+                <span className={styles.barValue}>{formatCurrency(cost)}</span>
+              </div>
+            ))}
+          </div>
+        )}
+      </div>
+      
       <div className={styles.chartSection}>
         <h4>Kosten nach Feature</h4>
         {Object.entries(statistics.costByFeature).length === 0 ? (
diff --git a/src/pages/billing/BillingDataView.tsx b/src/pages/billing/BillingDataView.tsx
index f11360d..504bedc 100644
--- a/src/pages/billing/BillingDataView.tsx
+++ b/src/pages/billing/BillingDataView.tsx
@@ -35,6 +35,7 @@ interface ViewStatistics {
   totalCost: number;
   transactionCount: number;
   costByProvider: Record<string, number>;
+  costByModel: Record<string, number>;
   costByFeature: Record<string, number>;
   costByMandate: Record<string, number>;
   timeSeries: Array<{ date: string; cost: number; count: number }>;
@@ -134,6 +135,7 @@ function _recordToChartData(record: Record<string, number>): ReportChartDataPoin
 
 function _buildOverviewSections(viewStats: ViewStatistics): ReportSection[] {
   const topProvider = Object.entries(viewStats.costByProvider).sort((a, b) => b[1] - a[1])[0];
+  const topModel = Object.entries(viewStats.costByModel || {}).sort((a, b) => b[1] - a[1])[0];
   const topFeature = Object.entries(viewStats.costByFeature).sort((a, b) => b[1] - a[1])[0];
 
   return [
@@ -150,15 +152,15 @@ function _buildOverviewSections(viewStats: ViewStatistics): ReportSection[] {
           value: Object.keys(viewStats.costByProvider).length,
           subtitle: topProvider ? `Top: ${topProvider[0]}` : 'Keine Nutzung'
         },
+        {
+          label: 'Modelle',
+          value: Object.keys(viewStats.costByModel || {}).length,
+          subtitle: topModel ? `Top: ${topModel[0]}` : 'Keine Nutzung'
+        },
         {
           label: 'Features',
           value: Object.keys(viewStats.costByFeature).length,
           subtitle: topFeature ? `Top: ${topFeature[0]}` : 'Keine Nutzung'
-        },
-        {
-          label: 'Mandanten',
-          value: Object.keys(viewStats.costByMandate).length,
-          subtitle: 'aktiv genutzt'
         }
       ]
     },
@@ -169,6 +171,13 @@ function _buildOverviewSections(viewStats: ViewStatistics): ReportSection[] {
       formatValue: _formatCurrency,
       span: 'half' as const
     },
+    {
+      type: 'horizontalBar',
+      title: 'Kosten nach Modell',
+      data: _recordToChartData(viewStats.costByModel || {}),
+      formatValue: _formatCurrency,
+      span: 'half' as const
+    },
     {
       type: 'horizontalBar',
       title: 'Kosten nach Feature',
@@ -206,6 +215,14 @@ function _buildStatisticsSections(viewStats: ViewStatistics): ReportSection[] {
       donut: true,
       span: 'half' as const
     },
+    {
+      type: 'pieChart',
+      title: 'Verteilung nach Modell',
+      data: _recordToChartData(viewStats.costByModel || {}),
+      formatValue: _formatCurrency,
+      donut: true,
+      span: 'half' as const
+    },
     {
       type: 'pieChart',
       title: 'Verteilung nach Feature',
@@ -234,6 +251,7 @@ function _buildStatisticsSections(viewStats: ViewStatistics): ReportSection[] {
         { metric: 'Transaktionen', value: String(viewStats.transactionCount) },
         { metric: 'Durchschnitt / Transaktion', value: _formatCurrency(avgCost) },
         { metric: 'Anbieter', value: String(Object.keys(viewStats.costByProvider).length) },
+        { metric: 'Modelle', value: String(Object.keys(viewStats.costByModel || {}).length) },
         { metric: 'Features', value: String(Object.keys(viewStats.costByFeature).length) },
         { metric: 'Mandanten', value: String(Object.keys(viewStats.costByMandate).length) }
       ]
@@ -304,8 +322,10 @@ export const BillingDataView: React.FC = () => {
       setTransactionsError(null);
       
       const params: any = {};
-      if (paginationParams) {
-        params.pagination = JSON.stringify(paginationParams);
+      // Only serialize if it's a plain pagination object (not a React event or other non-serializable object)
+      if (paginationParams && typeof paginationParams === 'object' && 'page' in paginationParams) {
+        const { page, pageSize, sortBy, sortDirection, search, filters } = paginationParams;
+        params.pagination = JSON.stringify({ page, pageSize, sortBy, sortDirection, search, filters });
       }
       
       const response = await api.get('/api/billing/view/users/transactions', { params });
@@ -353,6 +373,7 @@ export const BillingDataView: React.FC = () => {
     { key: 'transactionType', label: 'Typ', type: 'text' as any, sortable: true, filterable: true, width: 100 },
     { key: 'description', label: 'Beschreibung', type: 'text' as any, searchable: true, width: 250 },
     { key: 'aicoreProvider', label: 'Anbieter', type: 'text' as any, sortable: true, filterable: true, width: 120 },
+    { key: 'aicoreModel', label: 'Modell', type: 'text' as any, sortable: true, filterable: true, width: 150 },
     { key: 'featureCode', label: 'Feature', type: 'text' as any, sortable: true, filterable: true, width: 120 },
     { key: 'amount', label: 'Betrag (CHF)', type: 'number' as any, sortable: true, width: 120 },
   ], []);
diff --git a/src/pages/billing/BillingMandateView.tsx b/src/pages/billing/BillingMandateView.tsx
index 82d022f..f6792c6 100644
--- a/src/pages/billing/BillingMandateView.tsx
+++ b/src/pages/billing/BillingMandateView.tsx
@@ -146,6 +146,7 @@ const TransactionTable: React.FC<TransactionTableProps> = ({ transactions }) =>
             <th>Typ</th>
             <th>Beschreibung</th>
             <th>Anbieter</th>
+            <th>Modell</th>
             <th>Feature</th>
             <th style={{ textAlign: 'right' }}>Betrag</th>
           </tr>
@@ -162,6 +163,7 @@ const TransactionTable: React.FC<TransactionTableProps> = ({ transactions }) =>
               </td>
               <td>{t.description}</td>
               <td>{t.aicoreProvider || '-'}</td>
+              <td>{t.aicoreModel || '-'}</td>
               <td>{t.featureCode || '-'}</td>
               <td style={{ textAlign: 'right' }}>
                 {t.transactionType === 'DEBIT' ? '-' : '+'}{formatCurrency(t.amount)}
diff --git a/src/pages/billing/BillingTransactions.tsx b/src/pages/billing/BillingTransactions.tsx
index ddd7f2c..d64bde7 100644
--- a/src/pages/billing/BillingTransactions.tsx
+++ b/src/pages/billing/BillingTransactions.tsx
@@ -65,6 +65,7 @@ const TransactionRow: React.FC<TransactionRowProps> = ({ transaction }) => {
       </td>
       <td>{transaction.description}</td>
       <td>{transaction.aicoreProvider || '-'}</td>
+      <td>{transaction.aicoreModel || '-'}</td>
       <td>{transaction.featureCode || '-'}</td>
       <td style={{ textAlign: 'right' }}>
         {transaction.transactionType === 'DEBIT' ? '-' : '+'}{formatCurrency(transaction.amount)}
@@ -114,6 +115,7 @@ export const BillingTransactions: React.FC = () => {
                     <th>Typ</th>
                     <th>Beschreibung</th>
                     <th>Anbieter</th>
+                    <th>Modell</th>
                     <th>Feature</th>
                     <th style={{ textAlign: 'right' }}>Betrag</th>
                   </tr>
diff --git a/src/pages/billing/BillingUserView.tsx b/src/pages/billing/BillingUserView.tsx
index 194be7d..4587acc 100644
--- a/src/pages/billing/BillingUserView.tsx
+++ b/src/pages/billing/BillingUserView.tsx
@@ -228,6 +228,7 @@ const UserTransactionTable: React.FC<UserTransactionTableProps> = ({
             <th>Typ</th>
             <th>Beschreibung</th>
             <th>Anbieter</th>
+            <th>Modell</th>
             <th>Feature</th>
             <th style={{ textAlign: 'right' }}>Betrag</th>
           </tr>
@@ -245,6 +246,7 @@ const UserTransactionTable: React.FC<UserTransactionTableProps> = ({
               </td>
               <td>{t.description}</td>
               <td>{t.aicoreProvider || '-'}</td>
+              <td>{t.aicoreModel || '-'}</td>
               <td>{t.featureCode || '-'}</td>
               <td style={{ textAlign: 'right' }}>
                 {t.transactionType === 'DEBIT' ? '-' : '+'}{formatCurrency(t.amount)}