diff --git a/src/features/providers/sheets/forms/BaseProviderForm.tsx b/src/features/providers/sheets/forms/BaseProviderForm.tsx
index 8290634..b025e70 100644
--- a/src/features/providers/sheets/forms/BaseProviderForm.tsx
+++ b/src/features/providers/sheets/forms/BaseProviderForm.tsx
@@ -226,6 +226,13 @@ export function BaseProviderForm({
     return (resource.raw as { apiKey?: string } | undefined)?.apiKey ?? '';
   }, [brand, mode, resource]);
 
+  const fallbackAuthIndex = useMemo(() => {
+    if (mode !== 'edit' || !resource) return '';
+    return (
+      (resource.raw as { authIndex?: string } | undefined)?.authIndex ?? ''
+    );
+  }, [mode, resource]);
+
   const connectivityMessages = useMemo<ConnectivityErrorMessages>(
     () => ({
       baseUrlRequired: t('providersPage.connectivity.baseUrlRequired'),
@@ -249,6 +256,7 @@ export function BaseProviderForm({
       apiKeyEntries: form.apiKeyEntries,
       apiKey: form.apiKey,
       fallbackApiKey,
+      authIndex: fallbackAuthIndex,
     },
     connectivityMessages
   );
@@ -260,6 +268,7 @@ export function BaseProviderForm({
     apiKeyEntries: form.apiKeyEntries,
     apiKey: form.apiKey,
     fallbackApiKey,
+    authIndex: fallbackAuthIndex,
   });
   const [discoveryOpen, setDiscoveryOpen] = useState(false);
 
diff --git a/src/features/providers/sheets/forms/useConnectivityTest.ts b/src/features/providers/sheets/forms/useConnectivityTest.ts
index 900802c..b8c2f8d 100644
--- a/src/features/providers/sheets/forms/useConnectivityTest.ts
+++ b/src/features/providers/sheets/forms/useConnectivityTest.ts
@@ -77,6 +77,7 @@ export interface UseConnectivityTestArgs {
   apiKeyEntries?: ApiKeyEntryInput[];
   apiKey?: string;
   fallbackApiKey?: string;
+  authIndex?: string;
 }
 
 export interface ConnectivityErrorMessages {
@@ -110,6 +111,7 @@ export function useConnectivityTest(
     apiKeyEntries,
     apiKey,
     fallbackApiKey,
+    authIndex,
   } = args;
 
   const entriesCount = apiKeyEntries?.length ?? 0;
@@ -302,8 +304,9 @@ export function useConnectivityTest(
     const headerKey = resolveBearerToken(customHeaders);
     const hasApiKeyHeader = hasHeader(customHeaders, 'x-api-key');
     const resolvedKey = explicitKey || persistedKey || headerKey;
+    const resolvedAuthIndex = (authIndex ?? '').trim() || undefined;
 
-    if (!resolvedKey && !hasApiKeyHeader) {
+    if (!resolvedKey && !hasApiKeyHeader && !resolvedAuthIndex) {
       setClaudeStatus({ state: 'error', message: messages.apiKeyRequired });
       return;
     }
@@ -324,6 +327,7 @@ export function useConnectivityTest(
     try {
       const result = await apiCallApi.request(
         {
+          authIndex: resolvedAuthIndex,
           method: 'POST',
           url: endpoint,
           header: headerObj,
@@ -358,6 +362,7 @@ export function useConnectivityTest(
     }
   }, [
     apiKey,
+    authIndex,
     baseUrl,
     brand,
     fallbackApiKey,
diff --git a/src/features/providers/sheets/forms/useModelDiscovery.ts b/src/features/providers/sheets/forms/useModelDiscovery.ts
index 5014b7c..874b5cb 100644
--- a/src/features/providers/sheets/forms/useModelDiscovery.ts
+++ b/src/features/providers/sheets/forms/useModelDiscovery.ts
@@ -44,6 +44,7 @@ export interface UseModelDiscoveryArgs {
   apiKeyEntries?: ApiKeyEntryInput[];
   apiKey?: string;
   fallbackApiKey?: string;
+  authIndex?: string;
 }
 
 export interface UseModelDiscoveryResult {
@@ -59,8 +60,15 @@ export interface UseModelDiscoveryResult {
 export function useModelDiscovery(
   args: UseModelDiscoveryArgs
 ): UseModelDiscoveryResult {
-  const { brand, baseUrl, formHeaders, apiKeyEntries, apiKey, fallbackApiKey } =
-    args;
+  const {
+    brand,
+    baseUrl,
+    formHeaders,
+    apiKeyEntries,
+    apiKey,
+    fallbackApiKey,
+    authIndex,
+  } = args;
 
   const available = isModelDiscoveryBrand(brand);
   const [loading, setLoading] = useState(false);
@@ -74,27 +82,31 @@ export function useModelDiscovery(
     setError(null);
     try {
       const baseHeaders = buildHeaderObject(formHeaders);
+      const resolvedAuthIndex = (authIndex ?? '').trim() || undefined;
       let next: ModelInfo[] = [];
       if (brand === 'gemini') {
         const key = (apiKey ?? '').trim() || (fallbackApiKey ?? '').trim();
         next = await modelsApi.fetchGeminiModelsViaApiCall(
           baseUrl,
           key,
-          baseHeaders
+          baseHeaders,
+          resolvedAuthIndex
         );
       } else if (brand === 'codex') {
         const key = (apiKey ?? '').trim() || (fallbackApiKey ?? '').trim();
         next = await modelsApi.fetchV1ModelsViaApiCall(
           baseUrl,
           key,
-          baseHeaders
+          baseHeaders,
+          resolvedAuthIndex
         );
       } else if (brand === 'claude') {
         const key = (apiKey ?? '').trim() || (fallbackApiKey ?? '').trim();
         next = await modelsApi.fetchClaudeModelsViaApiCall(
           baseUrl,
           key,
-          baseHeaders
+          baseHeaders,
+          resolvedAuthIndex
         );
       } else if (brand === 'openaiCompatibility') {
         const firstEntry = (apiKeyEntries ?? []).find((e) =>
@@ -129,7 +141,16 @@ export function useModelDiscovery(
     } finally {
       setLoading(false);
     }
-  }, [available, apiKey, apiKeyEntries, baseUrl, brand, fallbackApiKey, formHeaders]);
+  }, [
+    available,
+    apiKey,
+    apiKeyEntries,
+    authIndex,
+    baseUrl,
+    brand,
+    fallbackApiKey,
+    formHeaders,
+  ]);
 
   const reset = useCallback(() => {
     setModels([]);
diff --git a/src/services/api/models.ts b/src/services/api/models.ts
index b18886d..9977d5b 100644
--- a/src/services/api/models.ts
+++ b/src/services/api/models.ts
@@ -108,7 +108,8 @@ export const modelsApi = {
   async fetchV1ModelsViaApiCall(
     baseUrl: string,
     apiKey?: string,
-    headers: Record<string, string> = {}
+    headers: Record<string, string> = {},
+    authIndex?: string
   ) {
     const endpoint = buildV1ModelsEndpoint(baseUrl);
     if (!endpoint) {
@@ -121,6 +122,7 @@ export const modelsApi = {
     }
 
     const result = await apiCallApi.request({
+      authIndex: authIndex?.trim() || undefined,
       method: 'GET',
       url: endpoint,
       header: Object.keys(resolvedHeaders).length ? resolvedHeaders : undefined
@@ -140,7 +142,8 @@ export const modelsApi = {
   async fetchModelsViaApiCall(
     baseUrl: string,
     apiKey?: string,
-    headers: Record<string, string> = {}
+    headers: Record<string, string> = {},
+    authIndex?: string
   ) {
     const endpoint = buildModelsEndpoint(baseUrl);
     if (!endpoint) {
@@ -153,6 +156,7 @@ export const modelsApi = {
     }
 
     const result = await apiCallApi.request({
+      authIndex: authIndex?.trim() || undefined,
       method: 'GET',
       url: endpoint,
       header: Object.keys(resolvedHeaders).length ? resolvedHeaders : undefined
@@ -185,7 +189,8 @@ export const modelsApi = {
   async fetchClaudeModelsViaApiCall(
     baseUrl: string,
     apiKey?: string,
-    headers: Record<string, string> = {}
+    headers: Record<string, string> = {},
+    authIndex?: string
   ) {
     const endpoint = buildClaudeModelsEndpoint(baseUrl);
     if (!endpoint) {
@@ -205,12 +210,14 @@ export const modelsApi = {
       resolvedHeaders['anthropic-version'] = DEFAULT_ANTHROPIC_VERSION;
     }
 
+    const trimmedAuthIndex = authIndex?.trim() || undefined;
     const signature = buildRequestSignature(endpoint, resolvedHeaders);
     const existing = CLAUDE_MODELS_IN_FLIGHT.get(signature);
     if (existing) return existing;
 
     const request = (async () => {
       const result = await apiCallApi.request({
+        authIndex: trimmedAuthIndex,
         method: 'GET',
         url: endpoint,
         header: Object.keys(resolvedHeaders).length ? resolvedHeaders : undefined
@@ -239,7 +246,8 @@ export const modelsApi = {
   async fetchGeminiModelsViaApiCall(
     baseUrl: string,
     apiKey?: string,
-    headers: Record<string, string> = {}
+    headers: Record<string, string> = {},
+    authIndex?: string
   ) {
     const endpoint = buildGeminiModelsEndpoint(baseUrl);
     if (!endpoint) {
@@ -252,6 +260,7 @@ export const modelsApi = {
       resolvedHeaders['x-goog-api-key'] = resolvedApiKey;
     }
 
+    const trimmedAuthIndex = authIndex?.trim() || undefined;
     const signature = buildRequestSignature(endpoint, resolvedHeaders);
     const existing = GEMINI_MODELS_IN_FLIGHT.get(signature);
     if (existing) return existing;
@@ -268,6 +277,7 @@ export const modelsApi = {
         }
 
         const result = await apiCallApi.request({
+          authIndex: trimmedAuthIndex,
           method: 'GET',
           url: url.toString(),
           header: Object.keys(resolvedHeaders).length ? resolvedHeaders : undefined