diff --git a/web-app/src/lib/__tests__/remoteModelCatalog.test.ts b/web-app/src/lib/__tests__/remoteModelCatalog.test.ts
index 07ae3b970a..9e6dc4964a 100644
--- a/web-app/src/lib/__tests__/remoteModelCatalog.test.ts
+++ b/web-app/src/lib/__tests__/remoteModelCatalog.test.ts
@@ -130,19 +130,14 @@ describe('fetchTopRemoteModels openai', () => {
     expect(gpt35.capabilities).toEqual(['completion', 'tools'])
   })
 
-  it('sends bearer + x-api-key headers', async () => {
+  it('sends only Authorization (no x-api-key) for OpenAI-compatible providers', async () => {
     const fetchImpl = vi.fn().mockResolvedValue(mkResponse({ data: [] }))
     await fetchTopRemoteModels(mkOpenAIProvider(), fetchImpl)
-    expect(fetchImpl).toHaveBeenCalledWith(
-      'https://api.openai.com/v1/models',
-      expect.objectContaining({
-        method: 'GET',
-        headers: expect.objectContaining({
-          Authorization: 'Bearer sk-test',
-          'x-api-key': 'sk-test',
-        }),
-      })
-    )
+    const headers = fetchImpl.mock.calls[0][1].headers as Record<string, string>
+    expect(headers.Authorization).toBe('Bearer sk-test')
+    // Sending both Authorization and x-api-key breaks upstreams that reject
+    // mixed auth (e.g. AWS Bedrock Mantle returns 401). See issue #8444.
+    expect(headers).not.toHaveProperty('x-api-key')
   })
 
   it('retries with fallback key on 401', async () => {
@@ -232,6 +227,14 @@ describe('fetchTopRemoteModels anthropic', () => {
     )
   })
 
+  it('sends only x-api-key (no Authorization) for anthropic providers', async () => {
+    const fetchImpl = vi.fn().mockResolvedValue(mkResponse({ data: [] }))
+    await fetchTopRemoteModels(mkAnthropicProvider(), fetchImpl)
+    const headers = fetchImpl.mock.calls[0][1].headers as Record<string, string>
+    expect(headers['x-api-key']).toBe('sk-ant')
+    expect(headers).not.toHaveProperty('Authorization')
+  })
+
   it('does not send anthropic headers for openai', async () => {
     const fetchImpl = vi.fn().mockResolvedValue(mkResponse({ data: [] }))
     await fetchTopRemoteModels(mkOpenAIProvider(), fetchImpl)
diff --git a/web-app/src/lib/remoteModelCatalog.ts b/web-app/src/lib/remoteModelCatalog.ts
index 632d64e604..cd5d9fa251 100644
--- a/web-app/src/lib/remoteModelCatalog.ts
+++ b/web-app/src/lib/remoteModelCatalog.ts
@@ -64,6 +64,26 @@ export function ensureAnthropicHeaders(
   setDefaultHeader(headers, ANTHROPIC_BROWSER_ACCESS_HEADER, 'true')
 }
 
+/// Attach the single auth header a provider expects on model-list / key-test
+/// requests. Anthropic-shaped providers authenticate via `x-api-key`; every
+/// other (OpenAI-compatible) provider uses `Authorization: *** Sending both
+/// at once breaks upstreams that reject mixed auth — e.g. AWS Bedrock Mantle
+/// answers `401 "request must not include both 'authorization' and
+/// 'x-api-key' headers"`. This mirrors how chat requests already pick one
+/// header per provider in model-factory.
+export function applyProviderAuthHeader(
+  provider: { provider?: string; base_url?: string; api_type?: string },
+  headers: Record<string, string>,
+  key: string | undefined
+): void {
+  if (!key) return
+  if (isAnthropicProvider(provider)) {
+    headers['x-api-key'] = key
+  } else {
+    headers['Authorization'] = `Bearer ${key}`
+  }
+}
+
 type CatalogKind = 'openai' | 'anthropic' | 'gemini'
 
 /// Resolve which catalog shape a provider speaks. `api_type` is authoritative
@@ -162,10 +182,7 @@ function inferAnthropicCapabilities(id: string): string[] | null {
 
 function buildHeaders(p: ProviderLike, key: string | undefined): Record<string, string> {
   const headers: Record<string, string> = { 'Content-Type': 'application/json' }
-  if (key) {
-    headers['x-api-key'] = key
-    headers['Authorization'] = `Bearer ${key}`
-  }
+  applyProviderAuthHeader(p, headers, key)
   if (p.custom_header) {
     for (const h of p.custom_header) headers[h.header] = h.value
   }
diff --git a/web-app/src/routes/settings/providers/$providerName.tsx b/web-app/src/routes/settings/providers/$providerName.tsx
index 25668e44d3..c229bf5835 100644
--- a/web-app/src/routes/settings/providers/$providerName.tsx
+++ b/web-app/src/routes/settings/providers/$providerName.tsx
@@ -51,6 +51,8 @@ import {
 import {
   supportsRemoteCatalog,
   fetchTopRemoteModels,
+  applyProviderAuthHeader,
+  ensureAnthropicHeaders,
 } from '@/lib/remoteModelCatalog'
 
 // as route.threadsDetail
@@ -450,15 +452,15 @@ function ProviderDetail() {
         if (!key) continue
         const headers: Record<string, string> = {
           'Content-Type': 'application/json',
-          'x-api-key': key,
-          Authorization: `Bearer ${key}`,
         }
+        applyProviderAuthHeader(provider, headers, key)
         if (
           provider.base_url.includes('localhost:') ||
           provider.base_url.includes('127.0.0.1:')
         ) {
           headers['Origin'] = 'tauri://localhost'
         }
+        ensureAnthropicHeaders(provider, headers)
 
         try {
           const response = await fetchImpl(`${provider.base_url}/models`, {
@@ -492,7 +494,7 @@ function ProviderDetail() {
     } finally {
       setIsTestingKeys(false)
     }
-  }, [apiKeysDraft, maskApiKey, provider?.base_url, serviceHub, t])
+  }, [apiKeysDraft, maskApiKey, provider, serviceHub, t])
 
   // Auto-refresh provider settings to get updated backend configuration
   const refreshSettings = useCallback(async () => {
diff --git a/web-app/src/services/providers/__tests__/tauri.test.ts b/web-app/src/services/providers/__tests__/tauri.test.ts
index e9496bbb2a..4d1bfbccec 100644
--- a/web-app/src/services/providers/__tests__/tauri.test.ts
+++ b/web-app/src/services/providers/__tests__/tauri.test.ts
@@ -338,7 +338,7 @@ describe('TauriProvidersService', () => {
       )
     })
 
-    it('adds auth headers when api key is available', async () => {
+    it('adds only the Authorization header for OpenAI-compatible providers', async () => {
       vi.mocked(providerRemoteApiKeyChain).mockReturnValue(['sk-test'])
       vi.mocked(fetchTauri).mockResolvedValueOnce({
         ok: true,
@@ -347,15 +347,31 @@ describe('TauriProvidersService', () => {
       } as any)
 
       await svc.fetchModelsFromProvider(baseProvider)
-      expect(fetchTauri).toHaveBeenCalledWith(
-        expect.any(String),
-        expect.objectContaining({
-          headers: expect.objectContaining({
-            'x-api-key': 'sk-test',
-            Authorization: 'Bearer sk-test',
-          }),
-        })
-      )
+      const headers = vi.mocked(fetchTauri).mock.calls[0][1]!
+        .headers as Record<string, string>
+      expect(headers.Authorization).toBe('Bearer sk-test')
+      // Both auth headers at once break upstreams that reject mixed auth
+      // (e.g. AWS Bedrock Mantle answers 401). See issue #8444.
+      expect(headers).not.toHaveProperty('x-api-key')
+    })
+
+    it('adds only the x-api-key header for anthropic-shaped providers', async () => {
+      vi.mocked(providerRemoteApiKeyChain).mockReturnValue(['sk-ant'])
+      vi.mocked(fetchTauri).mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: vi.fn().mockResolvedValue({ data: [] }),
+      } as any)
+
+      await svc.fetchModelsFromProvider({
+        ...baseProvider,
+        provider: 'my-gateway',
+        api_type: 'anthropic',
+      })
+      const headers = vi.mocked(fetchTauri).mock.calls[0][1]!
+        .headers as Record<string, string>
+      expect(headers['x-api-key']).toBe('sk-ant')
+      expect(headers).not.toHaveProperty('Authorization')
     })
 
     it('adds default anthropic-version header for anthropic-shaped custom providers', async () => {
diff --git a/web-app/src/services/providers/tauri.ts b/web-app/src/services/providers/tauri.ts
index fd2b25c157..97d84f813f 100644
--- a/web-app/src/services/providers/tauri.ts
+++ b/web-app/src/services/providers/tauri.ts
@@ -16,7 +16,7 @@ import {
   API_KEY_FALLBACKS_SETTING_KEY,
   providerRemoteApiKeyChain,
 } from '@/lib/provider-api-keys'
-import { ensureAnthropicHeaders } from '@/lib/remoteModelCatalog'
+import { ensureAnthropicHeaders, applyProviderAuthHeader } from '@/lib/remoteModelCatalog'
 
 export class TauriProvidersService extends DefaultProvidersService {
   fetch(): typeof fetch {
@@ -170,10 +170,7 @@ export class TauriProvidersService extends DefaultProvidersService {
           headers['Origin'] = 'tauri://localhost'
         }
 
-        if (key) {
-          headers['x-api-key'] = key
-          headers['Authorization'] = `Bearer ${key}`
-        }
+        applyProviderAuthHeader(provider, headers, key)
 
         if (provider.custom_header) {
           provider.custom_header.forEach((header) => {