diff --git a/app/admin-api/credentials/route.ts b/app/admin-api/credentials/route.ts
index a66170d..5cb6023 100644
--- a/app/admin-api/credentials/route.ts
+++ b/app/admin-api/credentials/route.ts
@@ -3,6 +3,7 @@ import {
   listCredentials,
   updateCredentialByIndex,
 } from '@/lib/server/domain/credentials';
+import { refreshCredentialModels } from '@/lib/server/domain/credential-models';
 import { getAdminSessionErrorResponse } from '@/lib/server/admin/session';
 import { getJsonBody } from '@/lib/server/shared/http';
 
@@ -33,12 +34,13 @@ export const POST = async (request: Request): Promise<Response> => {
       return Response.json(await updateCredentialByIndex(body.index, body));
     }
 
-    return Response.json(
-      await addCredential(
-        body,
-        typeof body.filename === 'string' ? body.filename : undefined,
-      ),
+    const saved = await addCredential(
+      body,
+      typeof body.filename === 'string' ? body.filename : undefined,
     );
+    await refreshCredentialModels(saved.filename);
+
+    return Response.json(saved);
   } catch (error) {
     return Response.json(
       {
diff --git a/app/page-loader.ts b/app/page-loader.ts
index af74012..49c439e 100644
--- a/app/page-loader.ts
+++ b/app/page-loader.ts
@@ -15,14 +15,12 @@ import type { AdminUsagePreferences } from '@/lib/server/admin/session';
 import { listAccessKeys } from '@/lib/server/domain/access-keys';
 import { getActiveConfig, getSettingLabels } from '@/lib/server/domain/config';
 import {
+  getCredentialSupportedModels,
   getCurrentCredentialInfo,
   listEligibleCredentialRecords,
   listCredentials,
 } from '@/lib/server/domain/credentials';
-import {
-  getModelsByCredential,
-  getModelsForCredentials,
-} from '@/lib/server/proxy/codebuddy';
+import { getModelsForCredentials } from '@/lib/server/proxy/codebuddy';
 import { getDebugSettings, listDebugLogs } from '@/lib/server/domain/debug';
 import { getUsageAnalytics } from '@/lib/server/domain/usage';
 import type { AppLocale } from '@/lib/i18n/routing';
@@ -145,19 +143,17 @@ export const getInitialData = async ({
     }
     case 'api-test': {
       const eligibleCredentials = await listEligibleCredentialRecords();
-      const [credentials, currentCredential, models, credentialModels] =
-        await Promise.all([
-          listCredentials(),
-          getCurrentCredentialInfo(),
-          getModelsForCredentials(eligibleCredentials),
-          getModelsByCredential(eligibleCredentials),
-        ]);
+      const [credentials, currentCredential, models] = await Promise.all([
+        listCredentials(),
+        getCurrentCredentialInfo(),
+        getModelsForCredentials(eligibleCredentials),
+      ]);
 
       return {
         credentialModels: Object.fromEntries(
-          Object.entries(credentialModels).map(([filename, value]) => [
-            filename,
-            value.models.map((model) => model.id),
+          eligibleCredentials.map((credential) => [
+            credential.filename,
+            getCredentialSupportedModels(credential.data),
           ]),
         ),
         credentials: credentials.credentials as unknown as CredentialSummary[],
diff --git a/app/page-shell.tsx b/app/page-shell.tsx
index 5bf0829..2f53b78 100644
--- a/app/page-shell.tsx
+++ b/app/page-shell.tsx
@@ -540,6 +540,10 @@ const AdminPageLayoutContent = ({
         modelsLoading: false,
       }));
       setApiTest((current) => {
+        if (filename && current.credentialFilename !== filename) {
+          return current;
+        }
+
         const selectedFilename = filename ?? current.credentialFilename;
         const models = modelRows[selectedFilename]?.models;
 
@@ -1044,9 +1048,6 @@ const AdminPageLayoutContent = ({
         name: result.data.filename ?? 'unknown',
       }),
     );
-    if (!isEditing && result.data.filename) {
-      await refreshCredentialModels(result.data.filename);
-    }
     await refreshAdminData();
   };
 
diff --git a/lib/server/domain/config.ts b/lib/server/domain/config.ts
index 97a3512..9e30804 100644
--- a/lib/server/domain/config.ts
+++ b/lib/server/domain/config.ts
@@ -6,6 +6,10 @@ import {
   readStorageJson,
   writeStorageJson,
 } from '../storage';
+import {
+  getCredentialSupportedModels,
+  listEligibleCredentialRecords,
+} from './credentials';
 
 export interface RuntimeConfig {
   CODEBUDDY_API_ENDPOINT: string;
@@ -168,7 +172,16 @@ export const getCodeBuddyApiEndpoint = async (): Promise<string> => {
     : 'https://copilot.tencent.com';
 };
 
-export const getDefaultModel = async (fallback = 'glm-5.1'): Promise<string> =>
-  fallback;
+export const getDefaultModel = async (
+  fallback = 'glm-5.1',
+): Promise<string> => {
+  const credentials = await listEligibleCredentialRecords();
+
+  return (
+    credentials
+      .flatMap((credential) => getCredentialSupportedModels(credential.data))
+      .sort((left, right) => left.localeCompare(right))[0] ?? fallback
+  );
+};
 
 export { getConfigDir, getConfigPath, getCredsDir, getFileStorageDir };
diff --git a/lib/server/domain/credential-models.ts b/lib/server/domain/credential-models.ts
index ef18e3a..18777e8 100644
--- a/lib/server/domain/credential-models.ts
+++ b/lib/server/domain/credential-models.ts
@@ -1,4 +1,5 @@
 import {
+  findCredentialRecordByFilename,
   getCredentialSupportedModels,
   listEligibleCredentialRecords,
   updateCredentialSupportedModels,
@@ -9,6 +10,30 @@ const globalCredentialModelRefreshState = globalThis as typeof globalThis & {
   __codebuddy2apiCredentialModelRefresh__?: Promise<void>;
 };
 
+export const refreshCredentialModels = async (
+  filename: string,
+): Promise<void> => {
+  try {
+    const credential = await findCredentialRecordByFilename(filename);
+
+    if (!credential) {
+      return;
+    }
+
+    const result = await getModelsByCredential([credential]);
+    const models = result[credential.filename]?.models ?? [];
+
+    if (models.length) {
+      await updateCredentialSupportedModels(
+        credential.filename,
+        models.map((model) => model.id),
+      );
+    }
+  } catch (error) {
+    console.warn('[CodeBuddy2API] Unable to refresh credential models', error);
+  }
+};
+
 export const refreshMissingCredentialModels = (): Promise<void> => {
   if (
     !globalCredentialModelRefreshState.__codebuddy2apiCredentialModelRefresh__
@@ -24,15 +49,7 @@ export const refreshMissingCredentialModels = (): Promise<void> => {
 
           await Promise.allSettled(
             missingModels.map(async (credential) => {
-              const result = await getModelsByCredential([credential]);
-              const models = result[credential.filename]?.models ?? [];
-
-              if (models.length) {
-                await updateCredentialSupportedModels(
-                  credential.filename,
-                  models.map((model) => model.id),
-                );
-              }
+              await refreshCredentialModels(credential.filename);
             }),
           );
         } catch (error) {
diff --git a/lib/server/domain/credentials.ts b/lib/server/domain/credentials.ts
index 203117e..2192fe4 100644
--- a/lib/server/domain/credentials.ts
+++ b/lib/server/domain/credentials.ts
@@ -690,16 +690,26 @@ export const resolveCredentialForRequest = async ({
   accessKeyId,
   affinityKey,
   allowedCredentialFilenames,
+  model,
 }: {
   accessKeyId?: string;
   affinityKey?: string;
   allowedCredentialFilenames?: string[];
+  model?: string;
 } = {}): Promise<CredentialRecord | null> => {
   const records = await readCredentialRecords();
+  const requestedModel = model?.trim();
   const eligibleRecords = getEligibleRecords(
     records,
     allowedCredentialFilenames,
-  );
+  ).filter((record) => {
+    if (!requestedModel) return true;
+
+    const supportedModels = getCredentialSupportedModels(record.data);
+    return (
+      supportedModels.length === 0 || supportedModels.includes(requestedModel)
+    );
+  });
 
   if (!eligibleRecords.length) {
     return null;
diff --git a/lib/server/proxy/codebuddy-auth.ts b/lib/server/proxy/codebuddy-auth.ts
index 3a9a816..0272381 100644
--- a/lib/server/proxy/codebuddy-auth.ts
+++ b/lib/server/proxy/codebuddy-auth.ts
@@ -1,5 +1,6 @@
 import { getCodeBuddyApiEndpoint } from '../domain/config';
 import { addCredential, type CredentialData } from '../domain/credentials';
+import { refreshCredentialModels } from '../domain/credential-models';
 import { getAdminSessionErrorResponse } from '../admin/session';
 
 const getAuthStateEndpoint = async (): Promise<string> =>
@@ -286,6 +287,7 @@ export const pollCodeBuddyAuth = async (
     };
     const credential = buildCredentialDataFromToken(tokenPayload);
     const saved = await addCredential(credential);
+    await refreshCredentialModels(saved.filename);
 
     return Response.json({
       access_token: tokenPayload.access_token,
diff --git a/lib/server/proxy/codebuddy.ts b/lib/server/proxy/codebuddy.ts
index 97cb07e..8527932 100644
--- a/lib/server/proxy/codebuddy.ts
+++ b/lib/server/proxy/codebuddy.ts
@@ -433,26 +433,11 @@ const normalizeMessages = (
     return applyPromptCacheControl(filtered);
   }
 
-  let hasSystemMessage = false;
-
-  const normalized = filtered.map((message, index) => {
-    if (message.role === 'system') {
-      hasSystemMessage = true;
-      return message;
-    }
-
+  const normalized = filtered.map((message) => {
     if (message.role !== 'developer') {
       return message;
     }
 
-    if (index === 0 && !hasSystemMessage) {
-      hasSystemMessage = true;
-      return {
-        ...message,
-        role: 'system',
-      };
-    }
-
     return {
       ...message,
       role: 'user',
@@ -466,12 +451,14 @@ const normalizeMessages = (
 
 export const resolveProxyContext = async (
   request: NextRequest,
+  model?: string,
 ): Promise<ProxyContext> => {
   const accessKey = await resolveRequestAccessKey(request);
   const credential = await resolveCredentialForRequest({
     accessKeyId: accessKey?.id,
     affinityKey: getCredentialAffinityKey(request, accessKey?.id ?? null),
     allowedCredentialFilenames: accessKey?.credentialFilenames,
+    model,
   });
 
   if (!credential) {
@@ -673,10 +660,13 @@ const buildUpstreamBody = async (
     context.preferences.firstMessageRoleToSystem,
   );
   const maxTokens = body.max_tokens ?? body.max_completion_tokens;
+  const credentialModels = getCredentialSupportedModels(
+    context.auth.credentialData,
+  );
   const model =
     typeof body.model === 'string' && body.model.trim()
       ? body.model
-      : await getDefaultModel();
+      : (credentialModels[0] ?? (await getDefaultModel()));
 
   return {
     model,
@@ -1315,7 +1305,8 @@ export const proxyChatCompletions = async (
   }
 
   try {
-    const resolvedContext = context ?? (await resolveProxyContext(request));
+    const resolvedContext =
+      context ?? (await resolveProxyContext(request, body.model));
     setDebugTraceCredential(debugTrace, resolvedContext.credentialFilename);
     const upstreamBody = await buildUpstreamBody(body, resolvedContext);
     const apiEndpoint = await getCodeBuddyApiEndpoint();
@@ -1430,7 +1421,12 @@ export const proxyResponsesUpstream = async (
   debugTrace?: DebugTrace,
 ): Promise<Response> => {
   try {
-    const resolvedContext = context ?? (await resolveProxyContext(request));
+    const resolvedContext =
+      context ??
+      (await resolveProxyContext(
+        request,
+        typeof body.model === 'string' ? body.model : undefined,
+      ));
     setDebugTraceCredential(debugTrace, resolvedContext.credentialFilename);
     const upstreamBody = {
       ...body,
diff --git a/lib/server/proxy/responses.ts b/lib/server/proxy/responses.ts
index 7def18a..dfb8af9 100644
--- a/lib/server/proxy/responses.ts
+++ b/lib/server/proxy/responses.ts
@@ -1,6 +1,7 @@
 import type { NextRequest } from 'next/server';
 
 import { getDefaultModel } from '../domain/config';
+import { getCredentialSupportedModels } from '../domain/credentials';
 import type { DebugTrace } from '../domain/debug';
 import {
   proxyChatCompletions,
@@ -1409,12 +1410,27 @@ export const handleResponsesRequest = async (
             requireEligible: true,
           },
         )
-      : await resolveProxyContext(request);
+      : await resolveProxyContext(
+          request,
+          typeof body.model === 'string' ? body.model : undefined,
+        );
+
+    const scopedBody =
+      !storedPreviousSession &&
+      (typeof body.model !== 'string' || !body.model.trim())
+        ? {
+            ...body,
+            model:
+              getCredentialSupportedModels(
+                proxyContext.auth.credentialData,
+              )[0] ?? (await getDefaultModel()),
+          }
+        : body;
 
     if (proxyContext.preferences.responsesPassthrough) {
       return proxyResponsesUpstream(
         request,
-        body as Record<string, unknown>,
+        scopedBody as Record<string, unknown>,
         proxyContext,
         debugTrace,
       );
@@ -1426,7 +1442,7 @@ export const handleResponsesRequest = async (
     );
 
     const prepared = await prepareTranscript(
-      body,
+      scopedBody,
       proxyContext.accessKeyId,
       previousSession,
     );
diff --git a/messages/en-US.json b/messages/en-US.json
index cb8dab3..70dd76e 100644
--- a/messages/en-US.json
+++ b/messages/en-US.json
@@ -233,8 +233,8 @@
       "credentialResponsesDirectHelp": "When enabled, `/v1/responses` requests using this credential are sent upstream directly instead of being converted into Chat Completions.",
       "credentialResponsesProxyTag": "Responses → Chat",
       "credentialRoleAsSystem": "Normalize developer messages for upstream",
-      "credentialRoleAsSystemHelp": "Sends the first developer message as system and later developer messages as user.",
-      "credentialRoleAsSystemTag": "developer → system",
+      "credentialRoleAsSystemHelp": "Sends every developer message as user.",
+      "credentialRoleAsSystemTag": "developer → user",
       "credentialRoleKeepDeveloper": "Keep developer",
       "credentialSectionTitle": "Saved credentials",
       "modelCredential": "Credential",
diff --git a/messages/ja-JP.json b/messages/ja-JP.json
index 18ad6a7..d34eb60 100644
--- a/messages/ja-JP.json
+++ b/messages/ja-JP.json
@@ -233,8 +233,8 @@
       "credentialResponsesDirectHelp": "有効にすると、この認証情報で処理される `/v1/responses` リクエストは Chat Completions へ変換せず上流へ直接送信されます。",
       "credentialResponsesProxyTag": "Responses → Chat",
       "credentialRoleAsSystem": "developer メッセージを上流向けに正規化",
-      "credentialRoleAsSystemHelp": "先頭の developer メッセージは system、それ以降の developer メッセージは user として送信します。",
-      "credentialRoleAsSystemTag": "developer → system",
+      "credentialRoleAsSystemHelp": "すべての developer メッセージを user として送信します。",
+      "credentialRoleAsSystemTag": "developer → user",
       "credentialRoleKeepDeveloper": "developer を保持",
       "credentialSectionTitle": "保存済み認証情報",
       "modelCredential": "認証情報",
diff --git a/messages/zh-CN.json b/messages/zh-CN.json
index c17d137..aa4987c 100644
--- a/messages/zh-CN.json
+++ b/messages/zh-CN.json
@@ -229,8 +229,8 @@
       "credentialResponsesDirectHelp": "启用后，使用此凭证的 `/v1/responses` 请求将直接发送到上游，不再转换为 Chat Completions。",
       "credentialResponsesProxyTag": "Responses → Chat",
       "credentialRoleAsSystem": "转换 developer 消息角色以兼容上游",
-      "credentialRoleAsSystemHelp": "首条 developer 消息作为 system 发送，其余 developer 消息作为 user 发送。",
-      "credentialRoleAsSystemTag": "developer → system",
+      "credentialRoleAsSystemHelp": "所有 developer 消息均作为 user 发送。",
+      "credentialRoleAsSystemTag": "developer → user",
       "credentialRoleKeepDeveloper": "保留 developer",
       "credentialSectionTitle": "已保存的凭证",
       "modelCredential": "凭据",
diff --git a/tests/admin/page-loader.test.ts b/tests/admin/page-loader.test.ts
index 1eea8ec..f3ce968 100644
--- a/tests/admin/page-loader.test.ts
+++ b/tests/admin/page-loader.test.ts
@@ -14,6 +14,7 @@ vi.mock('@/lib/server/domain/config', () => ({
 }));
 
 vi.mock('@/lib/server/domain/credentials', () => ({
+  getCredentialSupportedModels: vi.fn(),
   getCurrentCredentialInfo: vi.fn(),
   listEligibleCredentialRecords: vi.fn(),
   listCredentials: vi.fn(),
@@ -42,6 +43,7 @@ const { listAccessKeys } = await import('@/lib/server/domain/access-keys');
 const { getActiveConfig, getSettingLabels } =
   await import('@/lib/server/domain/config');
 const {
+  getCredentialSupportedModels,
   getCurrentCredentialInfo,
   listEligibleCredentialRecords,
   listCredentials,
@@ -81,6 +83,7 @@ describe('tab-scoped initial data', () => {
     vi.mocked(getCurrentCredentialInfo).mockResolvedValue({ status: 'empty' });
     vi.mocked(listCredentials).mockResolvedValue({ credentials: [] } as never);
     vi.mocked(listEligibleCredentialRecords).mockResolvedValue([]);
+    vi.mocked(getCredentialSupportedModels).mockReturnValue([]);
     vi.mocked(getModelsForCredentials).mockResolvedValue([]);
     vi.mocked(getModelsByCredential).mockResolvedValue({});
     vi.mocked(getDebugSettings).mockResolvedValue({
@@ -110,7 +113,6 @@ describe('tab-scoped initial data', () => {
         'getCurrentCredentialInfo',
         'listEligibleCredentialRecords',
         'getModelsForCredentials',
-        'getModelsByCredential',
       ],
     ],
     ['debug', ['getDebugSettings', 'listDebugLogs']],
@@ -143,6 +145,28 @@ describe('tab-scoped initial data', () => {
     },
   );
 
+  it('hydrates API-test credential models from saved values', async () => {
+    vi.mocked(listEligibleCredentialRecords).mockResolvedValue([
+      {
+        data: { supported_models: 'glm-5.1' },
+        filePath: '',
+        filename: 'credential-a.json',
+      },
+    ] as never);
+    vi.mocked(getCredentialSupportedModels).mockReturnValue(['glm-5.1']);
+
+    const initialData = await getInitialData({
+      locale: 'en-US',
+      tab: 'api-test',
+    });
+
+    expect(initialData).toMatchObject({
+      credentialModels: { 'credential-a.json': ['glm-5.1'] },
+      tab: 'api-test',
+    });
+    expect(getModelsByCredential).not.toHaveBeenCalled();
+  });
+
   it('restores persisted usage filters and refresh settings in the usage snapshot', async () => {
     vi.mocked(getUsageAnalytics).mockResolvedValue({
       callSeries: [],
diff --git a/tests/server/units.test.ts b/tests/server/units.test.ts
index c477bf6..224c98d 100644
--- a/tests/server/units.test.ts
+++ b/tests/server/units.test.ts
@@ -57,7 +57,11 @@ import {
   resetResponseSessions,
   translateResponsesToolsToChat,
 } from '@/lib/server/proxy/responses';
-import { updateSettings, getActiveConfig } from '@/lib/server/domain/config';
+import {
+  getActiveConfig,
+  getDefaultModel,
+  updateSettings,
+} from '@/lib/server/domain/config';
 import { getRequestHeaderMap } from '@/lib/server/shared/http';
 import { getUsageStats, resetUsageStats } from '@/lib/server/domain/stats';
 import {
@@ -492,6 +496,31 @@ describe('server units', () => {
       )?.filename,
     ).toBe(keyedCredential.filename);
 
+    const modelOneCredential = (await readCredentialRecords()).find(
+      (record) => record.data.user_id === 'one@example.com',
+    );
+    const modelTwoCredential = (await readCredentialRecords()).find(
+      (record) => record.data.user_id === 'two@example.com',
+    );
+    await updateCredentialSupportedModels(modelOneCredential?.filename ?? '', [
+      'glm-one',
+    ]);
+    await updateCredentialSupportedModels(modelTwoCredential?.filename ?? '', [
+      'glm-two',
+    ]);
+    expect(await getDefaultModel()).toBe('glm-one');
+    expect(
+      (
+        await resolveCredentialForRequest({
+          allowedCredentialFilenames: [
+            modelOneCredential?.filename ?? '',
+            modelTwoCredential?.filename ?? '',
+          ],
+          model: 'glm-two',
+        })
+      )?.filename,
+    ).toBe(modelTwoCredential?.filename);
+
     await recordUsageEvent({
       credentialFilename: 'cred-a',
       model: 'glm-5.1',
@@ -1510,7 +1539,7 @@ describe('server units', () => {
     expect(fourthBody.messages[0]?.content).toBe('short reply');
   });
 
-  it('normalizes developer messages for chat upstream based on position', async () => {
+  it('normalizes developer messages as user messages for chat upstream', async () => {
     await addCredential({
       bearer_token: 'token-dev-role',
       created_at: Math.floor(Date.now() / 1000),
@@ -1563,7 +1592,7 @@ describe('server units', () => {
     };
 
     expect(upstreamBody.messages).toEqual([
-      { role: 'system', content: 'first developer' },
+      { role: 'user', content: 'first developer' },
       { role: 'user', content: 'hello' },
       { role: 'user', content: 'later developer' },
       { role: 'system', content: 'existing system' },
@@ -1676,7 +1705,7 @@ describe('server units', () => {
 
     expect(firstBody.messages[0]?.role).toBe('developer');
     expect(secondBody.messages[0]?.role).toBe('developer');
-    expect(thirdBody.messages[0]?.role).toBe('system');
+    expect(thirdBody.messages[0]?.role).toBe('user');
     expect(fetchMock).toHaveBeenCalledTimes(3);
   });
 
@@ -2504,6 +2533,38 @@ describe('server units', () => {
     ]);
   });
 
+  it('uses the selected credential model for Responses requests without a model', async () => {
+    const credential = await addCredential({
+      bearer_token: 'credential-scoped-model-token',
+      responses_passthrough: true,
+      supported_models: 'glm-credential-scoped',
+      user_id: 'credential-scoped-model@example.com',
+    });
+    const accessKey = await createAccessKey({
+      credentialFilenames: [credential.filename],
+      name: 'Credential Scoped Responses Key',
+    });
+    const fetchMock = vi
+      .spyOn(globalThis, 'fetch')
+      .mockResolvedValue(makeJsonResponse({ output_text: 'ok' }));
+
+    const response = await handleResponsesRequest(
+      makeNextRequest('http://localhost/v1/responses', {
+        headers: { authorization: `Bearer ${accessKey.secret}` },
+        method: 'POST',
+      }),
+      { input: 'hello' },
+    );
+
+    expect(response.status).toBe(200);
+    expect(
+      JSON.parse(String((fetchMock.mock.calls[0]?.[1] as RequestInit).body)),
+    ).toMatchObject({
+      input: 'hello',
+      model: 'glm-credential-scoped',
+    });
+  });
+
   it('covers responses passthrough upstream non-ok and empty stream body branches', async () => {
     const createdCredential = await addCredential({
       bearer_token: 'token-branches',
@@ -3146,6 +3207,16 @@ describe('server units', () => {
             tenantId: 'tenant-456',
           },
         }),
+      )
+      // refreshCredentialModels after the credential is saved
+      .mockResolvedValueOnce(
+        makeJsonResponse({
+          code: 0,
+          data: {
+            agents: [{ models: ['glm-5.1'], name: 'cli' }],
+            models: [{ id: 'glm-5.1', name: 'GLM 5.1' }],
+          },
+        }),
       );
 
     const startResult = (await (await startCodeBuddyAuth()).json()) as Record<
@@ -3174,6 +3245,11 @@ describe('server units', () => {
       (credential) => credential.tenant_id === 'tenant-456',
     );
     expect(savedCredential?.tenant_id).toBe('tenant-456');
+    expect(
+      (await readCredentialRecords()).find(
+        (credential) => credential.filename === savedCredential?.filename,
+      )?.data.supported_models,
+    ).toBe('glm-5.1');
 
     const credInfo = await getCurrentCredentialInfo();
     expect(credInfo.status).toBe('round_robin');