diff --git a/src/server/__tests__/providers.test.ts b/src/server/__tests__/providers.test.ts
index 7381db61..0167e211 100644
--- a/src/server/__tests__/providers.test.ts
+++ b/src/server/__tests__/providers.test.ts
@@ -1263,6 +1263,42 @@ describe('ProviderService', () => {
       }
     })
 
+    test('requests non-stream OpenAI Chat responses during provider tests', async () => {
+      const originalFetch = globalThis.fetch
+      const calls: Array<{ body: Record<string, unknown> }> = []
+      globalThis.fetch = mock(async (_url: string | URL | Request, init?: RequestInit) => {
+        calls.push({ body: JSON.parse(String(init?.body)) as Record<string, unknown> })
+        return new Response(JSON.stringify({
+          id: 'chatcmpl-1',
+          object: 'chat.completion',
+          created: 0,
+          model: 'deepseek-v4-flash',
+          choices: [{ index: 0, message: { role: 'assistant', content: 'ok' }, finish_reason: 'stop' }],
+          usage: { prompt_tokens: 1, completion_tokens: 1, total_tokens: 2 },
+        }), {
+          status: 200,
+          headers: { 'Content-Type': 'application/json' },
+        })
+      }) as typeof fetch
+
+      try {
+        const svc = new ProviderService()
+        const result = await svc.testProviderConfig({
+          baseUrl: 'https://api.example.com',
+          apiKey: 'sk-api',
+          modelId: 'deepseek-v4-flash',
+          authStrategy: 'api_key',
+          apiFormat: 'openai_chat',
+        })
+
+        expect(result.connectivity.success).toBe(true)
+        expect(result.proxy?.success).toBe(true)
+        expect(calls.map((call) => call.body.stream)).toEqual([false, false])
+      } finally {
+        globalThis.fetch = originalFetch
+      }
+    })
+
     test('should use configured network timeout for provider tests', async () => {
       await fs.writeFile(
         path.join(tmpDir, 'settings.json'),
diff --git a/src/server/proxy/transform/anthropicToOpenaiChat.ts b/src/server/proxy/transform/anthropicToOpenaiChat.ts
index 123589b6..01a14385 100644
--- a/src/server/proxy/transform/anthropicToOpenaiChat.ts
+++ b/src/server/proxy/transform/anthropicToOpenaiChat.ts
@@ -53,7 +53,7 @@ export function anthropicToOpenaiChat(
   const result: OpenAIChatRequest = {
     model: body.model,
     messages,
-    stream: body.stream,
+    stream: body.stream === true,
   }
 
   // max_tokens — omit to let upstream provider use its own default/max.
diff --git a/src/server/services/providerService.ts b/src/server/services/providerService.ts
index 7d43fc33..3d3643b6 100644
--- a/src/server/services/providerService.ts
+++ b/src/server/services/providerService.ts
@@ -603,7 +603,7 @@ function buildDirectTestRequest(
     return {
       url: `${base}/v1/chat/completions`,
       headers: { 'Content-Type': 'application/json', Authorization: `Bearer ${apiKey}` },
-      body: { model: modelId, max_tokens: 16, messages: [{ role: 'user', content: prompt }] },
+      body: { model: modelId, max_tokens: 16, stream: false, messages: [{ role: 'user', content: prompt }] },
     }
   }
   if (format === 'openai_responses') {