fix(core): avoid passing undici agent to Anthropic SDK

Anthropic's Node client uses node-fetch, which expects a Node http(s).Agent.
Stop providing undici's Dispatcher as httpAgent; only set httpAgent when a proxy
is configured (via https-proxy-agent), otherwise let the SDK use its default.

packages/core/src/utils/runtimeFetchOptions.test.ts

@@ -30,6 +30,19 @@ vi.mock('undici', () => {
   };
 });
 
+vi.mock('https-proxy-agent', () => {
+  class MockHttpsProxyAgent {
+    proxyUrl: string;
+    constructor(proxyUrl: string) {
+      this.proxyUrl = proxyUrl;
+    }
+  }
+
+  return {
+    HttpsProxyAgent: MockHttpsProxyAgent,
+  };
+});
+
 describe('buildRuntimeFetchOptions (node runtime)', () => {
   it('disables undici timeouts for Agent in OpenAI options', () => {
     const result = buildRuntimeFetchOptions('openai');
@@ -60,17 +73,20 @@ describe('buildRuntimeFetchOptions (node runtime)', () => {
     });
   });
 
-  it('returns httpAgent with disabled timeouts for Anthropic options', () => {
+  it('returns empty object for Anthropic without proxy (uses SDK default agent)', () => {
     const result = buildRuntimeFetchOptions('anthropic');
 
+    expect(result).toEqual({});
+  });
+
+  it('returns HttpsProxyAgent for Anthropic with proxy', () => {
+    const result = buildRuntimeFetchOptions('anthropic', 'http://proxy.local');
+
     expect(result).toBeDefined();
     expect(result && 'httpAgent' in result).toBe(true);
 
-    const httpAgent = (result as { httpAgent?: { options?: UndiciOptions } })
+    const httpAgent = (result as { httpAgent?: { proxyUrl?: string } })
       .httpAgent;
-    expect(httpAgent?.options).toMatchObject({
-      headersTimeout: 0,
-      bodyTimeout: 0,
-    });
+    expect(httpAgent?.proxyUrl).toBe('http://proxy.local');
   });
 });

packages/core/src/utils/runtimeFetchOptions.ts

@@ -5,6 +5,7 @@
  */
 
 import { Agent, ProxyAgent, type Dispatcher } from 'undici';
+import { HttpsProxyAgent } from 'https-proxy-agent';
 
 /**
  * JavaScript runtime type
@@ -113,48 +114,42 @@ export function buildRuntimeFetchOptions(
     }
 
     case 'node': {
-      // Node.js: Use ProxyAgent when proxy is configured, otherwise Agent.
-      // undici timeouts are disabled to let SDK timeout control the request.
-      try {
-        const dispatcher = createDispatcher(proxyUrl);
-        if (sdkType === 'openai') {
-          return {
-            dispatcher,
-          };
-        } else {
-          return {
-            httpAgent: dispatcher,
-          };
-        }
-      } catch {
-        // If undici is not available, return appropriate default
-        if (sdkType === 'openai') {
+      // Node.js: Use appropriate agent based on SDK type.
+      // OpenAI SDK uses undici internally, so we use undici Dispatcher.
+      // Anthropic SDK uses node-fetch internally, so we use Node.js-compatible agents.
+      if (sdkType === 'openai') {
+        try {
+          const dispatcher = createDispatcher(proxyUrl);
+          return { dispatcher };
+        } catch {
           return undefined;
-        } else {
-          return {};
         }
+      } else {
+        // Anthropic SDK: use HttpsProxyAgent when proxy is set,
+        // otherwise let SDK use its default agentkeepalive agent.
+        // The SDK's timeout option controls request-level timeouts.
+        if (proxyUrl) {
+          return { httpAgent: new HttpsProxyAgent(proxyUrl) };
+        }
+        return {};
       }
     }
 
     default: {
-      // Unknown runtime: Use ProxyAgent when proxy is configured, otherwise Agent.
-      try {
-        const dispatcher = createDispatcher(proxyUrl);
-        if (sdkType === 'openai') {
-          return {
-            dispatcher,
-          };
-        } else {
-          return {
-            httpAgent: dispatcher,
-          };
-        }
-      } catch {
-        if (sdkType === 'openai') {
+      // Unknown runtime: treat as Node.js-like environment.
+      if (sdkType === 'openai') {
+        try {
+          const dispatcher = createDispatcher(proxyUrl);
+          return { dispatcher };
+        } catch {
           return undefined;
-        } else {
-          return {};
         }
+      } else {
+        // Anthropic SDK: use HttpsProxyAgent when proxy is set
+        if (proxyUrl) {
+          return { httpAgent: new HttpsProxyAgent(proxyUrl) };
+        }
+        return {};
       }
     }
   }