diff --git a/core/llm/index.ts b/core/llm/index.ts
index 1af44b25614..b3d0515cebd 100644
--- a/core/llm/index.ts
+++ b/core/llm/index.ts
@@ -1021,9 +1021,18 @@ export abstract class BaseLLM implements ILLM {
   }
 
   private canUseOpenAIResponses(options: CompletionOptions): boolean {
+    const isOfficialOpenAIAPI =
+      !this.apiBase ||
+      this.apiBase.trim() === "" ||
+      this.apiBase.replace(/\/$/, "") === "https://api.openai.com/v1";
+
+    const shouldUseResponses =
+      this._llmOptions.useResponsesApi === true ||
+      (this._llmOptions.useResponsesApi !== false && isOfficialOpenAIAPI);
+
     return (
       this.providerName === "openai" &&
-      this._llmOptions.useResponsesApi !== false &&
+      shouldUseResponses &&
       typeof (this as any)._streamResponses === "function" &&
       (this as any).isOSeriesOrGpt5PlusModel(options.model)
     );
diff --git a/core/llm/llms/OpenAI.test.ts b/core/llm/llms/OpenAI.test.ts
index 943e9436903..4658cf7a8f6 100644
--- a/core/llm/llms/OpenAI.test.ts
+++ b/core/llm/llms/OpenAI.test.ts
@@ -39,4 +39,51 @@ describe("OpenAI", () => {
     expect(openai.isOSeriesOrGpt5PlusModel("ao31")).toBeFalsy();
     expect(openai.isOSeriesOrGpt5PlusModel("1os")).toBeFalsy();
   });
+
+  describe("canUseOpenAIResponses", () => {
+    test("should use responses on official OpenAI apiBase for o-series model", () => {
+      const openai = new OpenAI({
+        model: "o3-mini",
+        apiKey: "test",
+      });
+      expect((openai as any).canUseOpenAIResponses({ model: "o3-mini" })).toBe(
+        true,
+      );
+    });
+
+    test("should not use responses on custom apiBase by default", () => {
+      const openai = new OpenAI({
+        model: "o3-mini",
+        apiKey: "test",
+        apiBase: "https://custom.openai-proxy.com/v1/",
+      });
+      expect((openai as any).canUseOpenAIResponses({ model: "o3-mini" })).toBe(
+        false,
+      );
+    });
+
+    test("should allow forcing responses on custom apiBase when useResponsesApi is true", () => {
+      const openai = new OpenAI({
+        model: "o3-mini",
+        apiKey: "test",
+        apiBase: "https://custom.openai-proxy.com/v1/",
+        useResponsesApi: true,
+      });
+      expect((openai as any).canUseOpenAIResponses({ model: "o3-mini" })).toBe(
+        true,
+      );
+    });
+
+    test("should disable responses when useResponsesApi is false", () => {
+      const openai = new OpenAI({
+        model: "o3-mini",
+        apiKey: "test",
+        apiBase: "https://api.openai.com/v1",
+        useResponsesApi: false,
+      });
+      expect((openai as any).canUseOpenAIResponses({ model: "o3-mini" })).toBe(
+        false,
+      );
+    });
+  });
 });
diff --git a/packages/openai-adapters/src/apis/OpenAI.ts b/packages/openai-adapters/src/apis/OpenAI.ts
index d0f8d30ca3a..2cec799b7dc 100644
--- a/packages/openai-adapters/src/apis/OpenAI.ts
+++ b/packages/openai-adapters/src/apis/OpenAI.ts
@@ -91,8 +91,17 @@ export class OpenAIApi implements BaseLlmApi {
     if (this.config.useResponsesApi === false) {
       return false;
     }
-    const isOfficialOpenAIAPI = this.apiBase === "https://api.openai.com/v1/";
-    return isOfficialOpenAIAPI && isResponsesModel(model);
+    if (!isResponsesModel(model)) {
+      return false;
+    }
+    if (this.config.useResponsesApi === true) {
+      return true;
+    }
+    const isOfficialOpenAIAPI =
+      !this.apiBase ||
+      this.apiBase.trim() === "" ||
+      this.apiBase.replace(/\/$/, "") === "https://api.openai.com/v1";
+    return isOfficialOpenAIAPI;
   }
 
   modifyCompletionBody<
diff --git a/packages/openai-adapters/src/test/openai-adapter.vitest.ts b/packages/openai-adapters/src/test/openai-adapter.vitest.ts
index 6abc49a3af4..ef72646fb38 100644
--- a/packages/openai-adapters/src/test/openai-adapter.vitest.ts
+++ b/packages/openai-adapters/src/test/openai-adapter.vitest.ts
@@ -1,4 +1,5 @@
-import { describe, vi } from "vitest";
+import { describe, expect, it, vi } from "vitest";
+import { OpenAIApi } from "../apis/OpenAI.js";
 import { createAdapterTests } from "./adapter-test-utils.js";
 
 // Mock the fetch package (not needed for OpenAI but required by the shared test utils)
@@ -25,4 +26,49 @@ describe("OpenAI Adapter Tests", () => {
       accept: "application/json",
     },
   });
+
+  describe("shouldUseResponsesEndpoint", () => {
+    it("should use responses endpoint on official apiBase for responses models", () => {
+      const api = new OpenAIApi({
+        provider: "openai",
+        apiKey: "test",
+        apiBase: "https://api.openai.com/v1/",
+      });
+      expect((api as any).shouldUseResponsesEndpoint("gpt-5")).toBe(true);
+      expect((api as any).shouldUseResponsesEndpoint("o3-mini")).toBe(true);
+      expect((api as any).shouldUseResponsesEndpoint("gpt-4o")).toBe(false);
+    });
+
+    it("should not use responses endpoint on custom apiBase by default", () => {
+      const api = new OpenAIApi({
+        provider: "openai",
+        apiKey: "test",
+        apiBase: "https://custom-openai-proxy.com/v1/",
+      });
+      expect((api as any).shouldUseResponsesEndpoint("gpt-5")).toBe(false);
+      expect((api as any).shouldUseResponsesEndpoint("o3-mini")).toBe(false);
+    });
+
+    it("should use responses endpoint on custom apiBase if useResponsesApi is true", () => {
+      const api = new OpenAIApi({
+        provider: "openai",
+        apiKey: "test",
+        apiBase: "https://custom-openai-proxy.com/v1/",
+        useResponsesApi: true,
+      });
+      expect((api as any).shouldUseResponsesEndpoint("gpt-5")).toBe(true);
+      expect((api as any).shouldUseResponsesEndpoint("o3-mini")).toBe(true);
+      expect((api as any).shouldUseResponsesEndpoint("gpt-4o")).toBe(false);
+    });
+
+    it("should not use responses endpoint when useResponsesApi is false", () => {
+      const api = new OpenAIApi({
+        provider: "openai",
+        apiKey: "test",
+        apiBase: "https://api.openai.com/v1/",
+        useResponsesApi: false,
+      });
+      expect((api as any).shouldUseResponsesEndpoint("gpt-5")).toBe(false);
+    });
+  });
 });