diff --git a/open-sse/executors/default.ts b/open-sse/executors/default.ts
index 03433011f3..cc2444602a 100644
--- a/open-sse/executors/default.ts
+++ b/open-sse/executors/default.ts
@@ -304,6 +304,21 @@ export class DefaultExecutor extends BaseExecutor {
         return `https://${resourceUrl || "portal.qwen.ai"}/v1/chat/completions`;
       }
       default: {
+        // Honor a user-supplied custom base URL (providerSpecificData.baseUrl) for
+        // OpenAI-format providers (e.g. the built-in "openai" provider pointed at a
+        // proxy/gateway). Without this, a configured custom base URL was silently
+        // ignored and requests always hit the hardcoded this.config.baseUrl
+        // (https://api.openai.com/v1/...). Scoped to openai-format providers so
+        // non-OpenAI default-branch providers keep their existing behavior.
+        const customBaseUrl =
+          typeof credentials?.providerSpecificData?.baseUrl === "string" &&
+          credentials.providerSpecificData.baseUrl.trim()
+            ? (credentials.providerSpecificData.baseUrl as string)
+            : null;
+        const isOpenAIFormat = !this.config.format || this.config.format === "openai";
+        if (customBaseUrl && isOpenAIFormat) {
+          return normalizeOpenAIChatUrl(customBaseUrl);
+        }
         const url = this.config.baseUrl;
         const entry = getRegistryEntry(this.provider);
         return entry?.urlSuffix ? `${url}${entry.urlSuffix}` : url;
diff --git a/tests/unit/executor-default-base.test.ts b/tests/unit/executor-default-base.test.ts
index 75d8c5fe84..6266bcb7fa 100644
--- a/tests/unit/executor-default-base.test.ts
+++ b/tests/unit/executor-default-base.test.ts
@@ -108,6 +108,40 @@ test("DefaultExecutor.buildUrl uses full chat endpoints for hosted OpenAI-compat
   assert.equal(crof.buildUrl("gpt-4.1", true), "https://crof.ai/v1/chat/completions");
 });
 
+test("DefaultExecutor.buildUrl honors a custom providerSpecificData.baseUrl for the built-in openai provider", () => {
+  const openai = new DefaultExecutor("openai");
+
+  // No override → hardcoded OpenAI endpoint (unchanged behavior).
+  assert.equal(
+    openai.buildUrl("gpt-4o", true),
+    "https://api.openai.com/v1/chat/completions"
+  );
+
+  // Custom base URL (e.g. a proxy/gateway) must be used instead of api.openai.com.
+  assert.equal(
+    openai.buildUrl("gpt-4o", true, 0, {
+      providerSpecificData: { baseUrl: "https://api.contactboxtools.me/v1" },
+    }),
+    "https://api.contactboxtools.me/v1/chat/completions"
+  );
+
+  // Trailing slash is normalized.
+  assert.equal(
+    openai.buildUrl("gpt-4o", true, 0, {
+      providerSpecificData: { baseUrl: "https://proxy.example/v1/" },
+    }),
+    "https://proxy.example/v1/chat/completions"
+  );
+
+  // A base URL already pointing at the chat endpoint is kept as-is.
+  assert.equal(
+    openai.buildUrl("gpt-4o", true, 0, {
+      providerSpecificData: { baseUrl: "https://proxy.example/v1/chat/completions" },
+    }),
+    "https://proxy.example/v1/chat/completions"
+  );
+});
+
 test("DefaultExecutor.buildUrl handles openai-compatible and anthropic-compatible providers", () => {
   const openAICompat = new DefaultExecutor("openai-compatible-test");
   const openAIResponsesCompat = new DefaultExecutor("openai-compatible-responses-test");