diff --git a/src/adapters/openai-responses-url.ts b/src/adapters/openai-responses-url.ts
index 15f28b03c5..aecc296c70 100644
--- a/src/adapters/openai-responses-url.ts
+++ b/src/adapters/openai-responses-url.ts
@@ -7,8 +7,10 @@ const TRAILING_V1 = /\/v1\/?$/;
  *  Custom `responsesPath` stays on the adapter; this helper is only the legacy /v1/responses branch.
  */
 export function openaiResponsesUrl(baseUrl: string): string {
-  const trimmed = baseUrl.trim().replace(TRAILING_SLASHES, "");
-  const withoutEndpoint = trimmed.replace(TRAILING_RESPONSES, "");
+  const url = new URL(baseUrl.trim());
+  const trimmedPath = url.pathname.replace(TRAILING_SLASHES, "");
+  const withoutEndpoint = trimmedPath.replace(TRAILING_RESPONSES, "");
   const withoutV1 = withoutEndpoint.replace(TRAILING_V1, "");
-  return `${withoutV1}/v1/responses`;
+  url.pathname = `${withoutV1}/v1/responses`;
+  return url.toString();
 }
diff --git a/tests/openai-responses-passthrough.test.ts b/tests/openai-responses-passthrough.test.ts
index a2b7d234c5..8521b05fe0 100644
--- a/tests/openai-responses-passthrough.test.ts
+++ b/tests/openai-responses-passthrough.test.ts
@@ -153,6 +153,7 @@ describe("OpenAI Responses key-auth URL construction", () => {
       ["https://api.openai.example/v1/", "https://api.openai.example/v1/responses"],
       ["https://api.openai.example/v1/responses", "https://api.openai.example/v1/responses"],
       ["https://api.openai.example/v1/responses/", "https://api.openai.example/v1/responses"],
+      ["https://responses", "https://responses/v1/responses"],
     ] as const) {
       expect(buildKeyAuthUrl(baseUrl)).toBe(expectedUrl);
     }