diff --git a/src/api/api.ts b/src/api/api.ts
index 98cbbd46..90c639ae 100644
--- a/src/api/api.ts
+++ b/src/api/api.ts
@@ -1,28 +1,12 @@
 import ky, { type HTTPError, type KyInstance, type Options } from "ky";
 import { env } from "@/lib/env";
+import { resolveApiPrefixUrl } from "@/lib/resolve-api-prefix-url";
 import { API_PATHS } from "@/api/api-paths";
 import { createApiErrorFromResponse, mapHttpStatusToMessage } from "@/api/api-error";
 import { queryKeys } from "@/infra/query-key/query-key";
 import queryClient from "@/lib/query/query-client";
 import { clearAuth, getAccessToken } from "@/lib/auth/auth-storage";
 
-function resolvePrefixUrl(url: string): string {
-  const trimmed = url.trim() || "/api";
-
-  if (trimmed.startsWith("http")) {
-    return trimmed.endsWith("/") ? trimmed : `${trimmed}/`;
-  }
-
-  const path = trimmed.startsWith("/") ? trimmed : `/${trimmed}`;
-  const pathWithSlash = path.endsWith("/") ? path : `${path}/`;
-  const origin =
-    typeof window !== "undefined" && window.location?.origin
-      ? window.location.origin
-      : "http://localhost:3000";
-
-  return `${origin}${pathWithSlash}`;
-}
-
 function clearSessionAndRedirectToLogin(): void {
   clearAuth();
   queryClient.setQueryData(queryKeys.auth.session(), null);
@@ -37,7 +21,7 @@ function isAuthLoginRequest(request: Request): boolean {
 }
 
 export const api: KyInstance = ky.create({
-  prefixUrl: resolvePrefixUrl(env.apiUrl),
+  prefixUrl: resolveApiPrefixUrl(env.apiUrl),
   headers: {
     Accept: "application/json",
     "Content-Type": "application/json",
diff --git a/src/domain/vendor-portal/api/vendor-portal-api.ts b/src/domain/vendor-portal/api/vendor-portal-api.ts
index 73801ce8..2630e86a 100644
--- a/src/domain/vendor-portal/api/vendor-portal-api.ts
+++ b/src/domain/vendor-portal/api/vendor-portal-api.ts
@@ -1,5 +1,6 @@
 import { API_PATHS } from "@/api/api-paths";
 import { env } from "@/lib/env";
+import { resolveApiBaseUrl } from "@/lib/resolve-api-prefix-url";
 import type {
   VendorPortalComment,
   VendorPortalDispatchDetail,
@@ -9,11 +10,8 @@ import type {
 } from "@/domain/vendor-portal/types/vendor-portal";
 
 function resolveVendorPortalUrl(path: string): string {
-  const base = env.apiUrl.trim().replace(/\/$/, "");
-  const prefix = base.startsWith("http")
-    ? base
-    : `${typeof window !== "undefined" ? window.location.origin : "http://localhost:3000"}${base.startsWith("/") ? base : `/${base}`}`;
-  return `${prefix}/${API_PATHS.vendorPortal.base}${path}`;
+  const base = resolveApiBaseUrl(env.apiUrl);
+  return `${base}/${API_PATHS.vendorPortal.base}${path}`;
 }
 
 async function vendorPortalCall<T>(
diff --git a/src/lib/resolve-api-prefix-url.ts b/src/lib/resolve-api-prefix-url.ts
new file mode 100644
index 00000000..0d42df45
--- /dev/null
+++ b/src/lib/resolve-api-prefix-url.ts
@@ -0,0 +1,35 @@
+const DEFAULT_ORIGIN = "http://localhost:3000";
+
+function getDefaultOrigin(): string {
+  if (typeof window !== "undefined" && window.location?.origin) {
+    return window.location.origin;
+  }
+
+  return DEFAULT_ORIGIN;
+}
+
+/**
+ * Resolves VITE_API_URL to a Ky-compatible prefixUrl (always ends with /).
+ *
+ * Contract:
+ * - Relative paths (e.g. `/api`) are joined with the app origin.
+ * - Absolute URLs are used as-is; they must already include `/api` for production.
+ * - API_PATHS do not include `/api` — only this prefix supplies it.
+ */
+export function resolveApiPrefixUrl(url: string, origin = getDefaultOrigin()): string {
+  const trimmed = url.trim() || "/api";
+
+  if (trimmed.startsWith("http")) {
+    return trimmed.endsWith("/") ? trimmed : `${trimmed}/`;
+  }
+
+  const path = trimmed.startsWith("/") ? trimmed : `/${trimmed}`;
+  const pathWithSlash = path.endsWith("/") ? path : `${path}/`;
+
+  return `${origin}${pathWithSlash}`;
+}
+
+/** API base without trailing slash (for fetch URLs outside Ky). */
+export function resolveApiBaseUrl(url: string, origin = getDefaultOrigin()): string {
+  return resolveApiPrefixUrl(url, origin).replace(/\/$/, "");
+}