diff --git a/config/api-url-contract.ts b/config/api-url-contract.ts
new file mode 100644
index 00000000..85f44af6
--- /dev/null
+++ b/config/api-url-contract.ts
@@ -0,0 +1,21 @@
+/** Absolute VITE_API_URL values must end with this segment (API_PATHS omit /api). */
+export const ABSOLUTE_API_URL_SUFFIX = /\/api\/?$/;
+
+/**
+ * Fails fast when an absolute VITE_API_URL omits the /api segment.
+ * Relative values (e.g. /api) are valid and skipped.
+ */
+export function assertAbsoluteApiUrlHasApiSegment(apiUrl: string | undefined): void {
+  const trimmed = apiUrl?.trim();
+  if (!trimmed?.startsWith("http")) {
+    return;
+  }
+
+  if (!ABSOLUTE_API_URL_SUFFIX.test(trimmed)) {
+    throw new Error(
+      `VITE_API_URL absolute URLs must end with /api (got: ${trimmed}). ` +
+        "API route paths do not include /api — set VITE_API_URL to the full API base, e.g. https://api.example.com/api. " +
+        "See README.md and .env.example.",
+    );
+  }
+}
diff --git a/tsconfig.node.json b/tsconfig.node.json
index a02b1983..3e43f464 100644
--- a/tsconfig.node.json
+++ b/tsconfig.node.json
@@ -16,5 +16,10 @@
     "outDir": "./node_modules/.tmp/tsconfig.node",
     "types": ["node"]
   },
-  "include": ["vite.config.ts", "vitest.config.ts"]
+  "include": [
+    "vite.config.ts",
+    "vitest.config.ts",
+    "config/api-url-contract.ts",
+    "config/api-url-contract.test.ts"
+  ]
 }
diff --git a/vite.config.ts b/vite.config.ts
index c25e540b..204b1ce1 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -2,10 +2,13 @@ import react from "@vitejs/plugin-react";
 import tailwindcss from "@tailwindcss/vite";
 import path from "node:path";
 import { defineConfig, loadEnv, transformWithEsbuild } from "vite";
+import { assertAbsoluteApiUrlHasApiSegment } from "./config/api-url-contract";
 
 export default defineConfig(({ mode }) => {
   const env = loadEnv(mode, process.cwd(), "");
 
+  assertAbsoluteApiUrlHasApiSegment(env.VITE_API_URL);
+
   return {
     plugins: [
       {