diff --git a/backend/src/llm-gemini-header.test.ts b/backend/src/llm-gemini-header.test.ts
new file mode 100644
index 0000000..5a090b2
--- /dev/null
+++ b/backend/src/llm-gemini-header.test.ts
@@ -0,0 +1,32 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { summarizeVideo } from "./llm.js";
+
+test("summarizeVideo sends Gemini API key via x-goog-api-key header and not URL query parameter", async () => {
+  let capturedUrl = "";
+  let capturedHeaders: Record<string, string> = {};
+
+  const originalFetch = globalThis.fetch;
+  globalThis.fetch = (async (input: RequestInfo | URL, init?: RequestInit) => {
+    capturedUrl = input.toString();
+    if (init?.headers) {
+      capturedHeaders = init.headers as Record<string, string>;
+    }
+    return {
+      ok: true,
+      json: async () => ({
+        candidates: [{ content: { parts: [{ text: JSON.stringify({ summary: "test summary" }) }] } }],
+      }),
+    } as Response;
+  }) as typeof fetch;
+
+  try {
+    process.env.GEMINI_API_KEY = "test-secret-gemini-key";
+    await summarizeVideo({ url: "https://www.youtube.com/watch?v=dQw4w9WgXcQ" });
+
+    assert.ok(!capturedUrl.includes("key="), "Request URL must not contain key= query parameter");
+    assert.equal(capturedHeaders["x-goog-api-key"], "test-secret-gemini-key", "x-goog-api-key header must contain the API key");
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
diff --git a/backend/src/llm.ts b/backend/src/llm.ts
index 4974871..e8ab96d 100644
--- a/backend/src/llm.ts
+++ b/backend/src/llm.ts
@@ -754,11 +754,13 @@ export async function summarizeVideo(input: {
     });
 
     const endpoint =
-      `https://generativelanguage.googleapis.com/v1beta/models/${config.gemini.model}:generateContent` +
-      `?key=${encodeURIComponent(config.gemini.apiKey)}`;
+      `https://generativelanguage.googleapis.com/v1beta/models/${config.gemini.model}:generateContent`;
     const res = await fetch(endpoint, {
       method: "POST",
-      headers: { "Content-Type": "application/json" },
+      headers: {
+        "Content-Type": "application/json",
+        "x-goog-api-key": config.gemini.apiKey,
+      },
       body: JSON.stringify({
         systemInstruction: { parts: [{ text: VIDEO_SYSTEM }] },
         contents: [{ role: "user", parts }],