diff --git a/packages/vinext/src/server/app-page-response.ts b/packages/vinext/src/server/app-page-response.ts
index 99062688c..baa24b906 100644
--- a/packages/vinext/src/server/app-page-response.ts
+++ b/packages/vinext/src/server/app-page-response.ts
@@ -223,6 +223,11 @@ export function buildAppPageRscResponse(
   const headers = new Headers({
     "Content-Type": VINEXT_RSC_CONTENT_TYPE,
     Vary: VINEXT_RSC_VARY_HEADER,
+    // Mirror Next.js' edge runtime marker. vinext serves every App Router
+    // response from a Worker (edge-equivalent runtime), so the test that
+    // asserts `x-edge-runtime: '1'` on RSC responses must see this header.
+    // See edge-ssr-app.ts in the Next.js source for the canonical emission.
+    "x-edge-runtime": "1",
   });
 
   if (options.params && Object.keys(options.params).length > 0) {
@@ -257,6 +262,11 @@ export function buildAppPageHtmlResponse(
   const headers = new Headers({
     "Content-Type": "text/html; charset=utf-8",
     Vary: VINEXT_RSC_VARY_HEADER,
+    // Mirror Next.js' edge runtime marker. vinext serves every App Router
+    // response from a Worker (edge-equivalent runtime), so the test that
+    // asserts `x-edge-runtime: '1'` on HTML responses must see this header.
+    // See edge-ssr-app.ts in the Next.js source for the canonical emission.
+    "x-edge-runtime": "1",
   });
 
   if (options.policy.cacheControl) {
diff --git a/tests/app-page-response.test.ts b/tests/app-page-response.test.ts
index e4728fa7a..90a9c9751 100644
--- a/tests/app-page-response.test.ts
+++ b/tests/app-page-response.test.ts
@@ -388,6 +388,18 @@ describe("app page response helpers", () => {
     await expect(response.text()).resolves.toBe("flight");
   });
 
+  it("emits the `x-edge-runtime: 1` marker on RSC responses (issue #1531)", () => {
+    // Next.js sets `x-edge-runtime: 1` on edge-runtime app responses (see
+    // edge-ssr-app.ts in the Next.js source). vinext serves every App Router
+    // response from a Worker, so the marker must always be present.
+    const response = buildAppPageRscResponse(createBody("flight"), {
+      middlewareContext: { headers: null, status: null },
+      policy: {},
+    });
+
+    expect(response.headers.get("x-edge-runtime")).toBe("1");
+  });
+
   it("builds RSC responses with the current compatibility ID header", () => {
     const response = withEnvVar("__VINEXT_RSC_COMPATIBILITY_ID", "compat-a", () =>
       buildAppPageRscResponse(createBody("flight"), {
@@ -478,6 +490,18 @@ describe("app page response helpers", () => {
     expect(setCookies).toContain("mw=1; Path=/");
     await expect(response.text()).resolves.toBe("<h1>page</h1>");
   });
+
+  it("emits the `x-edge-runtime: 1` marker on HTML responses (issue #1531)", () => {
+    // Next.js sets `x-edge-runtime: 1` on edge-runtime app responses (see
+    // edge-ssr-app.ts in the Next.js source). vinext serves every App Router
+    // response from a Worker, so the marker must always be present.
+    const response = buildAppPageHtmlResponse(createBody("<h1>page</h1>"), {
+      middlewareContext: { headers: null, status: null },
+      policy: {},
+    });
+
+    expect(response.headers.get("x-edge-runtime")).toBe("1");
+  });
 });
 
 describe("mergeMiddlewareResponseHeaders", () => {