diff --git a/packages/common/first-referrer-cookie.test.ts b/packages/common/first-referrer-cookie.test.ts
index 18f54214620..4fa97a92751 100644
--- a/packages/common/first-referrer-cookie.test.ts
+++ b/packages/common/first-referrer-cookie.test.ts
@@ -232,6 +232,33 @@ describe('first-referrer-cookie', () => {
         has_existing_cookie: false,
       })
     })
+
+    it('parses URL-encoded value from Next.js response.cookies.set()', () => {
+      const header = `${MW_DIAG_COOKIE_NAME}=hit%3D1%26would_stamp%3D1%26has_cookie%3D0`
+      expect(parseMwDiagCookie(header)).toEqual({
+        hit: true,
+        would_stamp: true,
+        has_existing_cookie: false,
+      })
+    })
+
+    it('parses URL-encoded value with has_cookie=1', () => {
+      const header = `${MW_DIAG_COOKIE_NAME}=hit%3D1%26would_stamp%3D0%26has_cookie%3D1`
+      expect(parseMwDiagCookie(header)).toEqual({
+        hit: true,
+        would_stamp: false,
+        has_existing_cookie: true,
+      })
+    })
+
+    it('parses URL-encoded value among multiple cookies', () => {
+      const header = `session=abc; ${MW_DIAG_COOKIE_NAME}=hit%3D1%26would_stamp%3D0%26has_cookie%3D0; theme=dark`
+      expect(parseMwDiagCookie(header)).toEqual({
+        hit: true,
+        would_stamp: false,
+        has_existing_cookie: false,
+      })
+    })
   })
 
   describe('shouldRefreshCookie', () => {
diff --git a/packages/common/first-referrer-cookie.ts b/packages/common/first-referrer-cookie.ts
index 4f9a8436fb6..2fef56d656b 100644
--- a/packages/common/first-referrer-cookie.ts
+++ b/packages/common/first-referrer-cookie.ts
@@ -290,8 +290,8 @@ export function parseMwDiagCookie(cookieHeader: string): MwDiagData | null {
 
     if (!match) return null
 
-    const value = match.slice(`${MW_DIAG_COOKIE_NAME}=`.length)
-    const params = new URLSearchParams(value)
+    const rawValue = match.slice(`${MW_DIAG_COOKIE_NAME}=`.length)
+    const params = new URLSearchParams(decodeURIComponent(rawValue))
 
     if (params.get('hit') !== '1') return null