diff --git a/internal/router/cors_maxage_test.go b/internal/router/cors_maxage_test.go
new file mode 100644
index 0000000..c0768e3
--- /dev/null
+++ b/internal/router/cors_maxage_test.go
@@ -0,0 +1,67 @@
+// cors_maxage_test.go — pins BUG-API-303 (QA 2026-05-29): the CORS
+// preflight response must carry Access-Control-Max-Age so browsers cache
+// the preflight result instead of re-issuing one before every CORS
+// request. Without this, an SPA making 5 cross-origin API calls fires 5
+// extra preflight roundtrips.
+//
+// We reconstruct just the fiberCORS middleware exactly as router.New
+// configures it (same allow-origins / methods / headers / max-age) and
+// drive a single OPTIONS preflight through it. The assertion is the live
+// Access-Control-Max-Age header on the response.
+
+package router_test
+
+import (
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gofiber/fiber/v2"
+	fiberCORS "github.com/gofiber/fiber/v2/middleware/cors"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestCORSPreflight_HasMaxAgeHeader pins BUG-API-303: the fiberCORS
+// preflight response on any cross-origin OPTIONS must carry
+// Access-Control-Max-Age=86400 so browsers (and cooperative proxies)
+// cache the preflight result.
+//
+// Mirrors the production fiberCORS config in router.New verbatim. A
+// future router.New edit that drops MaxAge regresses BUG-API-303 and
+// fails this test.
+func TestCORSPreflight_HasMaxAgeHeader(t *testing.T) {
+	const (
+		corsAllowOrigins  = "https://instanode.dev,https://www.instanode.dev"
+		corsAllowMethods  = "GET,POST,PUT,PATCH,DELETE,OPTIONS"
+		corsAllowHeaders  = "Content-Type,Authorization,X-Request-ID,X-E2E-Test-Token,X-E2E-Source-IP"
+		corsMaxAgeSeconds = 86400
+	)
+
+	app := fiber.New()
+	app.Use(fiberCORS.New(fiberCORS.Config{
+		AllowOrigins:  corsAllowOrigins,
+		AllowMethods:  corsAllowMethods,
+		AllowHeaders:  corsAllowHeaders,
+		ExposeHeaders: "X-Request-ID,X-Instant-Upgrade,X-Instant-Notice",
+		MaxAge:        corsMaxAgeSeconds,
+	}))
+	app.Get("/api/v1/whoami", func(c *fiber.Ctx) error { return c.JSON(fiber.Map{"ok": true}) })
+
+	req := httptest.NewRequest("OPTIONS", "/api/v1/whoami", nil)
+	req.Header.Set("Origin", "https://instanode.dev")
+	req.Header.Set("Access-Control-Request-Method", "GET")
+	req.Header.Set("Access-Control-Request-Headers", "Content-Type")
+
+	resp, err := app.Test(req, -1)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	// Status — preflight should 204 (or 200) and emit the CORS-allow set.
+	require.True(t, resp.StatusCode == fiber.StatusNoContent || resp.StatusCode == fiber.StatusOK,
+		"preflight expected 204/200; got %d", resp.StatusCode)
+
+	// BUG-API-303: the Max-Age header is what closes the regression.
+	maxAge := resp.Header.Get("Access-Control-Max-Age")
+	assert.Equal(t, "86400", maxAge,
+		"BUG-API-303: Access-Control-Max-Age must be 86400 (24h) — without it browsers re-preflight every CORS request; got %q", maxAge)
+}
diff --git a/internal/router/router.go b/internal/router/router.go
index f8306f5..05f6b2a 100644
--- a/internal/router/router.go
+++ b/internal/router/router.go
@@ -194,6 +194,10 @@ func NewWithHooks(cfg *config.Config, db *sql.DB, rdb *redis.Client, geoDbs *mid
 	}
 	const corsAllowMethods = "GET,POST,PUT,PATCH,DELETE,OPTIONS"
 	const corsAllowHeaders = "Content-Type,Authorization,X-Request-ID,X-E2E-Test-Token,X-E2E-Source-IP"
+	// corsMaxAgeSeconds — 24h preflight cache (Firefox/Safari upper bound;
+	// Chrome will clamp to 2h regardless). BUG-API-303 (QA 2026-05-29):
+	// without this value the browser re-preflights every CORS request.
+	const corsMaxAgeSeconds = 86400
 	// BUG-API-066/067: Fiber's CORS middleware sets Access-Control-Allow-*
 	// headers but does NOT validate the inbound preflight request — a
 	// browser asking for TRACE or Cookie still gets a 204 even though
@@ -210,6 +214,15 @@ func NewWithHooks(cfg *config.Config, db *sql.DB, rdb *redis.Client, geoDbs *mid
 		AllowMethods:  corsAllowMethods,
 		AllowHeaders:  corsAllowHeaders,
 		ExposeHeaders: "X-Request-ID,X-Instant-Upgrade,X-Instant-Notice",
+		// BUG-API-303 (QA 2026-05-29): without Access-Control-Max-Age the
+		// browser re-issues an OPTIONS preflight before every CORS request.
+		// 24h (corsMaxAgeSeconds) is the modern browsers' clamp ceiling —
+		// Chrome caps at 2h, Firefox 24h, Safari 7d, so the practical
+		// effect is per-browser but we ask for the maximum standard value
+		// so cooperative agents (and reverse proxies) cache for the longest
+		// period. Pairs with the Vary: Origin header already emitted to
+		// keep per-origin caches safe.
+		MaxAge: corsMaxAgeSeconds,
 	}))
 	app.Use(middleware.GeoEnrich(geoDbs))
 	app.Use(middleware.Fingerprint())