diff --git a/internal/router/cors_credentials_test.go b/internal/router/cors_credentials_test.go
new file mode 100644
index 00000000..049b5b1e
--- /dev/null
+++ b/internal/router/cors_credentials_test.go
@@ -0,0 +1,120 @@
+// cors_credentials_test.go — pins the AUTH-004 follow-up fix
+// (instanode-web#150 / #151, prod 2026-05-30): the fiberCORS middleware
+// MUST emit `Access-Control-Allow-Credentials: true` on the actual-CORS
+// response so the dashboard SPA's `fetch('/auth/exchange', {credentials:
+// 'include'})` can READ the response. Without it, the browser blocks
+// the read with:
+//
+//   Access to fetch at 'https://api.instanode.dev/auth/exchange' from
+//   origin 'https://instanode.dev' has been blocked by CORS policy:
+//   The value of the 'Access-Control-Allow-Credentials' header in the
+//   response is '' which must be 'true' when the request's credentials
+//   mode is 'include'.
+//
+// We reconstruct the fiberCORS middleware exactly as router.New
+// configures it (same allow-origins / methods / headers / credentials)
+// and drive a single actual cross-origin GET. The assertion is the live
+// Access-Control-Allow-Credentials header on the response.
+//
+// A future router.New edit that drops AllowCredentials regresses prod
+// login and fails this test.
+
+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"
+)
+
+// TestCORS_AllowCredentialsHeader pins the AUTH-004 follow-up.
+// The fiberCORS config on an allowed cross-origin request must respond
+// with `Access-Control-Allow-Credentials: true`.
+func TestCORS_AllowCredentialsHeader(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,
+		AllowCredentials: true,
+	}))
+	app.Post("/auth/exchange", func(c *fiber.Ctx) error {
+		return c.JSON(fiber.Map{"token": "stub"})
+	})
+
+	// Actual cross-origin POST (not preflight) — the response must carry
+	// Access-Control-Allow-Credentials: true.
+	req := httptest.NewRequest("POST", "/auth/exchange", nil)
+	req.Header.Set("Origin", "https://instanode.dev")
+
+	resp, err := app.Test(req, -1)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	allowCreds := resp.Header.Get("Access-Control-Allow-Credentials")
+	assert.Equal(t, "true", allowCreds,
+		"AUTH-004 follow-up: Access-Control-Allow-Credentials must be 'true' so the dashboard SPA's `credentials: 'include'` POSTs can read the response. Without it the browser blocks the read with a generic 'blocked by CORS policy' error. Got %q", allowCreds)
+
+	// Sanity: Access-Control-Allow-Origin must NOT be `*` when credentials
+	// are allowed (CORS spec forbids that combination). It should be the
+	// exact origin instead.
+	allowOrigin := resp.Header.Get("Access-Control-Allow-Origin")
+	assert.Equal(t, "https://instanode.dev", allowOrigin,
+		"with AllowCredentials:true, Allow-Origin must be the exact requesting origin (not `*`); got %q", allowOrigin)
+}
+
+// TestCORSPreflight_HasAllowCredentialsHeader — same assertion but on
+// the OPTIONS preflight path. Browsers check the preflight response for
+// `Access-Control-Allow-Credentials: true` BEFORE issuing the actual
+// request when the JS uses `credentials: 'include'`. Without it, the
+// preflight is treated as "credentials not permitted" and the real
+// request never fires.
+func TestCORSPreflight_HasAllowCredentialsHeader(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,
+		AllowCredentials: true,
+	}))
+	app.Post("/auth/exchange", func(c *fiber.Ctx) error {
+		return c.JSON(fiber.Map{"token": "stub"})
+	})
+
+	req := httptest.NewRequest("OPTIONS", "/auth/exchange", nil)
+	req.Header.Set("Origin", "https://instanode.dev")
+	req.Header.Set("Access-Control-Request-Method", "POST")
+
+	resp, err := app.Test(req, -1)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	require.True(t, resp.StatusCode == fiber.StatusNoContent || resp.StatusCode == fiber.StatusOK,
+		"preflight expected 204/200; got %d", resp.StatusCode)
+
+	allowCreds := resp.Header.Get("Access-Control-Allow-Credentials")
+	assert.Equal(t, "true", allowCreds,
+		"preflight Allow-Credentials must be 'true' for `credentials:include` requests; got %q", allowCreds)
+}
diff --git a/internal/router/router.go b/internal/router/router.go
index 8a9155e0..8f533069 100644
--- a/internal/router/router.go
+++ b/internal/router/router.go
@@ -260,6 +260,17 @@ 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",
+		// AUTH-004 follow-up (2026-05-30): /auth/exchange is called from the
+		// dashboard SPA with `credentials: 'include'` so the browser sends
+		// the HttpOnly `instanode_session_exchange` cookie cross-origin
+		// (instanode.dev → api.instanode.dev). For the browser to ALLOW the
+		// SPA to read the response body, the response must carry
+		// `Access-Control-Allow-Credentials: true`. Without it, fetch
+		// rejects the read with the generic "Failed to fetch" / "blocked
+		// by CORS policy" error. Safe because AllowOrigins is an explicit
+		// allowlist (no `*` — the CORS spec forbids credentials + wildcard
+		// origin precisely to prevent rogue sites from siphoning cookies).
+		AllowCredentials: true,
 		// 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 —