diff --git a/packages/ui/components/tooltip.wrn b/packages/ui/components/tooltip.wrn
index 2c2f895e..8e980ad6 100644
--- a/packages/ui/components/tooltip.wrn
+++ b/packages/ui/components/tooltip.wrn
@@ -189,6 +189,12 @@ id: string = ""
       pointer-events: none;
     }
 
+    /* Closed until hovered or focused, including in server HTML before the
+       client applies data-show; otherwise every tooltip flashes on load. */
+    .wrn-tooltip[data-open="false"] .wrn-tooltip__content {
+      display: none;
+    }
+
     .wrn-tooltip[data-interactive="true"] .wrn-tooltip__content {
       pointer-events: auto;
     }
diff --git a/packages/ui/test/tooltip-closed.test.ts b/packages/ui/test/tooltip-closed.test.ts
new file mode 100644
index 00000000..cbb4f0e9
--- /dev/null
+++ b/packages/ui/test/tooltip-closed.test.ts
@@ -0,0 +1,8 @@
+import { expect, test } from "bun:test";
+
+// Server HTML renders tooltip content before the client applies data-show, so
+// a closed tooltip must also be hidden by CSS or it flashes on every page load.
+test("closed tooltip content is hidden by CSS before hydration", async () => {
+  const source = await Bun.file(new URL("../components/tooltip.wrn", import.meta.url)).text();
+  expect(source).toMatch(/\.wrn-tooltip\[data-open="false"\] \.wrn-tooltip__content \{\s*display: none;/);
+});