diff --git a/.changeset/swift-clocks-ring.md b/.changeset/swift-clocks-ring.md
new file mode 100644
index 0000000000..641166e7d3
--- /dev/null
+++ b/.changeset/swift-clocks-ring.md
@@ -0,0 +1,5 @@
+---
+"@stackoverflow/stacks-svelte": patch
+---
+
+Pre-bundle dayjs plugins with .js extension in optimizeDeps
diff --git a/packages/stacks-svelte/vite.config.ts b/packages/stacks-svelte/vite.config.ts
index b849fdcedb..ecb7a82b89 100644
--- a/packages/stacks-svelte/vite.config.ts
+++ b/packages/stacks-svelte/vite.config.ts
@@ -21,7 +21,9 @@ export default defineConfig({
             "@stackoverflow/stacks-icons-legacy/icons",
             "dayjs",
             "dayjs/plugin/relativeTime",
+            "dayjs/plugin/relativeTime.js",
             "dayjs/plugin/updateLocale",
+            "dayjs/plugin/updateLocale.js",
             // svelte-sonner must be pre-bundled to ensure its virtual CSS modules
             // (from <style global> blocks) are properly resolved during test execution.
             // Without pre-bundling, CI environments may trigger mid-test re-optimization,