diff --git a/frontend/jest.config.ts b/frontend/jest.config.ts
index 13ffe01bcc..d8c38d99ae 100644
--- a/frontend/jest.config.ts
+++ b/frontend/jest.config.ts
@@ -54,8 +54,11 @@ const config: Config = {
   // remark/micromark/unified/mdast/hast/unist dependencies are ESM-only, as are
   // react-router and its cookie-es and @remix-run/route-pattern dependencies.
   // They must be transformed; everything else in node_modules stays ignored.
+  // Keep @fluentui out of this list: it ships CommonJS (lib-commonjs), and
+  // transforming it sends thousands of files per suite through the transform
+  // cache, which is slow and races into EPERM reads on Windows under parallel runs.
   transformIgnorePatterns: [
-    "/node_modules/(?!(@fluentui|axios|react-markdown|react-router|@remix-run/route-pattern|cookie-es|remark-.*|rehype-.*|micromark.*|mdast-.*|hast-.*|unist-.*|unified|bail|trough|vfile.*|is-plain-obj|trim-lines|property-information|comma-separated-tokens|space-separated-tokens|decode-named-character-reference|character-entities.*|html-url-attributes|devlop|zwitch|longest-streak|markdown-table|ccount|escape-string-regexp|estree-util-.*|hastscript|web-namespaces|stringify-entities|inline-style-parser|style-to-object)/)",
+    "/node_modules/(?!(axios|react-markdown|react-router|@remix-run/route-pattern|cookie-es|remark-.*|rehype-.*|micromark.*|mdast-.*|hast-.*|unist-.*|unified|bail|trough|vfile.*|is-plain-obj|trim-lines|property-information|comma-separated-tokens|space-separated-tokens|decode-named-character-reference|character-entities.*|html-url-attributes|devlop|zwitch|longest-streak|markdown-table|ccount|escape-string-regexp|estree-util-.*|hastscript|web-namespaces|stringify-entities|inline-style-parser|style-to-object)/)",
   ],
   globals: {},
 };