diff --git a/tests/harness/npm.test.ts b/tests/harness/npm.test.ts
index 2df0d87ef..795e42846 100644
--- a/tests/harness/npm.test.ts
+++ b/tests/harness/npm.test.ts
@@ -112,9 +112,10 @@ const KNOWN: Record<string, Known> = {
 const cases = shardSelect(npmCases(fixturesRoot), (c) => c.name);
 
 /** Per-case timeouts past the default 120 s: the undici cases compile the
- * built-in undici implementation's module graph, which a sanitized build
- * on a CI runner takes minutes to compile. */
-const TIMEOUTS: Record<string, number> = { undici5: 600_000, undici7: 600_000 };
+ * built-in undici implementation's module graph, and zod3 compiles zod 3's
+ * 3,700-line types module. A sanitized build of either takes minutes on a
+ * CI runner. */
+const TIMEOUTS: Record<string, number> = { undici5: 600_000, undici7: 600_000, zod3: 600_000 };
 
 interface RunResult {
   stdout: Buffer;