diff --git a/lib/src/legacy/importer.ts b/lib/src/legacy/importer.ts
index 892d2efa..0267c0f5 100644
--- a/lib/src/legacy/importer.ts
+++ b/lib/src/legacy/importer.ts
@@ -70,6 +70,7 @@ export class LegacyFileImporterWrapper<sync extends 'sync' | 'async'>
  * new-API `Importer` and `FileImporter`.
  */
 export class LegacyImportersWrapper<sync extends 'sync' | 'async'> {
+  private id = 0;
   private importerResult?: ImporterResult;
   private fileUrl?: URL;
 
@@ -130,6 +131,9 @@ export class LegacyImportersWrapper<sync extends 'sync' | 'async'> {
             ? (result as {file: string}).file
             : p.join(p.dirname(parentPath), path)
         );
+        if (!('file' in result)) {
+          canonicalUrl.searchParams.set('id', '' + this.id++);
+        }
         this.importerResult = {
           contents: result.contents || '',
           syntax: canonicalUrl.pathname.endsWith('.sass')