diff --git a/js/src/singletons/fetch.ts b/js/src/singletons/fetch.ts
index 65b30c8d7..27ef19616 100644
--- a/js/src/singletons/fetch.ts
+++ b/js/src/singletons/fetch.ts
@@ -12,7 +12,7 @@ export const setFetchImplementation = (fetch: (...args: any[]) => any) => {
   (globalThis as any)[LANGSMITH_FETCH_IMPLEMENTATION_KEY] = fetch;
 };
 
-export const getFetchImplementation = () => {
+export const getFetchImplementation: () => (...args: any[]) => any = () => {
   return (
     (globalThis as any)[LANGSMITH_FETCH_IMPLEMENTATION_KEY] ??
     DEFAULT_FETCH_IMPLEMENTATION
diff --git a/js/src/utils/async_caller.ts b/js/src/utils/async_caller.ts
index a00350718..2771d2239 100644
--- a/js/src/utils/async_caller.ts
+++ b/js/src/utils/async_caller.ts
@@ -153,8 +153,9 @@ export class AsyncCaller {
 
   fetch(...args: Parameters<typeof fetch>): ReturnType<typeof fetch> {
     return this.call(() =>
-      getFetchImplementation()(...args).then((res) =>
-        res.ok ? res : Promise.reject(res)
+      getFetchImplementation()(...args).then(
+        (res: Awaited<ReturnType<typeof fetch>>) =>
+          res.ok ? res : Promise.reject(res)
       )
     );
   }