feat(js): Adds time to first token for new AI SDK wrapper (langchain-ai#1930)

Author: jacoblee93

js/package.json

@@ -1,6 +1,6 @@
 {
   "name": "langsmith",
-  "version": "0.3.58",
+  "version": "0.3.59",
   "description": "Client library to connect to the LangSmith LLM Tracing and Evaluation Platform.",
   "packageManager": "yarn@1.22.19",
   "files": [

js/src/experimental/vercel/middleware.ts

@@ -186,6 +186,20 @@ export function LangSmithMiddleware(config?: {
         const chunks: LanguageModelV2StreamPart[] = [];
         const transformStream = new TransformStream({
           async transform(chunk: LanguageModelV2StreamPart, controller) {
+            if (
+              chunk.type === "tool-input-start" ||
+              chunk.type === "text-start"
+            ) {
+              // Only necessary to log the first token event
+              if (
+                runTree?.events == null ||
+                (Array.isArray(runTree.events) && runTree.events.length === 0)
+              ) {
+                runTree?.addEvent({ name: "new_token" });
+              }
+            } else if (chunk.type === "finish") {
+              runTree?.addEvent({ name: "end" });
+            }
             chunks.push(chunk);
             controller.enqueue(chunk);
           },

js/src/index.ts

@@ -20,4 +20,4 @@ export { overrideFetchImplementation } from "./singletons/fetch.js";
 export { getDefaultProjectName } from "./utils/project.js";
 
 // Update using yarn bump-version
-export const __version__ = "0.3.58";
+export const __version__ = "0.3.59";