fix(core): Use a single shared client instance for all runs (#7110)

langchain-core/src/callbacks/manager.ts

@@ -1267,36 +1267,8 @@ export function ensureHandler(
 }
 
 /**
- * @example
- * ```typescript
- * const prompt = PromptTemplate.fromTemplate(`What is the answer to {question}?`);
- *
- * // Example of using LLMChain to process a series of questions
- * const chain = new LLMChain({
- *   llm: new ChatOpenAI({ temperature: 0.9 }),
- *   prompt,
- * });
- *
- * // Process questions using the chain
- * const processQuestions = async (questions) => {
- *   for (const question of questions) {
- *     const result = await chain.call({ question });
- *     console.log(result);
- *   }
- * };
- *
- * // Example questions
- * const questions = [
- *   "What is your name?",
- *   "What is your quest?",
- *   "What is your favorite color?",
- * ];
- *
- * // Run the example
-const logFunction = handler.raiseError ? console.error : console.warn; 
-* processQuestions(questions).catch(consolelogFunction;
- *
- * ```
+ * @deprecated Use [`traceable`](https://docs.smith.langchain.com/observability/how_to_guides/tracing/annotate_code)
+ * from "langsmith" instead.
  */
 export class TraceGroup {
   private runManager?: CallbackManagerForChainRun;

langchain-core/src/singletons/tracer.ts

@@ -0,0 +1,18 @@
+import { Client } from "langsmith";
+import { getEnvironmentVariable } from "../utils/env.js";
+
+let client: Client;
+
+export const getDefaultLangChainClientSingleton = () => {
+  if (client === undefined) {
+    const clientParams =
+      getEnvironmentVariable("LANGCHAIN_CALLBACKS_BACKGROUND") === "false"
+        ? {
+            // LangSmith has its own backgrounding system
+            blockOnRootRunFinalization: true,
+          }
+        : {};
+    client = new Client(clientParams);
+  }
+  return client;
+};

langchain-core/src/tracers/tracer_langchain.ts

@@ -11,6 +11,7 @@ import {
 import { getEnvironmentVariable, getRuntimeEnvironment } from "../utils/env.js";
 import { BaseTracer } from "./base.js";
 import { BaseCallbackHandlerInput } from "../callbacks/base.js";
+import { getDefaultLangChainClientSingleton } from "../singletons/tracer.js";
 
 export interface Run extends BaseRun {
   id: string;
@@ -59,14 +60,7 @@ export class LangChainTracer
       getEnvironmentVariable("LANGCHAIN_PROJECT") ??
       getEnvironmentVariable("LANGCHAIN_SESSION");
     this.exampleId = exampleId;
-    const clientParams =
-      getEnvironmentVariable("LANGCHAIN_CALLBACKS_BACKGROUND") === "false"
-        ? {
-            // LangSmith has its own backgrounding system
-            blockOnRootRunFinalization: true,
-          }
-        : {};
-    this.client = client ?? new Client(clientParams);
+    this.client = client ?? getDefaultLangChainClientSingleton();
 
     const traceableTree = LangChainTracer.getTraceableRunTree();
     if (traceableTree) {