feat(core): Generalize streaming usage for language models based on passed callback handlers

langchain-core/src/callbacks/base.ts

@@ -285,6 +285,19 @@ abstract class BaseCallbackHandlerMethodsClass {
  */
 export type CallbackHandlerMethods = BaseCallbackHandlerMethodsClass;
 
+/**
+ * Interface for handlers that can indicate a preference for streaming responses.
+ * When implemented, this allows the handler to signal whether it prefers to receive
+ * streaming responses from language models rather than complete responses.
+ */
+export interface CallbackHandlerPrefersStreaming {
+  readonly lc_prefer_streaming: boolean;
+}
+
+export function callbackHandlerPrefersStreaming(x: BaseCallbackHandler) {
+  return "lc_prefer_streaming" in x && x.lc_prefer_streaming;
+}
+
 /**
  * Abstract base class for creating callback handlers in the LangChain
  * framework. It provides a set of optional methods that can be overridden

langchain-core/src/language_models/chat_models.ts

@@ -44,11 +44,10 @@ import {
   RunnableSequence,
   RunnableToolLike,
 } from "../runnables/base.js";
-import { isStreamEventsHandler } from "../tracers/event_stream.js";
-import { isLogStreamHandler } from "../tracers/log_stream.js";
 import { concat } from "../utils/stream.js";
 import { RunnablePassthrough } from "../runnables/passthrough.js";
 import { isZodSchema } from "../utils/types/is_zod_schema.js";
+import { callbackHandlerPrefersStreaming } from "../callbacks/base.js";
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 export type ToolChoice = string | Record<string, any> | "auto" | "any";
@@ -370,9 +369,9 @@ export abstract class BaseChatModel<
     // Even if stream is not explicitly called, check if model is implicitly
     // called from streamEvents() or streamLog() to get all streamed events.
     // Bail out if _streamResponseChunks not overridden
-    const hasStreamingHandler = !!runManagers?.[0].handlers.find((handler) => {
-      return isStreamEventsHandler(handler) || isLogStreamHandler(handler);
-    });
+    const hasStreamingHandler = !!runManagers?.[0].handlers.find(
+      callbackHandlerPrefersStreaming
+    );
     if (
       hasStreamingHandler &&
       baseMessages.length === 1 &&

langchain-core/src/language_models/llms.ts

@@ -24,9 +24,8 @@ import {
 } from "./base.js";
 import type { RunnableConfig } from "../runnables/config.js";
 import type { BaseCache } from "../caches/base.js";
-import { isStreamEventsHandler } from "../tracers/event_stream.js";
-import { isLogStreamHandler } from "../tracers/log_stream.js";
 import { concat } from "../utils/stream.js";
+import { callbackHandlerPrefersStreaming } from "../callbacks/base.js";
 
 export type SerializedLLM = {
   _model: string;
@@ -270,9 +269,9 @@ export abstract class BaseLLM<
     // Even if stream is not explicitly called, check if model is implicitly
     // called from streamEvents() or streamLog() to get all streamed events.
     // Bail out if _streamResponseChunks not overridden
-    const hasStreamingHandler = !!runManagers?.[0].handlers.find((handler) => {
-      return isStreamEventsHandler(handler) || isLogStreamHandler(handler);
-    });
+    const hasStreamingHandler = !!runManagers?.[0].handlers.find(
+      callbackHandlerPrefersStreaming
+    );
     let output: LLMResult;
     if (
       hasStreamingHandler &&

langchain-core/src/tracers/event_stream.ts

@@ -2,6 +2,7 @@ import { BaseTracer, type Run } from "./base.js";
 import {
   BaseCallbackHandler,
   BaseCallbackHandlerInput,
+  CallbackHandlerPrefersStreaming,
 } from "../callbacks/base.js";
 import { IterableReadableStream } from "../utils/stream.js";
 import { AIMessageChunk } from "../messages/ai.js";
@@ -145,7 +146,10 @@ export const isStreamEventsHandler = (
  * handler that logs the execution of runs and emits `RunLog` instances to a
  * `RunLogStream`.
  */
-export class EventStreamCallbackHandler extends BaseTracer {
+export class EventStreamCallbackHandler
+  extends BaseTracer
+  implements CallbackHandlerPrefersStreaming
+{
   protected autoClose = true;
 
   protected includeNames?: string[];
@@ -172,6 +176,8 @@ export class EventStreamCallbackHandler extends BaseTracer {
 
   name = "event_stream_tracer";
 
+  lc_prefer_streaming = true;
+
   constructor(fields?: EventStreamCallbackHandlerInput) {
     super({ _awaitHandler: true, ...fields });
     this.autoClose = fields?.autoClose ?? true;

langchain-core/src/tracers/log_stream.ts

@@ -6,6 +6,7 @@ import { BaseTracer, type Run } from "./base.js";
 import {
   BaseCallbackHandler,
   BaseCallbackHandlerInput,
+  CallbackHandlerPrefersStreaming,
   HandleLLMNewTokenCallbackFields,
 } from "../callbacks/base.js";
 import { IterableReadableStream } from "../utils/stream.js";
@@ -210,7 +211,10 @@ function isChatGenerationChunk(
  * handler that logs the execution of runs and emits `RunLog` instances to a
  * `RunLogStream`.
  */
-export class LogStreamCallbackHandler extends BaseTracer {
+export class LogStreamCallbackHandler
+  extends BaseTracer
+  implements CallbackHandlerPrefersStreaming
+{
   protected autoClose = true;
 
   protected includeNames?: string[];
@@ -241,6 +245,8 @@ export class LogStreamCallbackHandler extends BaseTracer {
 
   name = "log_stream_tracer";
 
+  lc_prefer_streaming = true;
+
   constructor(fields?: LogStreamCallbackHandlerInput) {
     super({ _awaitHandler: true, ...fields });
     this.autoClose = fields?.autoClose ?? true;