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How to stream chat model responses

All chat models implement the Runnable interface, which comes with a default implementations of standard runnable methods (i.e. invoke, batch, stream, streamEvents).

The default streaming implementation provides an AsyncIterator that yields a single value: the final output from the underlying chat model provider.

tip

The default implementation does not provide support for token-by-token streaming, but it ensures that the the model can be swapped in for any other model as it supports the same standard interface.

The ability to stream the output token-by-token depends on whether the provider has implemented proper streaming support.

See which integrations support token-by-token streaming here.

Streaming​

Below we use a --- to help visualize the delimiter between tokens.

Pick your chat model:

Install dependencies

tip

See this section for general instructions on installing integration packages.

yarn add @langchain/openai

Add environment variables

OPENAI_API_KEY=your-api-key

Instantiate the model

import { ChatOpenAI } from "@langchain/openai";

const model = new ChatOpenAI({
  model: "gpt-3.5-turbo-0125",
  temperature: 0
});
for await (const chunk of await model.stream(
  "Write me a 1 verse song about goldfish on the moon"
)) {
  console.log(`${chunk.content}
---`);
}

---
Here
---
 is
---
 a
---

---
1
---

---
verse
---
 song
---
 about
---
 gol
---
dfish
---
 on
---
 the
---
 moon
---
:
---


Gol
---
dfish
---
 on
---
 the
---
 moon
---
,
---
 swimming
---
 through
---
 the
---
 sk
---
ies
---
,
---

Floating
---
 in
---
 the
---
 darkness
---
,
---
 beneath
---
 the
---
 lunar
---
 eyes
---
.
---

Weight
---
less
---
 as
---
 they
---
 drift
---
,
---
 through
---
 the
---
 endless
---
 voi
---
d,
---

D
---
rif
---
ting
---
,
---
 swimming
---
,
---
 exploring
---
,
---
 this
---
 new
---
 worl
---
d unexp
---
lo
---
ye
---
d.
---

---

---

Stream events​

Chat models also support the standard astream events method.

This method is useful if you’re streaming output from a larger LLM application that contains multiple steps (e.g., an LLM chain composed of a prompt, llm and parser).

let idx = 0;

for await (const event of model.streamEvents(
  "Write me a 1 verse song about goldfish on the moon",
  {
    version: "v1",
  }
)) {
  idx += 1;
  if (idx >= 5) {
    console.log("...Truncated");
    break;
  }
  console.log(event);
}
{
  run_id: "a84e1294-d281-4757-8f3f-dc4440612949",
  event: "on_llm_start",
  name: "ChatAnthropic",
  tags: [],
  metadata: {},
  data: { input: "Write me a 1 verse song about goldfish on the moon" }
}
{
  event: "on_llm_stream",
  run_id: "a84e1294-d281-4757-8f3f-dc4440612949",
  tags: [],
  metadata: {},
  name: "ChatAnthropic",
  data: {
    chunk: AIMessageChunk {
      lc_serializable: true,
      lc_kwargs: {
        content: "",
        additional_kwargs: {
          id: "msg_01DqDQ9in33ZhmrCzdZaRNMZ",
          type: "message",
          role: "assistant",
          model: "claude-3-haiku-20240307"
        },
        tool_calls: [],
        invalid_tool_calls: [],
        tool_call_chunks: [],
        response_metadata: {}
      },
      lc_namespace: [ "langchain_core", "messages" ],
      content: "",
      name: undefined,
      additional_kwargs: {
        id: "msg_01DqDQ9in33ZhmrCzdZaRNMZ",
        type: "message",
        role: "assistant",
        model: "claude-3-haiku-20240307"
      },
      response_metadata: {},
      tool_calls: [],
      invalid_tool_calls: [],
      tool_call_chunks: []
    }
  }
}
{
  event: "on_llm_stream",
  run_id: "a84e1294-d281-4757-8f3f-dc4440612949",
  tags: [],
  metadata: {},
  name: "ChatAnthropic",
  data: {
    chunk: AIMessageChunk {
      lc_serializable: true,
      lc_kwargs: {
        content: "Here",
        additional_kwargs: {},
        tool_calls: [],
        invalid_tool_calls: [],
        tool_call_chunks: [],
        response_metadata: {}
      },
      lc_namespace: [ "langchain_core", "messages" ],
      content: "Here",
      name: undefined,
      additional_kwargs: {},
      response_metadata: {},
      tool_calls: [],
      invalid_tool_calls: [],
      tool_call_chunks: []
    }
  }
}
{
  event: "on_llm_stream",
  run_id: "a84e1294-d281-4757-8f3f-dc4440612949",
  tags: [],
  metadata: {},
  name: "ChatAnthropic",
  data: {
    chunk: AIMessageChunk {
      lc_serializable: true,
      lc_kwargs: {
        content: " is",
        additional_kwargs: {},
        tool_calls: [],
        invalid_tool_calls: [],
        tool_call_chunks: [],
        response_metadata: {}
      },
      lc_namespace: [ "langchain_core", "messages" ],
      content: " is",
      name: undefined,
      additional_kwargs: {},
      response_metadata: {},
      tool_calls: [],
      invalid_tool_calls: [],
      tool_call_chunks: []
    }
  }
}
...Truncated

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