How to add values to a chain's state

Prerequisites

This guide assumes familiarity with the following concepts:

• LangChain Expression Language (LCEL)
• Chaining runnables
• Calling runnables in parallel
• Custom functions
• Passing data through

An alternate way of passing data through steps of a chain is to leave the current values of the chain state unchanged while assigning a new value under a given key. The RunnablePassthrough.assign() static method takes an input value and adds the extra arguments passed to the assign function.

This is useful in the common LangChain Expression Language pattern of additively creating a dictionary to use as input to a later step.

Here’s an example:

import {
  RunnableParallel,
  RunnablePassthrough,
} from "@langchain/core/runnables";

const runnable = RunnableParallel.from({
  extra: RunnablePassthrough.assign({
    mult: (input: { num: number }) => input.num * 3,
    modified: (input: { num: number }) => input.num + 1,
  }),
});

await runnable.invoke({ num: 1 });

{ extra: { num: 1, mult: 3, modified: 2 } }

Let’s break down what’s happening here.

• The input to the chain is {"num": 1}. This is passed into a RunnableParallel, which invokes the runnables it is passed in parallel with that input.
• The value under the extra key is invoked. RunnablePassthrough.assign() keeps the original keys in the input dict ({"num": 1}), and assigns a new key called mult. The value is lambda x: x["num"] * 3), which is 3. Thus, the result is {"num": 1, "mult": 3}.
• {"num": 1, "mult": 3} is returned to the RunnableParallel call, and is set as the value to the key extra.
• At the same time, the modified key is called. The result is 2, since the lambda extracts a key called "num" from its input and adds one.

Thus, the result is {'extra': {'num': 1, 'mult': 3}, 'modified': 2}.

Streaming

One convenient feature of this method is that it allows values to pass through as soon as they are available. To show this off, we’ll use RunnablePassthrough.assign() to immediately return source docs in a retrieval chain:

tip

See this section for general instructions on installing integration packages.

npm

npm i @langchain/openai

yarn

yarn add @langchain/openai

pnpm

pnpm add @langchain/openai

import { StringOutputParser } from "@langchain/core/output_parsers";
import { ChatPromptTemplate } from "@langchain/core/prompts";
import {
  RunnablePassthrough,
  RunnableSequence,
} from "@langchain/core/runnables";
import { ChatOpenAI, OpenAIEmbeddings } from "@langchain/openai";
import { MemoryVectorStore } from "langchain/vectorstores/memory";

const vectorstore = await MemoryVectorStore.fromDocuments(
  [{ pageContent: "harrison worked at kensho", metadata: {} }],
  new OpenAIEmbeddings()
);

const retriever = vectorstore.asRetriever();

const template = `Answer the question based only on the following context:
{context}

Question: {question}
`;

const prompt = ChatPromptTemplate.fromTemplate(template);

const model = new ChatOpenAI({ model: "gpt-4o" });

const generationChain = prompt.pipe(model).pipe(new StringOutputParser());

const retrievalChain = RunnableSequence.from([
  {
    context: retriever.pipe((docs) => docs[0].pageContent),
    question: new RunnablePassthrough(),
  },
  RunnablePassthrough.assign({ output: generationChain }),
]);

const stream = await retrievalChain.stream("where did harrison work?");

for await (const chunk of stream) {
  console.log(chunk);
}

{ question: "where did harrison work?" }
{ context: "harrison worked at kensho" }
{ output: "" }
{ output: "H" }
{ output: "arrison" }
{ output: " worked" }
{ output: " at" }
{ output: " Kens" }
{ output: "ho" }
{ output: "." }
{ output: "" }

We can see that the first chunk contains the original "question" since that is immediately available. The second chunk contains "context" since the retriever finishes second. Finally, the output from the generation_chain streams in chunks as soon as it is available.

Next steps

Now you’ve learned how to pass data through your chains to help to help format the data flowing through your chains.

To learn more, see the other how-to guides on runnables in this section.