@langchain/svelte

Svelte SDK for building AI-powered applications with Deep Agents, LangChain and LangGraph. It provides a useStream function that manages streaming, state, branching, and interrupts with a Svelte 5 runes-compatible reactive API.

Installation

npm install @langchain/svelte @langchain/core

Peer dependencies: svelte (^5.0.0), @langchain/core (^1.0.1)

Quick Start

<script lang="ts">
  import { useStream } from "@langchain/svelte";

  const stream = useStream({
    assistantId: "agent",
    apiUrl: "http://localhost:2024",
  });
</script>

<div>
  {#each stream.messages as msg, i (msg.id ?? i)}
    <div>{msg.content}</div>
  {/each}

  <button
    disabled={stream.isLoading}
    onclick={() =>
      void stream.submit({ messages: [{ type: "human", content: "Hello!" }] })}
  >
    Send
  </button>
</div>

All reactive properties (messages, isLoading, values, etc.) are accessed directly on the returned object — no $ prefix needed. Avoid destructuring reactive properties; use stream.messages instead of const { messages } = stream to keep reactivity intact.

useStream Options

| Option | Type | Description | |---|---|---| | assistantId | string | Required. The assistant/graph ID to stream from. | | apiUrl | string | Base URL of the LangGraph API. | | client | Client | Pre-configured Client instance (alternative to apiUrl). | | messagesKey | string | State key containing messages. Defaults to "messages". | | initialValues | StateType | Initial state values before any stream data arrives. | | fetchStateHistory | boolean \| { limit: number } | Fetch thread history on stream completion. Enables branching. | | throttle | boolean \| number | Throttle state updates for performance. | | onFinish | (state, error?) => void | Called when the stream completes. | | onError | (error, state?) => void | Called on stream errors. | | onThreadId | (threadId) => void | Called when a new thread is created. | | onUpdateEvent | (event) => void | Receive update events from the stream. | | onCustomEvent | (event) => void | Receive custom events from the stream. | | onStop | () => void | Called when the stream is stopped by the user. |

Return Values

All reactive properties are exposed as getters on the returned object. They update automatically and can be read directly in Svelte 5 templates without the $ prefix.

| Property | Type | Description | |---|---|---| | values | StateType | Current graph state. | | messages | BaseMessage[] | Messages from the current state. | | isLoading | boolean | Whether a stream is currently active. | | error | unknown | The most recent error, if any. | | interrupt | Interrupt \| undefined | Current interrupt requiring user input. | | branch | string | Active branch identifier. | | submit(values, options?) | function | Submit new input to the graph. When called while a stream is active, the run is created on the server with multitaskStrategy: "enqueue" and queued automatically. | | stop() | function | Cancel the active stream. | | setBranch(branch) | function | Switch to a different conversation branch. | | getMessagesMetadata(msg, index?) | function | Get branching and checkpoint metadata for a message. | | switchThread(id) | (id: string \| null) => void | Switch to a different thread. Pass null to start a new thread on next submit. | | queue.entries | ReadonlyArray<QueueEntry> | Pending server-side runs. Each entry has id (server run ID), values, options, and createdAt. | | queue.size | number | Number of pending runs on the server. | | queue.cancel(id) | (id: string) => Promise<boolean> | Cancel a pending run on the server by its run ID. | | queue.clear() | () => Promise<void> | Cancel all pending runs on the server. |

Type Safety

Provide your state type as a generic parameter:

<script lang="ts">
  import { useStream } from "@langchain/svelte";
  import type { BaseMessage } from "langchain";

  interface MyState {
    messages: BaseMessage[];
    context?: string;
  }

  const stream = useStream<MyState>({
    assistantId: "my-graph",
    apiUrl: "http://localhost:2024",
  });
</script>

Typed Interrupts

<script lang="ts">
  import { useStream } from "@langchain/svelte";
  import type { BaseMessage } from "langchain";

  const stream = useStream<
    { messages: BaseMessage[] },
    { InterruptType: { question: string } }
  >({
    assistantId: "my-graph",
    apiUrl: "http://localhost:2024",
  });
</script>

Handling Interrupts

<script lang="ts">
  import { useStream } from "@langchain/svelte";
  import type { BaseMessage } from "langchain";

  const stream = useStream<
    { messages: BaseMessage[] },
    { InterruptType: { question: string } }
  >({
    assistantId: "agent",
    apiUrl: "http://localhost:2024",
  });
</script>

<div>
  {#each stream.messages as msg, i (msg.id ?? i)}
    <div>{msg.content}</div>
  {/each}

  {#if stream.interrupt}
    <div>
      <p>{stream.interrupt.value.question}</p>
      <button onclick={() => void stream.submit(null, { command: { resume: "Approved" } })}>
        Approve
      </button>
    </div>
  {/if}

  <button
    onclick={() =>
      void stream.submit({ messages: [{ type: "human", content: "Hello" }] })}
  >
    Send
  </button>
</div>

Branching

Enable conversation branching with fetchStateHistory: true:

<script lang="ts">
  import { useStream } from "@langchain/svelte";

  const stream = useStream({
    assistantId: "agent",
    apiUrl: "http://localhost:2024",
    fetchStateHistory: true,
  });
</script>

<div>
  {#each stream.messages as msg, i (msg.id ?? i)}
    {@const metadata = stream.getMessagesMetadata(msg, i)}
    {@const branchOptions = metadata?.branchOptions}
    {@const currentBranch = metadata?.branch}

    <div>
      <p>{msg.content}</p>

      {#if branchOptions && currentBranch}
        <button onclick={() => {
          const prev = branchOptions[branchOptions.indexOf(currentBranch) - 1];
          if (prev) stream.setBranch(prev);
        }}>
          Previous
        </button>
        <span>
          {branchOptions.indexOf(currentBranch) + 1} / {branchOptions.length}
        </span>
        <button onclick={() => {
          const next = branchOptions[branchOptions.indexOf(currentBranch) + 1];
          if (next) stream.setBranch(next);
        }}>
          Next
        </button>
      {/if}
    </div>
  {/each}

  <button
    onclick={() =>
      void stream.submit({ messages: [{ type: "human", content: "Hello" }] })}
  >
    Send
  </button>
</div>

Server-Side Queuing

When submit() is called while a stream is already active, the SDK automatically creates the run on the server with multitaskStrategy: "enqueue". The pending runs are tracked in queue and processed in order as each finishes:

<script lang="ts">
  import { useStream } from "@langchain/svelte";

  const stream = useStream({
    assistantId: "agent",
    apiUrl: "http://localhost:2024",
  });
</script>

<div>
  {#each stream.messages as msg, i (msg.id ?? i)}
    <div>{msg.content}</div>
  {/each}

  {#if stream.queue.size > 0}
    <div>
      <p>{stream.queue.size} message(s) queued</p>
      <button onclick={() => void stream.queue.clear()}>Clear Queue</button>
    </div>
  {/if}

  <button
    disabled={stream.isLoading}
    onclick={() =>
      void stream.submit({ messages: [{ type: "human", content: "Hello!" }] })}
  >
    Send
  </button>
  <button onclick={() => stream.switchThread(null)}>New Thread</button>
</div>

Switching threads via switchThread() cancels all pending runs and clears the queue.

Stream Context

Use setStreamContext and getStreamContext to share a single useStream instance across a component tree without prop drilling. This uses Svelte's built-in setContext / getContext under the hood.

Setting context in a parent

Call setStreamContext during component initialisation to provide the stream to all descendants:

<script lang="ts">
  import { useStream, setStreamContext } from "@langchain/svelte";
  import ChatMessages from "./ChatMessages.svelte";
  import ChatInput from "./ChatInput.svelte";

  const stream = useStream({
    assistantId: "agent",
    apiUrl: "http://localhost:2024",
  });

  setStreamContext(stream);
</script>

<ChatMessages />
<ChatInput />

Consuming context in a child

Call getStreamContext in any descendant to retrieve the stream. The returned value has the same shape and types as the useStream return value:

<script lang="ts">
  import { getStreamContext } from "@langchain/svelte";

  const stream = getStreamContext();
</script>

{#each stream.messages as msg, i (msg.id ?? i)}
  <div>{msg.content}</div>
{/each}

{#if stream.isLoading}
  <p>Thinking…</p>
{/if}

<script lang="ts">
  import { getStreamContext } from "@langchain/svelte";

  const stream = getStreamContext();

  let input = $state("");
</script>

<form onsubmit={(e) => { e.preventDefault(); void stream.submit({ messages: [{ type: "human", content: input }] }); input = ""; }}>
  <input bind:value={input} />
  <button type="submit">Send</button>
</form>

Type safety

getStreamContext accepts the same generic parameters as useStream so child components can be fully typed:

<script lang="ts">
  import { getStreamContext } from "@langchain/svelte";
  import type { BaseMessage } from "@langchain/core/messages";

  interface MyState {
    messages: BaseMessage[];
    context?: string;
  }

  const stream = getStreamContext<MyState>();
</script>

setStreamContext returns the stream it was given, so you can inline both calls:

<script lang="ts">
  import { useStream, setStreamContext } from "@langchain/svelte";

  const stream = setStreamContext(
    useStream({ assistantId: "agent", apiUrl: "http://localhost:2024" }),
  );
</script>

Note: Both functions must be called during component initialisation (i.e. at the top level of a <script> block), just like Svelte's own setContext / getContext. Calling getStreamContext without a parent setStreamContext throws an error.

Custom Transport

Instead of connecting to a LangGraph API, you can provide your own streaming transport. Pass a transport object instead of assistantId to use a custom backend:

<script lang="ts">
  import { useStream, FetchStreamTransport } from "@langchain/svelte";
  import type { BaseMessage } from "langchain";

  const stream = useStream<{ messages: BaseMessage[] }>({
    transport: new FetchStreamTransport({
      url: "https://my-api.example.com/stream",
    }),
    threadId: null,
    onThreadId: (id) => console.log("Thread created:", id),
  });
</script>

<div>
  {#each stream.messages as msg, i (msg.id ?? i)}
    {@const metadata = stream.getMessagesMetadata(msg, i)}
    <div>
      <p>{msg.content}</p>
      {#if metadata?.streamMetadata}
        <span>Node: {metadata.streamMetadata.langgraph_node}</span>
      {/if}
    </div>
  {/each}

  <p>Current branch: {stream.branch}</p>

  <button
    disabled={stream.isLoading}
    onclick={() =>
      void stream.submit({ messages: [{ type: "human", content: "Hello!" }] })}
  >
    Send
  </button>
</div>

The custom transport interface returns the same properties as the standard useStream function, including getMessagesMetadata, branch, setBranch, switchThread, and all message/interrupt/subagent helpers. When using a custom transport, getMessagesMetadata returns stream metadata sent alongside messages during streaming; branch and setBranch provide local branch state management. onFinish is also supported and receives a synthetic ThreadState built from the final locally streamed values; the run metadata argument is undefined.

Sharing State with provideStream

When multiple components need access to the same stream (a message list, a header, an input bar), use provideStream and getStream to share a single stream instance via Svelte's context API:

<!-- ChatContainer.svelte -->
<script lang="ts">
  import { provideStream } from "@langchain/svelte";
  import ChatHeader from "./ChatHeader.svelte";
  import MessageList from "./MessageList.svelte";
  import MessageInput from "./MessageInput.svelte";

  provideStream({
    assistantId: "agent",
    apiUrl: "http://localhost:2024",
  });
</script>

<ChatHeader />
<MessageList />
<MessageInput />

<!-- MessageList.svelte -->
<script lang="ts">
  import { getStream } from "@langchain/svelte";

  const stream = getStream();
</script>

{#each stream.messages as msg (msg.id)}
  <div>{msg.content}</div>
{/each}

<!-- MessageInput.svelte -->
<script lang="ts">
  import { getStream } from "@langchain/svelte";

  const stream = getStream();
  let input = $state("");

  function send() {
    stream.submit({ messages: [{ type: "human", content: input }] });
    input = "";
  }
</script>

<form onsubmit={send}>
  <textarea bind:value={input}></textarea>
  <button disabled={stream.isLoading} type="submit">Send</button>
</form>

<!-- ChatHeader.svelte -->
<script lang="ts">
  import { getStream } from "@langchain/svelte";

  const stream = getStream();
</script>

<header>
  <h1>Chat</h1>
  {#if stream.isLoading}
    <span>Thinking...</span>
  {/if}
  {#if stream.error}
    <span>Error occurred</span>
  {/if}
</header>

Multiple Agents

Nest provideStream calls for multi-agent scenarios — Svelte's context scoping ensures each subtree gets its own stream:

<!-- ResearchPanel.svelte -->
<script lang="ts">
  import { provideStream } from "@langchain/svelte";
  provideStream({ assistantId: "researcher", apiUrl: "http://localhost:2024" });
</script>

<MessageList />
<MessageInput />

Playground

For complete end-to-end examples with full agentic UIs, visit the LangChain UI Playground.

License

MIT