Type safety

@langchain/svelte is designed around agent-brand inference: pass typeof agent to the hook and get typed state, tool calls, and (for DeepAgents) subagent state maps back. Plain state generics still work for graphs that aren't built with the agent helpers.

Agent-brand inference

Pass typeof agent to infer state, tool-call union, and (for DeepAgents) subagent state map:

import type { agent } from "./agent";
import { useStream } from "@langchain/svelte";

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

  // stream.values is inferred
  // stream.toolCalls is a discriminated union of your tools
  // stream.subagents carries per-subagent state types
}

The same inference works on getStream<typeof agent>() and on the companion selector hooks when you pass a scoped target:

const messages = useMessages(stream); // BaseMessage[]
const tools = useToolCalls<typeof agent>(stream);
const state = useValues<typeof agent>(stream);

Custom state types

For plain state shapes, pass the state directly. The second and third generic slots are InterruptType and ConfigurableType:

import { useStream } from "@langchain/svelte";
import type { BaseMessage } from "@langchain/core/messages";

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

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

Prop-drilling the stream

Two type aliases cover the two common prop shapes:

• UseStreamReturn<T> — fully-resolved return type. Use for { stream: UseStreamReturn<typeof agent> } props when the child needs typed access to state / tool calls. (UseStreamResult<T> is an alias of the same type.)
• AnyStream — type-erased handle (UseStreamReturn<any, any, any>). Use when the child only forwards stream into selector hooks and doesn't care about the underlying state shape.

import type { AnyStream, UseStreamReturn } from "@langchain/svelte";
import type { agent } from "./agent";

function ChatPanel({ stream }: { stream: UseStreamReturn<typeof agent> }) {
  // stream.values is typed here
}

function SubgraphCard({
  stream,
  subgraph,
}: {
  stream: AnyStream;
  subgraph: SubgraphDiscoverySnapshot;
}) {
  // stream is only used to feed selector hooks — no generic required
}

Type helpers

Helper	Use
InferStateType<T>	Unwraps a compiled graph / agent brand / agent tool array into its state shape.
InferToolCalls<T>	Derives a discriminated union of tool-call shapes.
InferToolOutput<T>	Derives a single tool's output type.
InferSubagentStates<T>	{ name: State, … } map derived from a DeepAgent brand.
WidenUpdateMessages<T>	Widens messages so both wire-format and BaseMessage instances typecheck in submit().
StreamSubmitOptions<State, Configurable>	Options shape accepted by submit().
AgentServerAdapter / HttpAgentServerAdapter	Custom-transport interface + convenience class. See Transports.
SelectorTarget / SubagentDiscoverySnapshot / SubgraphDiscoverySnapshot	For components that render scoped views. See Selectors / Subagents.
AssembledToolCall, ToolCallStatus	For rendering tool-call UI.
MessageMetadata, UseSubmissionQueueReturn, SubmissionQueueEntry	Companion-hook return shapes. See Selectors.

API reference