Tool calling

Display agent tool calls with rich, type-safe UI cards

Agents can invoke external tools like weather APIs, calculators, web search, database queries, and more. The results are in raw JSON. This pattern shows you how to render structured, type-safe UI cards for every tool call your agent makes, complete with loading states and error handling.

How tool calling works

When a LangGraph agent decides it needs external data, it emits one or more tool calls as part of an AI message. Each tool call includes:

• name: the tool being invoked (e.g. "get_weather", "calculator")
• args: the structured arguments passed to the tool
• id: a unique identifier linking the call to its result

The agent runtime executes the tool, and the result comes back as a ToolMessage. The useStream hook unifies all of this into a single toolCalls array you can render directly.

Setting up useStream

The first step is wiring up useStream to your agent backend. The hook returns reactive state including a toolCalls array that updates in real time as the agent streams.

The code examples use useStream<typeof myAgent> for type-safe stream state. See Type inference for Python or JavaScript backends.

React

import { useStream } from "@langchain/react";

const AGENT_URL = "http://localhost:2024";

export function Chat() {
  const stream = useStream<typeof myAgent>({
    apiUrl: AGENT_URL,
    assistantId: "tool_calling",
  });

  return (
    <div>
      {stream.messages.map((msg) => (
        <Message key={msg.id} message={msg} toolCalls={stream.toolCalls} />
      ))}
    </div>
  );
}

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

const AGENT_URL = "http://localhost:2024";

const stream = useStream<typeof myAgent>({
  apiUrl: AGENT_URL,
  assistantId: "tool_calling",
});
</script>

<template>
  <div>
    <Message
      v-for="msg in stream.messages.value"
      :key="msg.id"
      :message="msg"
      :tool-calls="stream.toolCalls.value"
    />
  </div>
</template>

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

  const AGENT_URL = "http://localhost:2024";

  const stream = useStream<typeof myAgent>({
    apiUrl: AGENT_URL,
    assistantId: "tool_calling",
  });
</script>

<div>
  {#each stream.messages as msg (msg.id)}
    <Message message={msg} toolCalls={stream.toolCalls} />
  {/each}
</div>

import { Component } from "@angular/core";
import { injectStream } from "@langchain/angular";

const AGENT_URL = "http://localhost:2024";

@Component({
  selector: "app-chat",
  template: `
    @for (msg of stream.messages(); track msg.id) {
      <app-message [message]="msg" [toolCalls]="stream.toolCalls()" />
    }
  `,
})
export class ChatComponent {
  stream = injectStream<typeof myAgent>({
    apiUrl: AGENT_URL,
    assistantId: "tool_calling",
  });
}

The AssembledToolCall type

Each entry in the toolCalls array is an AssembledToolCall object:

interface AssembledToolCall<
  TName extends string = string,
  TInput = unknown,
  TOutput = unknown,
> {
  name: TName;
  callId: string;
  id: string;
  namespace: string[];
  input: TInput;
  args: TInput;
  output: TOutput | null;
  status: "running" | "finished" | "error";
  error: string | undefined;
}

Property	Description
name	The name of the tool (e.g. "get_weather")
callId	Unique ID matching the AI message’s tool_calls entry
id	Alias for callId, matching message-level tool calls
namespace	Namespace where the tool call was emitted
input	Structured arguments the agent passed to the tool
args	Alias for input, matching message-level tool calls
output	Tool output after a successful call, or null while running or after an error
status	Lifecycle state: "running", "finished", or "error"
error	Error details when the tool call fails

Filtering tool calls per message

An AI message may trigger multiple tool calls, and your chat may contain many AI messages. To render the right tool cards under each message, filter by matching callId against the message’s tool_calls array:

function Message({
  message,
  toolCalls,
}: {
  message: AIMessage;
  toolCalls: AssembledToolCall[];
}) {
  const messageToolCalls = toolCalls.filter((tc) =>
    message.tool_calls?.find((t) => t.id === tc.callId)
  );

  return (
    <div>
      <p>{message.text}</p>
      {messageToolCalls.map((tc) => (
        <ToolCard key={tc.callId} toolCall={tc} />
      ))}
    </div>
  );
}

Building specialized tool cards

Rather than dumping raw JSON, build dedicated UI components for each tool. Use name to select the right card:

function ToolCard({ toolCall }: { toolCall: AssembledToolCall }) {
  if (toolCall.status === "running") {
    return <LoadingCard name={toolCall.name} />;
  }

  if (toolCall.status === "error") {
    return <ErrorCard name={toolCall.name} error={toolCall.error} />;
  }

  switch (toolCall.name) {
    case "get_weather":
      return <WeatherCard input={toolCall.input} output={toolCall.output} />;
    case "calculator":
      return (
        <CalculatorCard input={toolCall.input} output={toolCall.output} />
      );
    case "web_search":
      return <SearchCard input={toolCall.input} output={toolCall.output} />;
    default:
      return <GenericToolCard toolCall={toolCall} />;
  }
}

Weather card example

function WeatherCard({
  input,
  output,
}: {
  input: { location: string };
  output: { temperature: number; condition: string };
}) {
  return (
    <div className="rounded-lg border p-4">
      <div className="flex items-center gap-2">
        <CloudIcon />
        <h3 className="font-semibold">{input.location}</h3>
      </div>
      <div className="mt-2 text-3xl font-bold">{output.temperature}°F</div>
      <p className="text-muted-foreground">{output.condition}</p>
    </div>
  );
}

Loading and error states

Always handle the pending and error states to give users clear feedback:

function LoadingCard({ name }: { name: string }) {
  return (
    <div className="flex items-center gap-2 rounded-lg border p-4 animate-pulse">
      <Spinner />
      <span>Running {name}...</span>
    </div>
  );
}

function ErrorCard({ name, error }: { name: string; error?: unknown }) {
  return (
    <div className="rounded-lg border border-red-300 bg-red-50 p-4">
      <h3 className="font-semibold text-red-700">Error in {name}</h3>
      <p className="text-sm text-red-600">
        {String(error ?? "Tool execution failed")}
      </p>
    </div>
  );
}

Type-safe tool arguments

If your tools are defined with structured schemas, you can use the ToolCallFromTool utility type to get fully typed args:

import { tool } from "@langchain/core/tools";
import { z } from "zod";

const getWeather = tool(async ({ location }) => { /* ... */ }, {
  name: "get_weather",
  description: "Get the current weather for a location",
  schema: z.object({
    location: z.string().describe("City name"),
  }),
});

type WeatherToolCall = ToolCallFromTool<typeof getWeather>;
// WeatherToolCall.input and WeatherToolCall.args are now { location: string }

Using ToolCallFromTool gives you compile-time safety. If the tool schema changes, your UI components will flag type errors immediately.

Rendering tool calls inline with streaming text

Tool calls often arrive interleaved with streamed text. The useStream hook keeps toolCalls in sync with the stream, so pending cards appear as soon as the agent emits the call, before the tool has finished executing.

This means users see:

1. The AI’s text as it streams in
2. A loading card the moment a tool call is emitted
3. The card updates to show the result once the tool completes

Tool calls update in place. The same callId transitions from "running" to "finished" (or "error"), so your UI re-renders the same component with new state.

Handling multiple concurrent tool calls

Agents can invoke several tools in parallel. The toolCalls array will contain multiple entries with status: "running" simultaneously. Each resolves independently, so your UI should handle partial completion gracefully:

function ToolCallList({ toolCalls }: { toolCalls: AssembledToolCall[] }) {
  const pending = toolCalls.filter((tc) => tc.status === "running");
  const completed = toolCalls.filter((tc) => tc.status === "finished");

  return (
    <div className="space-y-2">
      {completed.map((tc) => (
        <ToolCard key={tc.callId} toolCall={tc} />
      ))}
      {pending.map((tc) => (
        <LoadingCard key={tc.callId} name={tc.name} />
      ))}
    </div>
  );
}

Best practices

Follow these guidelines when building tool call UIs:

• Always handle all three states: running, finished, and error. Users should never see a blank card.
• Validate results safely. Tool outputs are typed as unknown until you narrow them for a specific card.
• Provide a generic fallback. Not every tool needs a bespoke card. Render a collapsible JSON view for unknown tool names.
• Show the tool name and args during loading. Users want to know what the agent is doing, even before the result arrives.
• Keep cards compact. Tool cards sit inline with chat messages. Avoid overwhelming the conversation with oversized widgets.

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