AI

Type Error with onToolCall

When using the onToolCall callback with TypeScript, you may encounter type errors when trying to pass tool properties directly to addToolResult.

Problem

TypeScript cannot automatically narrow the type of toolCall.toolName when you have both static and dynamic tools, leading to type errors:

// ❌ This causes a TypeScript error
const { messages, sendMessage, addToolResult } = useChat({
  async onToolCall({ toolCall }) {
    addToolResult({
      tool: toolCall.toolName, // Type 'string' is not assignable to type '"yourTool" | "yourOtherTool"'
      toolCallId: toolCall.toolCallId,
      output: someOutput,
    });
  },
});

The error occurs because:

  • Static tools have specific literal types for their names (e.g., "getWeatherInformation")
  • Dynamic tools have toolName as a generic string
  • TypeScript can't guarantee that toolCall.toolName matches your specific tool names

Solution

Check if the tool is dynamic first to enable proper type narrowing:

// ✅ Correct approach with type narrowing
const { messages, sendMessage, addToolResult } = useChat({
  async onToolCall({ toolCall }) {
    // Check if it's a dynamic tool first
    if (toolCall.dynamic) {
      return;
    }


    // Now TypeScript knows this is a static tool with the correct type
    addToolResult({
      tool: toolCall.toolName, // No type error!
      toolCallId: toolCall.toolCallId,
      output: someOutput,
    });
  },
});