Tool Invocation Missing Result Error

Issue

When using generateText() or streamText(), you may encounter the error "ToolInvocation must have a result" when a tool without an execute function is called.

Cause

The error occurs when you define a tool without an execute function and don't provide the result through other means (like useChat's onToolCall or addToolResult functions).

Each time a tool is invoked, the model expects to receive a result before continuing the conversation. Without a result, the model cannot determine if the tool call succeeded or failed and the conversation state becomes invalid.

Solution

You have two options for handling tool results:

Server-side execution using tools with an execute function:

const tools = {
  weather: tool({
    description: 'Get the weather in a location',
    parameters: z.object({
      location: z
        .string()
        .describe('The city and state, e.g. "San Francisco, CA"'),
    }),
    execute: async ({ location }) => {
      // Fetch and return weather data
      return { temperature: 72, conditions: 'sunny', location };
    },
  }),
};

Client-side execution with useChat (omitting the execute function), but you must provide results in one of these ways:

const { messages } = useChat({
  // Option 1: Handle using onToolCall
  onToolCall: async ({ toolCall }) => {
    if (toolCall.toolName === 'getLocation') {
      const result = await getLocationData();
      return result; // This becomes the tool result
    }
  },
});

}

// Option 2: Use addToolResult (e.g. with interactive UI elements)
const { messages, addToolResult } = useChat();

// Inside your JSX, when rendering tool calls:
<button
  onClick={() =>
    addToolResult({
      toolCallId, // must provide tool call ID
      result: {
        /* your tool result */
      },
    })
  }
>
  Confirm
</button>;

Whether handling tools on the server or client, each tool call must have a corresponding result before the conversation can continue.