Tool definition in `streamUI` should not have `render` prope

Error message

Tool definition in `streamUI` should not have `render` property. Use `generate` instead. Found in tool: ${name}

What it means

In streamUI, server-side tool definitions must use `generate` for non-UI tools; a tool carrying a `render` property is a UI-rendering mistake and is rejected. The error names the offending tool to make it easy to locate.

Source

Thrown at packages/rsc/src/stream-ui/stream-ui.tsx:197

  if (typeof model === 'string') {
    throw new Error(
      '`model` cannot be a string in `streamUI`. Use the actual model instance instead.',
    );
  }
  if ('functions' in settings) {
    throw new Error(
      '`functions` is not supported in `streamUI`, use `tools` instead.',
    );
  }
  if ('provider' in settings) {
    throw new Error(
      '`provider` is no longer needed in `streamUI`. Use `model` instead.',
    );
  }
  if (tools) {
    for (const [name, tool] of Object.entries(tools)) {
      if ('render' in tool) {
        throw new Error(
          'Tool definition in `streamUI` should not have `render` property. Use `generate` instead. Found in tool: ' +
            name,
        );
      }
    }
  }

  const ui = createStreamableUI(initial);

  // The default text renderer just returns the content as string.
  const textRender = text || defaultTextRenderer;

  let finished: Promise<void> | undefined;

  let finishEvent: {
    finishReason: FinishReason;
    usage: LanguageModelV4Usage;
    warnings?: CallWarning[];

View on GitHub (pinned to 69428b1f8b)

Solutions

Rename `render` to `generate` in the tool definition for server-executed tools
If the tool should render UI, move that tool definition to the client side or follow the current streamUI tools schema
Loop through your tools object and remove/replace any entry containing `render`

Example fix

// before
const tools = { weather: { render: async (args) => <Weather args={args} /> } };
// after
const tools = { weather: { generate: async (args) => data } };

Defensive patterns

Strategy: validation

Validate before calling

for (const [name, tool] of Object.entries(tools)) {
  if ('render' in tool) throw new TypeError(`Tool ${name} must use 'generate', not 'render'`);
}

Prevention

Keep server tools (generate) and client UI tools separate
Type tools with the streamUI Tools type
Review tool definitions after copying from client-side examples

When it happens

Trigger: Passing a tools object where any entry has a `render` key, e.g. reusing tool definitions from a client component or from an older API shape that used `render`.

Common situations: Copying tool definitions from useChat/useCompletion client examples that use `render`; reusing tools shared between server and client; migrating from an earlier API where render was the property name.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/2e23bd2085fab287. Report an issue: GitHub.