`functions` is not supported in `streamUI`, use `tools` inst

Error message

`functions` is not supported in `streamUI`, use `tools` instead.

What it means

The RSC streamUI API renamed `functions` to `tools`. During the experimental phase it detects a `functions` key in the settings object and throws to force the migration. The behavior is now expressed via the `tools` option.

Source

Thrown at packages/rsc/src/stream-ui/stream-ui.tsx:185

      /**
       * Optional response data.
       */
      response?: {
        /**
         * Response headers.
         */
        headers?: Record<string, string>;
      };
    }) => Promise<void> | void;
  }): Promise<RenderResult> {
  // TODO: Remove these errors after the experimental phase.
  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,
        );
      }
    }
  }

View on GitHub (pinned to 69428b1f8b)

Solutions

Rename the `functions` option to `tools`
Update tool definitions to the tools shape (schema/execute/render style used by streamUI)
Check the current ai/rsc documentation for the tools API

Example fix

// before
streamUI({ model, functions: { myTool } });
// after
streamUI({ model, tools: { myTool } });

Defensive patterns

Strategy: validation

Validate before calling

if ('functions' in settings) throw new TypeError('Use `tools` instead of `functions`');

Prevention

Search the codebase for 'functions:' when migrating to streamUI
Follow current ai/rsc docs for the tools option shape
Enable TS excess property checks with a typed options object

When it happens

Trigger: Calling streamUI({ ..., functions: {...} }) — the check `'functions' in settings` throws before any execution.

Common situations: Code written against very early experimental versions of `ai/rsc`; copy-pasted snippets from old blog posts or docs; partial migration where model was updated to an instance but the old `functions` option was left.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/7bbbba30667de24e. Report an issue: GitHub.