`model` cannot be a string in `streamUI`. Use the actual mod

Error message

`model` cannot be a string in `streamUI`. Use the actual model instance instead.

What it means

streamUI in the experimental RSC API requires a real LanguageModelV1 instance for its `model` option. Passing a provider string id (like 'openai:gpt-4'), which older versions of the underlying streamText accepted, is rejected with this error during the experimental phase.

Source

Thrown at packages/rsc/src/stream-ui/stream-ui.tsx:180

      value: ReactNode;
      /**
       * Warnings from the model provider (e.g. unsupported settings)
       */
      warnings?: CallWarning[];
      /**
       * 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: ' +

View on GitHub (pinned to 69428b1f8b)

Solutions

Create a model instance via a provider function, e.g. openai('gpt-4o'), and pass that to streamUI
Import the provider package (@ai-sdk/openai, @ai-sdk/anthropic, ...) and use its model factory
Remove any string-based model id configuration

Example fix

// before
streamUI({ model: 'gpt-4o', prompt });
// after
import { openai } from '@ai-sdk/openai';
streamUI({ model: openai('gpt-4o'), prompt });

Defensive patterns

Strategy: validation

Validate before calling

if (typeof model === 'string') throw new TypeError('streamUI requires a model instance, got a string');

Type guard

function isModelInstance(m: unknown) { return typeof m === 'object' && m !== null && 'doGenerate' in m; }

Prevention

Always create models with provider factories: openai('gpt-4o')
Never pass provider id strings from config files directly as model
Type the model param as LanguageModelV1 so strings fail at compile time

When it happens

Trigger: Calling streamUI({ model: '<some-string>', ... }) — the first validation checks `typeof model === 'string'` and throws immediately before any streaming starts.

Common situations: Migrating code from an older API where model ids were strings; copying generateText examples that use provider id strings; misunderstanding provider options like `openai('gpt-4')` (an instance) vs the raw string 'gpt-4'.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/2821f40679cf0cfc. Report an issue: GitHub.