vercel/ai · error · UnsupportedFunctionalityError

'${functionality}' functionality not supported.

Error message

'${functionality}' functionality not supported.

What it means

streamText throws an UnsupportedFunctionalityError when you access the `elementStream` property but the configured output specification does not support element streams (its createElementStreamTransform returns null). Element streams are only available for output specifications that define per-element streaming (e.g. array/partial-object style outputs); in plain 'text' mode there is no element concept, so the stream cannot be created. The error names the mode so you can see which output configuration lacks the feature.

Source

Thrown at packages/ai/src/generate-text/stream-text.ts:2897

        new TransformStream<
          EnrichedStreamPart<TOOLS, InferPartialOutput<OUTPUT>>,
          InferPartialOutput<OUTPUT>
        >({
          transform({ partialOutput }, controller) {
            if (partialOutput != null) {
              controller.enqueue(partialOutput);
            }
          },
        }),
      ),
    );
  }

  get elementStream(): AsyncIterableStream<InferElementOutput<OUTPUT>> {
    const transform = this.outputSpecification?.createElementStreamTransform();

    if (transform == null) {
      throw new UnsupportedFunctionalityError({
        functionality: `element streams in ${
          this.outputSpecification?.name ?? 'text'
        } mode`,
      });
    }

    return createAsyncIterableStream(this.teeStream().pipeThrough(transform));
  }

  private getOutputPromise(): Promise<InferCompleteOutput<OUTPUT>> {
    if (this.outputPromise == null) {
      this.outputPromise = this.finalStep.then(step => {
        const output = this.outputSpecification ?? text();
        return output.parseCompleteOutput(
          { text: step.text },
          {
            response: step.response,
            usage: step.usage,

View on GitHub (pinned to 69428b1f8b)

Solutions

1. Configure streamText with an output specification that supports element streams (e.g. an array Output) before reading elementStream.
2. If you only need full text, read `result.textStream` instead of `result.elementStream`.
3. If you need a structured object (not streamed element-by-element), use `result.output`/object stream consumption instead of elementStream.
4. Check `this.outputSpecification`/mode at runtime and branch to the correct stream accessor.

Example fix

// before
const result = streamText({ model, prompt });
for await (const element of result.elementStream) { ... }

// after
const result = streamText({
  model,
  prompt,
  experimental_output: Output.array(Output.string()),
});
for await (const element of result.elementStream) { ... }

const supportsElements =
  outputSpecification != null &&
  typeof outputSpecification.createElementStreamTransform === 'function';
if (!supportsElements) {
  // consume result.textStream or result.output instead of elementStream
}

function supportsElementStream(spec: unknown): spec is { createElementStreamTransform: () => unknown } {
  return (
    typeof spec === 'object' && spec !== null &&
    'createElementStreamTransform' in spec &&
    typeof (spec as any).createElementStreamTransform === 'function'
  );
}

try {
  for await (const el of result.elementStream) handle(el);
} catch (error) {
  if (UnsupportedFunctionalityError.isInstance(error)) {
    for await (const text of result.textStream) handleText(text); // fallback
  } else throw error;
}

When it happens

Trigger: Accessing `result.elementStream` on the DefaultStreamTextResult returned by streamText when no outputSpecification is set (default 'text' mode) or when using an output specification that does not implement createElementStreamTransform.

Common situations: Copying example code that iterates elementStream into a project that calls streamText without `experimental_output`/output configuration; using an output mode (e.g. plain text or object output) that predates or lacks element streaming support; upgrading the SDK and switching output specs without updating the consumption code.

Related errors

• Chunking must be "word", "line", a RegExp, an Intl.Segmenter
• Invalid argument for parameter model: model ${model.provider
• 'element streams in no-schema mode' functionality not suppor
• 'element streams in object mode' functionality not supported
• Unsupported chunk type: ${_exhaustiveCheck}

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/66f31a0449209619. Report an issue: GitHub.