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Consuming Streams

Overview

streamText returns a result object with two async iterables for consuming the stream: textStream (text chunks only) and fullStream (all part types). Which one you use depends on whether you need just the text or the full picture including tool calls, reasoning, and metadata.

The result also exposes aggregated promises that resolve when the stream completes: text, usage, totalUsage, finishReason, and steps. These let you await final values without manually accumulating stream chunks.

Both textStream and fullStream are consumed with for await...of. Once consumed, the stream is exhausted — you cannot iterate it again. If you need both text streaming and final aggregates, use the promises alongside the stream.

textStream — Text Only

import { streamText } from 'ai';
import { anthropic } from '@ai-sdk/anthropic';

const result = streamText({
  model: anthropic('claude-sonnet-4-5-20250514'),
  prompt: 'Explain streaming in 3 sentences.',
});

for await (const chunk of result.textStream) {
  process.stdout.write(chunk);
}

fullStream — All Part Types

for await (const part of result.fullStream) {
  switch (part.type) {
    case 'text-delta':
      process.stdout.write(part.textDelta);
      break;
    case 'reasoning':
      console.log('Reasoning:', part.textDelta);
      break;
    case 'tool-call':
      console.log('Tool:', part.toolName, part.args);
      break;
    case 'tool-result':
      console.log('Result:', part.result);
      break;
    case 'step-start':
      console.log('Step started');
      break;
    case 'step-finish':
      console.log('Step done:', part.finishReason);
      break;
  }
}

Stream Part Types

Type	Key Properties	Description
`text-delta`	`textDelta`	Incremental text chunk
`reasoning`	`textDelta`	Chain-of-thought reasoning chunk
`source`	`source`	Source citation URL
`tool-call`	`toolName`, `args`	Complete tool invocation
`tool-call-streaming-start`	`toolCallId`, `toolName`	Tool call begins streaming
`tool-call-delta`	`toolCallId`, `argsTextDelta`	Incremental tool call arguments
`tool-result`	`toolName`, `result`	Tool execution result
`step-start`	—	Generation step begins
`step-finish`	`finishReason`, `usage`, `isContinued`	Generation step completes

Aggregated Promises

const result = streamText({ model, prompt });

// Consume the stream first
for await (const chunk of result.textStream) {
  process.stdout.write(chunk);
}

// Then await aggregated values
const finalText = await result.text;
const usage = await result.usage;
const totalUsage = await result.totalUsage;
const reason = await result.finishReason;
const steps = await result.steps;

See also

Challenge 1.4: Streaming Text — first streaming exercise
Level 7: Production Streaming — advanced streaming patterns

Sources