fix(ai): skip stringifying text when streaming partial text by aayush-kapoor · Pull Request #14123 · vercel/ai

aayush-kapoor · 2026-04-03T20:05:37Z

Background

streamText with the default text output called JSON.stringify on the full accumulated text on every single streaming chunk that creates increasingly large string copies per stream casuing memory issues

Summary

skip JSON.stringify when the partial output is already a string
structured outputs still go through stringify as before since they need serialization to compare.
compare the text directly
no extra full-string serialization per chunk

Manual Verification

tried reproducing via

repro

import { openai } from '@ai-sdk/openai';
import { streamText } from 'ai';
import { run } from '../../lib/run';

run(async () => {
  const result = streamText({
    model: openai.responses('gpt-4o-mini'),
    prompt:
      'Write an extremely detailed 5000-word essay about the history of computing. Include every detail you can.',
  });

  let chunks = 0;
  for await (const textPart of result.textStream) {
    chunks++;
    if (chunks % 100 === 0) {
      const mb = (process.memoryUsage().heapUsed / 1024 / 1024).toFixed(1);
      console.log(`chunk ${chunks} — heap: ${mb}MB`);
    }
  }

  console.log(`\nTotal chunks: ${chunks}`);
  console.log(
    `Final heap: ${(process.memoryUsage().heapUsed / 1024 / 1024).toFixed(1)}MB`,
  );
});

Checklist

Tests have been added / updated (for bug fixes / features)
Documentation has been added / updated (for bug fixes / features)
A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
I have reviewed this pull request (self-review)

Related Issues

fixes #13839

lgrammel · 2026-04-07T08:43:28Z

packages/ai/src/generate-text/stream-text.ts

      if (result !== undefined) {
        // only send new json if it has changed:
-        const currentJson = JSON.stringify(result.partial);
+        const currentJson =


worth adding a comment why the logic is here

also this can lead to downstream issues, because currentJson is not JSON in those cases (the variable name is now misleading)

concern found

## Background #13839 streamText with the default text output called JSON.stringify on the full accumulated text on every single streaming chunk that creates increasingly large string copies per stream casuing memory issues ## Summary - skip `JSON.stringify` when the partial output is already a string - structured outputs still go through stringify as before since they need serialization to compare. - compare the text directly - no extra full-string serialization per chunk ## Manual Verification tried reproducing via <details> <summary>repro</summary> ```ts import { openai } from '@ai-sdk/openai'; import { streamText } from 'ai'; import { run } from '../../lib/run'; run(async () => { const result = streamText({ model: openai.responses('gpt-4o-mini'), prompt: 'Write an extremely detailed 5000-word essay about the history of computing. Include every detail you can.', }); let chunks = 0; for await (const textPart of result.textStream) { chunks++; if (chunks % 100 === 0) { const mb = (process.memoryUsage().heapUsed / 1024 / 1024).toFixed(1); console.log(`chunk ${chunks} — heap: ${mb}MB`); } } console.log(`\nTotal chunks: ${chunks}`); console.log( `Final heap: ${(process.memoryUsage().heapUsed / 1024 / 1024).toFixed(1)}MB`, ); }); ``` </details> ## Checklist - [x] Tests have been added / updated (for bug fixes / features) - [ ] Documentation has been added / updated (for bug fixes / features) - [x] A _patch_ changeset for relevant packages has been added (for bug fixes / features - run `pnpm changeset` in the project root) - [x] I have reviewed this pull request (self-review) ## Related Issues fixes #13839

vercel-ai-sdk · 2026-04-07T18:35:47Z

✅ Backport PR created: #14200

@aayush-kapoor

…#14200) This is an automated backport of #14123 to the release-v6.0 branch. FYI @aayush-kapoor Co-authored-by: Aayush Kapoor <83492835+aayush-kapoor@users.noreply.github.com>

## Background #13839 streamText with the default text output called JSON.stringify on the full accumulated text on every single streaming chunk that creates increasingly large string copies per stream casuing memory issues ## Summary - skip `JSON.stringify` when the partial output is already a string - structured outputs still go through stringify as before since they need serialization to compare. - compare the text directly - no extra full-string serialization per chunk ## Manual Verification tried reproducing via <details> <summary>repro</summary> ```ts import { openai } from '@ai-sdk/openai'; import { streamText } from 'ai'; import { run } from '../../lib/run'; run(async () => { const result = streamText({ model: openai.responses('gpt-4o-mini'), prompt: 'Write an extremely detailed 5000-word essay about the history of computing. Include every detail you can.', }); let chunks = 0; for await (const textPart of result.textStream) { chunks++; if (chunks % 100 === 0) { const mb = (process.memoryUsage().heapUsed / 1024 / 1024).toFixed(1); console.log(`chunk ${chunks} — heap: ${mb}MB`); } } console.log(`\nTotal chunks: ${chunks}`); console.log( `Final heap: ${(process.memoryUsage().heapUsed / 1024 / 1024).toFixed(1)}MB`, ); }); ``` </details> ## Checklist - [x] Tests have been added / updated (for bug fixes / features) - [ ] Documentation has been added / updated (for bug fixes / features) - [x] A _patch_ changeset for relevant packages has been added (for bug fixes / features - run `pnpm changeset` in the project root) - [x] I have reviewed this pull request (self-review) ## Related Issues fixes #13839

vercel-ai-sdk · 2026-04-07T20:39:49Z

🚀 Published in:

Package	Version
`ai`	`7.0.0-beta.71`
`@ai-sdk/angular`	`3.0.0-beta.71`
`@ai-sdk/gateway`	`4.0.0-beta.40`
`@ai-sdk/langchain`	`3.0.0-beta.71`
`@ai-sdk/llamaindex`	`3.0.0-beta.71`
`@ai-sdk/otel`	`1.0.0-beta.17`
`@ai-sdk/react`	`4.0.0-beta.71`
`@ai-sdk/rsc`	`3.0.0-beta.72`
`@ai-sdk/svelte`	`5.0.0-beta.71`
`@ai-sdk/vue`	`4.0.0-beta.71`

fix(ai): skip stringifying text when streaming partial text

c2f937c

github-actions bot assigned aayush-kapoor Apr 3, 2026

tigent bot added ai/core core functions like generateText, streamText, etc. Provider utils, and provider spec. bug Something isn't working as documented reproduction provided labels Apr 3, 2026

fix + cs

63605b0

vercel bot deployed to Preview April 3, 2026 20:07 View deployment

aayush-kapoor mentioned this pull request Apr 6, 2026

fix: avoid quadratic memory growth in streamText text() without breaking chunk streaming #14119

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lgrammel reviewed Apr 7, 2026

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lgrammel previously approved these changes Apr 7, 2026

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lgrammel self-requested a review April 7, 2026 08:44

lgrammel approved these changes Apr 7, 2026

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rename var + comment

bc9e8f3

vercel bot deployed to Preview April 7, 2026 18:19 View deployment

aayush-kapoor and others added 2 commits April 7, 2026 14:22

empty

c87f409

Merge branch 'main' into aayush/stream-text-stringify

82fecd6

aayush-kapoor added the backport Admins only: add this label to a pull request in order to backport it to the prior version label Apr 7, 2026

vercel bot deployed to Preview April 7, 2026 18:24 View deployment

aayush-kapoor merged commit e68be55 into main Apr 7, 2026
21 checks passed

aayush-kapoor deleted the aayush/stream-text-stringify branch April 7, 2026 18:34

vercel-ai-sdk bot mentioned this pull request Apr 7, 2026

Backport: fix(ai): skip stringifying text when streaming partial text #14200

Merged

vercel-ai-sdk bot removed the backport Admins only: add this label to a pull request in order to backport it to the prior version label Apr 7, 2026

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fix(ai): skip stringifying text when streaming partial text#14123

fix(ai): skip stringifying text when streaming partial text#14123
aayush-kapoor merged 5 commits intomainfrom
aayush/stream-text-stringify

aayush-kapoor commented Apr 3, 2026 •

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aayush-kapoor commented Apr 3, 2026 •

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