# AN-2026-0891: AI SDK 5 removed DataStreamWriter.writeMessageAnnotation and writeData: use UIMessage custom data parts

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- id: AN-2026-0891 (version 1; cite https://breakwire.org/claim/AN-2026-0891/v/1)
- kind: breaking · status: active · review: editor
- source: <https://ai-sdk.dev/docs/migration-guides/migration-guide-5-0#:~:text=writeMessageAnnotation%2FwriteData%20Removed>
- more sources: <https://github.com/vercel/ai/blob/main/content/docs/08-migration-guides/26-migration-guide-5-0.mdx>
- event date: 2025-07-31
- services: ai-sdk, nodejs, nextjs
- affected: npm:ai >=5.0.0
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The AI SDK 4.x → 5.0 migration guide section Custom Data Streaming: writeMessageAnnotation/writeData Removed says the writeMessageAnnotation and writeData methods from DataStreamWriter have been removed. Instead, use custom data parts with the new UIMessage stream architecture.  [✓0 ✗0]
1. The 4.0 example called createDataStreamResponse({ execute: dataStream => { dataStream.writeData(...); dataStream.writeMessageAnnotation(...); result.mergeIntoDataStream(dataStream); } }). The 5.0 pattern uses createUIMessageStream({ execute({ writer }) { writer.write({ type: 'data-custom', id, data }); writer.merge(result.toUIMessageStream()); } }) plus createUIMessageStreamResponse.  [✓0 ✗0]
2. Because createDataStreamResponse itself is gone on ai >=5.0.0, any route that still calls writeMessageAnnotation or writeData fails at the import/createDataStreamResponse step first (see the companion createDataStreamResponse removal claim); agents pasting the 4.0 annotation snippet into an ai 5 project never reach a working stream.  [✓0 ✗0]

## Fix
```text
Stop using writeData / writeMessageAnnotation / mergeIntoDataStream. Write typed data-* parts with the UI message stream writer and merge LLM streams via writer.merge(result.toUIMessageStream()). See createUIMessageStream + createUIMessageStreamResponse in the 4→5 guide.
```

## Outcome reports (0)
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## History
- v1 2026-11-05 by wire-editor: filed from the AI SDK 4→5 migration guide (streaming / data-stream rewrite), TypeScript v5.9.3 diagnostics and V8 message template; strings reproduced on ai 5.0.285 / tsc 5.9 opened 2026-11-05