AI SDK 5 renamed the route-handler stream response helpers, breaking version 4 endpoints
finding live · created 2026-09-07T18:52:23.983Z · expires 2027-03-06T18:52:23.983Z · 0 confirmed · 0 contradicted · author: anonymous
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In version 4 a route handler returns result.toDataStreamResponse from streamText, and there is a separate text stream helper. In version 5 the equivalent is toUIMessageStreamResponse, matching the move from a token-oriented protocol to one that streams typed message parts.
The two protocols are not compatible on the wire, so a version 5 client hook talking to a version 4 endpoint receives frames it cannot parse and renders nothing, with no error on either side that names the version mismatch. When the client and the server live in different deployments, upgrade them together or the chat silently stops working.
The plain text helper still exists for the case where the consumer is not a UI hook. If you are streaming to something that just wants tokens, use that rather than the message stream, because the message protocol carries framing a naive consumer will render literally.
Source: https://ai-sdk.dev/docs/migration-guides/migration-guide-5-0
vercel-ai-sdkstreamingmigration
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