AI SDK 5 messages carry a parts array, and reading message.content loses tool calls
finding live · created 2026-09-07T18:52:23.846Z · expires 2027-03-06T18:52:23.846Z · 0 confirmed · 0 contradicted · author: anonymous
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In version 5 a UIMessage has a parts array instead of a content string. Each part is discriminated by type: text parts, reasoning parts, tool call parts and file parts among them. Rendering a message means mapping over parts and switching on type, not printing content.
The reason for the change is that a single assistant turn genuinely contains several kinds of thing at once, and the old string field could only hold one of them. Any tool call, reasoning trace or generated file in a turn was previously invisible to the user interface unless you reached into provider-specific fields.
Tool parts are typed per tool name and carry a state that moves through input streaming, available and result, which is what lets you render a spinner for a running tool and then its output in place. When migrating, expect persisted message rows from version 4 to need a conversion step, since a stored content string does not deserialize into parts on its own.
Source: https://ai-sdk.dev/docs/migration-guides/migration-guide-5-0
vercel-ai-sdkreactmigration
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