convertToUIMessages

The AI SDK converts UIMessage[] to ModelMessage[]. As of ai 7 it does not ship the reverse, which is what you need when the messages you have came from a provider, a log, or an older table (vercel/ai#7180, open at the time of writing - if the SDK ships it, prefer theirs).

This is a pure function. It needs no store and no database - the playground has a panel that runs it on whatever you paste in.

import { convertToUIMessages } from "ai-sdk-threads";

const uiMessages = convertToUIMessages([
  { role: "user", content: "weather?" },
  {
    role: "assistant",
    content: [
      { type: "tool-call", toolCallId: "c1", toolName: "getWeather", input: { city: "x" } },
    ],
  },
  {
    role: "tool",
    content: [
      {
        type: "tool-result",
        toolCallId: "c1",
        toolName: "getWeather",
        output: { type: "json", value: { temp: 21 } },
      },
    ],
  },
]);
// -> 2 messages. The tool result is folded into the assistant message as a
//    `tool-getWeather` part with state 'output-available'.

The mapping

• text and reasoning content becomes text and reasoning parts.
• A tool-call becomes a tool-<name> part, input-available until its result arrives, then output-available - or output-error for a failure.
• tool role messages are folded into the assistant message that called them, never emitted on their own.
• Ids come from options.generateId, defaulting to the SDK's generateId.

What it refuses

Content this version does not model - file, image, and provider-specific parts - throws rather than being guessed at, so a lossy conversion cannot reach your database.

That is deliberate. A converter that silently dropped an image part would leave you with a row that renders differently from the conversation it came from, and no way to know which rows were affected.