`uiMessageToModelMessages()` drops stable message IDs

[kolaworld]

TanStack AI version

0.43.0

Framework/Library version

@tanstack/ai-persistence: 0.1.0, @tanstack/ai-client`: 0.23.0, @tanstack/ai-svelte: 0.16.0

Describe the bug and the steps to reproduce it

ModelMessage.id is documented as the stable identity used for a persistence and hydration round trip.

modelMessagesToUIMessages() honors that contract and reuses ModelMessage.id, but the reverse
uiMessageToModelMessages() conversion does not consistently copy UIMessage.id into the generated model messages.

Bug — UI → model drops IDs:

• The one-to-one user-message path returns only role and content in messages.ts#L178-L190.
• Assistant segments and derived tool-result messages also omit IDs in messages.ts#L230-L295.
• The remaining derived tool-result paths have the same omission in messages.ts#L343-L376.

Correct — model → UI restores IDs:

• Persisted IDs are deliberately reused in messages.ts#L585-L636, matching the ModelMessage.id contract in types.ts#L358-L375.

The asymmetric conversion makes a live UI message lose its identity before its persisted. Hydration must then generate a new ID, which breaks message deduplication, metadata joins, and in-place rendering across reloads.

Your Minimal, Reproducible Example - (Sandbox Highly Recommended)

https://stackblitz.com/edit/vitejs-vite-yjppr1f7?file=package.json,index.html,src%2Fmain.ts

const [modelMessage] = uiMessageToModelMessages({
  id: 'message-1',
  role: 'user',
  createdAt: new Date(),
  parts: [{ type: 'text', content: 'Hello' }],
})

expect(modelMessage.id).toBe('message-1') // currently undefined

The same loss occurs when an assistant UI message is converted into assistant segments and tool-result ModelMessage objects.

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