useUIMessages streaming saved tool calls issue

[franckdsf]

When saving a custom tool call using the saveMessage function AS PART OF A SUBWORKFLOW triggered by an llm, the message is successfully saved in the database and streamed to the client.
However, the useUIMessages hook overwrites previous tool calls from the current message until the message has finished streaming.

Steps to Reproduce

Consider the following sequence from an LLM message:

1. LLM triggers Tool Call A (started)
2. Tool A internally triggers custom functions B and C, which each call saveMessage
3. Finally, Tool Call A (success) is completed

Expected Behavior

The final UI message content should accumulate all tool calls progressively, like so:

{
  content: [ A(started),  B,  C,  A(result) ]
}

Actual Behavior

Instead, useUIMessages overwrites prior message states on each saveMessage update.
As a result, the UI briefly displays intermediate states like:

[A] → [B] → [C] → { content: [A(started), B, C, A(result)] }

This causes previously saved tool calls to disappear from the UI until the message fully completes.

Notes

• The issue occurs only during message streaming.
• Once the message is complete, the final combined message is correct.
• Likely related to how useUIMessages merges or replaces partial message updates.

[franckdsf]

const data = useThreadMessages(api.llm.threads.getPaginated.unauthed, { threadId }, { initialNumItems: 50, stream: true });
const results = toUIMessages(data.results ?? []);

using useThreadMessages instead of useUIMessages works. So it truly comes from the useUIMessages hook.

Obviously, I had to remove the toUIMessages from the convex return type.

[franckdsf]

Something like that works

// Backend
export const getPaginated = query({
  args: { threadId: v.string(), paginationOpts: paginationOptsValidator, streamArgs: vStreamArgs },
  handler: async (ctx, args) => {
    const streams = await syncStreams(ctx, components.agent, args);
    const paginated = await listMessages(ctx, components.agent, args);

    return { ...paginated, streams};
  },
});

// Front
const data = useThreadMessages(api.llm.threads.getPaginated.unauthed, { threadId }, { initialNumItems: 50, stream: true });
const results = toUIMessages(data.results ?? []);