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Suggest follow-up questions after AI responses, shown only on the last message.

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Overview

After the AI responds, generate contextual follow-up questions using a fast model. These are streamed as data-* parts which are UI-only - they’re filtered out before sending context to the LLM. File convention: lib/ai/followup-suggestions.ts → components/followup-suggestions.tsx

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How it works

  1. Generate suggestions after the main response using a cheap/fast model
  2. Stream as data-followupSuggestions part (data-* prefix = UI-only)
  3. Ignore data-* parts via convertToModelMessages({ convertDataPart: () => undefined })
  4. Render only on the last message

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Code

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1. Generate & Stream Suggestions

lib/ai/followup-suggestions.ts
import { type ModelMessage, Output, streamText } from "ai";
import { z } from "zod";
import { getLanguageModel } from "@/lib/ai/providers";
import type { StreamWriter } from "@/lib/ai/types";
import { config } from "@/lib/config";
import { generateUUID } from "@/lib/utils";

export async function generateFollowupSuggestions(
  modelMessages: ModelMessage[]
) {
  return streamText({
    model: await getLanguageModel(
      config.models.defaults.followupSuggestions
    ),
    messages: [
      ...modelMessages,
      {
        role: "user",
        content:
          "What question should I ask next? Return an array of 3-5 suggestions, max 80 chars each.",
      },
    ],
    output: Output.object({
      schema: z.object({
        suggestions: z.array(z.string()).min(3).max(5),
      }),
    }),
  });
}

export async function streamFollowupSuggestions({
  followupSuggestionsResult,
  writer,
}: {
  followupSuggestionsResult: ReturnType<typeof generateFollowupSuggestions>;
  writer: StreamWriter;
}) {
  const dataPartId = generateUUID();
  const result = await followupSuggestionsResult;

  for await (const chunk of result.partialOutputStream) {
    writer.write({
      id: dataPartId,
      type: "data-followupSuggestions", // data-* = UI-only, filtered from LLM context
      data: {
        suggestions:
          chunk.suggestions?.filter((s): s is string => s !== undefined) ?? [],
      },
    });
  }
}

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2. Call After Response

app/(chat)/api/chat/route.ts
// Inside createUIMessageStream execute callback, after result.consumeStream()
await result.consumeStream();

const response = await result.response;
const responseMessages = response.messages;

// Generate and stream follow-up suggestions
const followupSuggestionsResult = generateFollowupSuggestions([
  ...contextForLLM,
  ...responseMessages,
]);
await streamFollowupSuggestions({
  followupSuggestionsResult,
  writer: dataStream,
});

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3. Ignore Data Parts in Conversion

lib/ai/core-chat-agent.ts
import { convertToModelMessages } from "ai";

// Convert to model messages, ignoring data-* parts (UI-only)
const modelMessages = await convertToModelMessages(messages, {
  convertDataPart: () => undefined, // Ignores all data-* parts
});

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4. Render on Last Message Only

components/followup-suggestions.tsx
"use client";

import { useChatStoreApi } from "@ai-sdk-tools/store";
import { useMessageIds } from "@/lib/stores/hooks-base";
import {
  useMessagePartByPartIdx,
  useMessagePartTypesById,
} from "@/lib/stores/hooks-message-parts";
import type { ChatMessage } from "@/lib/ai/types";
import { generateUUID } from "@/lib/utils";

export function FollowUpSuggestionsParts({ messageId }: { messageId: string }) {
  const types = useMessagePartTypesById(messageId);
  const ids = useMessageIds();
  const isLastMessage = ids.at(-1) === messageId;

  // Only show on the last message
  if (!isLastMessage) {
    return null;
  }

  const partIdx = types.indexOf("data-followupSuggestions");
  if (partIdx === -1) {
    return null;
  }

  return <FollowUpSuggestionsPart messageId={messageId} partIdx={partIdx} />;
}

function FollowUpSuggestionsPart({
  messageId,
  partIdx,
}: {
  messageId: string;
  partIdx: number;
}) {
  const part = useMessagePartByPartIdx(
    messageId,
    partIdx,
    "data-followupSuggestions"
  );

  return <FollowUpSuggestions suggestions={part.data.suggestions} />;
}

export function FollowUpSuggestions({
  suggestions,
}: {
  suggestions: string[];
}) {
  const storeApi = useChatStoreApi();

  const handleClick = (suggestion: string) => {
    const sendMessage = storeApi.getState().sendMessage;
    const parentMessageId = storeApi.getState().getLastMessageId();

    const message: ChatMessage = {
      id: generateUUID(),
      role: "user",
      parts: [{ type: "text", text: suggestion }],
      metadata: {
        createdAt: new Date(),
        parentMessageId,
        selectedModel: "your-default-model", // Get from context
      },
    };

    sendMessage(message);
  };

  if (!suggestions?.length) return null;

  return (
    <div className="mt-2 flex flex-col gap-2">
      <div className="text-xs text-muted-foreground">Related</div>
      <div className="flex flex-wrap gap-1.5">
        {suggestions.map((s) => (
          <button
            key={s}
            onClick={() => handleClick(s)}
            className="rounded-md border px-2 py-1 text-sm hover:bg-muted"
          >
            {s}
          </button>
        ))}
      </div>
    </div>
  );
}