unsloth/studio/frontend/src/features/chat/utils/parse-assistant-content.ts

// SPDX-License-Identifier: AGPL-3.0-only
// Copyright 2026-present the Unsloth AI Inc. team. All rights reserved. See /studio/LICENSE.AGPL-3.0

import type { ChatModelRunResult } from "@assistant-ui/react";

type ContentPart = NonNullable<ChatModelRunResult["content"]>[number];

const THINK_OPEN_TAG = "<think>";
const THINK_CLOSE_TAG = "</think>";

// ContentPart from @assistant-ui/react has readonly fields, so coalescing via
// `last.text += text` fails (TS2540). Instead replace the last element with a
// fresh merged object: same allocation cost as mutation but type-safe.

function appendTextPart(parts: ContentPart[], text: string): void {
  if (!text) return;
  const last = parts.at(-1);
  if (last?.type === "text") {
    parts[parts.length - 1] = { type: "text", text: last.text + text };
    return;
  }
  parts.push({ type: "text", text });
}

function appendReasoningPart(parts: ContentPart[], text: string): void {
  if (!text) return;
  const last = parts.at(-1);
  if (last?.type === "reasoning") {
    parts[parts.length - 1] = { type: "reasoning", text: last.text + text };
    return;
  }
  parts.push({ type: "reasoning", text });
}

export function parseAssistantContent(
  raw: string,
): ContentPart[] {
  const parts: ContentPart[] = [];
  if (!raw) {
    return parts;
  }

  let cursor = 0;
  while (cursor < raw.length) {
    const openIndex = raw.indexOf(THINK_OPEN_TAG, cursor);
    if (openIndex === -1) {
      appendTextPart(parts, raw.slice(cursor));
      break;
    }

    appendTextPart(parts, raw.slice(cursor, openIndex));

    const reasoningStart = openIndex + THINK_OPEN_TAG.length;
    const closeIndex = raw.indexOf(THINK_CLOSE_TAG, reasoningStart);
    if (closeIndex === -1) {
      appendReasoningPart(parts, raw.slice(reasoningStart));
      break;
    }

    appendReasoningPart(parts, raw.slice(reasoningStart, closeIndex));
    cursor = closeIndex + THINK_CLOSE_TAG.length;
  }

  return parts;
}

export function hasClosedThinkTag(raw: string): boolean {
  return raw.includes(THINK_CLOSE_TAG);
}