server/chatSessionSerializer.js

const ROOT_JSON_KEY = '__historyRootJson';
const VERSION_META_FIELDS = ['toolCalls', 'responseEdits', 'responseSegments', 'error'];

function validateAndRepairTree(rootMessage) {
  const repair = (msg) => {
    if (!msg) return;
    if (msg.content === undefined || msg.content === null) {
      msg.content = '';
    }
    if (msg.versions && Array.isArray(msg.versions)) {
      for (const version of msg.versions) {
        if (version.content === undefined || version.content === null) {
          version.content = '';
        }
        if (version.tail && Array.isArray(version.tail)) {
          for (const tailMsg of version.tail) {
            repair(tailMsg);
          }
        }
      }
    }
  };
  repair(rootMessage);
  return rootMessage;
}

function cloneAndRepairTree(rootMessage) {
  return validateAndRepairTree(JSON.parse(JSON.stringify(rootMessage)));
}

function getActiveVersion(message) {
  if (!message) return null;
  const versions = Array.isArray(message.versions) ? message.versions : [];
  if (!versions.length) {
    message.versions = [{ content: message.content ?? '', tail: [], timestamp: Date.now() }];
    message.currentVersionIdx = 0;
    return message.versions[0];
  }
  const currentVersionIdx = Number.isInteger(message.currentVersionIdx)
    ? Math.max(0, Math.min(message.currentVersionIdx, versions.length - 1))
    : 0;
  message.currentVersionIdx = currentVersionIdx;
  if (!Array.isArray(message.versions[currentVersionIdx].tail)) {
    message.versions[currentVersionIdx].tail = [];
  }
  if (message.versions[currentVersionIdx].content === undefined || message.versions[currentVersionIdx].content === null) {
    message.versions[currentVersionIdx].content = message.content ?? '';
  }
  return message.versions[currentVersionIdx];
}

function cloneVersionMetaValue(value) {
  if (value === undefined) return undefined;
  return JSON.parse(JSON.stringify(value));
}

function syncMessageFromActiveVersion(message) {
  if (!message) return message;
  const currentVersion = getActiveVersion(message);
  if (!currentVersion) return message;
  message.content = currentVersion.content ?? message.content ?? '';
  VERSION_META_FIELDS.forEach((key) => {
    if (key in currentVersion) {
      message[key] = cloneVersionMetaValue(currentVersion[key]);
    } else {
      delete message[key];
    }
  });
  return message;
}

function cloneJson(value) {
  return JSON.parse(JSON.stringify(value));
}

export function extractFlatHistory(rootMessage) {
  if (!rootMessage) return [];

  const toFlatEntry = (message) => {
    const cloned = cloneJson(message);
    if (cloned.content === undefined || cloned.content === null) {
      cloned.content = '';
    }
    syncMessageFromActiveVersion(cloned);
    if (Array.isArray(cloned.versions)) {
      cloned.versions = cloned.versions.map((version) => ({
        ...version,
        tail: [],
      }));
    }
    return cloned;
  };

  const history = [toFlatEntry(rootMessage)];
  const currentVerIdx = rootMessage.currentVersionIdx ?? 0;

  if (!Array.isArray(rootMessage.versions) || currentVerIdx >= rootMessage.versions.length) {
    return history;
  }

  const currentTail = rootMessage.versions[currentVerIdx]?.tail;
  if (currentTail && Array.isArray(currentTail)) {
    const walkTail = (tail) => {
      for (let i = 0; i < tail.length; i++) {
        const msg = tail[i];
        if (msg?.content === undefined || msg?.content === null) {
          msg.content = '';
        }
        syncMessageFromActiveVersion(msg);
        history.push(toFlatEntry(msg));
        const ver = msg.versions?.[msg.currentVersionIdx ?? 0];
        if (ver?.tail && Array.isArray(ver.tail)) {
          walkTail(ver.tail);
        }
        if (msg.role === 'user' && Array.isArray(msg.versions) && msg.versions.length > 1) {
          break;
        }
      }
    };
    walkTail(currentTail);
  }

  return history;
}

export function serializeSessionForClient(session) {
  const rootMessage = Array.isArray(session?.history) && session.history[0]
    ? cloneAndRepairTree(session.history[0])
    : null;
  return {
    id: session?.id,
    name: session?.name,
    created: session?.created,
    history: rootMessage ? extractFlatHistory(rootMessage) : [],
    model: session?.model || null,
    updatedAt: session?.updatedAt || null,
    [ROOT_JSON_KEY]: rootMessage ? JSON.stringify(rootMessage) : null,
  };
}