mongodb/node-mongodb-native · error · MongoCompatibilityError

Current topology does not support sessions

Error message

Current topology does not support sessions

What it means

Thrown when the user explicitly created a ClientSession (session.startSession()) and passed it to an operation, but the topology the connection belongs to does not support sessions. Standalone servers (pre-3.6 or any standalone without replica sets) and very old sharded clusters do not support logical sessions, so the driver rejects the explicit session as a MongoCompatibilityError rather than silently dropping it.

Source

Thrown at src/cmap/connection.ts:398

      const { version, strict, deprecationErrors } = this.serverApi;
      cmd.apiVersion = version;
      if (strict != null) cmd.apiStrict = strict;
      if (deprecationErrors != null) cmd.apiDeprecationErrors = deprecationErrors;
    }

    if (this.hasSessionSupport && session) {
      if (
        session.clusterTime &&
        clusterTime &&
        session.clusterTime.clusterTime.greaterThan(clusterTime.clusterTime)
      ) {
        clusterTime = session.clusterTime;
      }

      const sessionError = applySession(session, cmd, options);
      if (sessionError) throw sessionError;
    } else if (session?.explicit) {
      throw new MongoCompatibilityError('Current topology does not support sessions');
    }

    // if we have a known cluster time, gossip it
    if (clusterTime) {
      cmd.$clusterTime = clusterTime;
    }

    // For standalone, drivers MUST NOT set $readPreference.
    if (this.description.type !== ServerType.Standalone) {
      if (
        !isSharded(this) &&
        !this.description.loadBalanced &&
        this.supportsOpMsg &&
        options.directConnection === true &&
        readPreference?.mode === 'primary'
      ) {
        // For mongos and load balancers with 'primary' mode, drivers MUST NOT set $readPreference.
        // For all other types with a direct connection, if the read preference is 'primary'

View on GitHub (pinned to 3366c21a63)

Solutions

1. Connect to a replica set or sharded cluster instead of a standalone if you need sessions/transactions.
2. Stop passing an explicit session for operations against a standalone.
3. For local dev, start mongod with --replSet and initialize the replica set.
4. Ensure the server version is >= 3.6.

Example fix

// before (directConnection to standalone)
const client = new MongoClient('mongodb://localhost:27017/?directConnection=true');
const s = client.startSession();
await coll.find({}, { session: s });

// after
const client = new MongoClient('mongodb://localhost:27017/?replicaSet=rs0');

import { MongoClient } from 'mongodb';
async function topologySupportsSessions(uri: string): Promise<boolean> {
  const c = new MongoClient(uri);
  try {
    await c.connect();
    const hello = await c.db('admin').command({ hello: 1 });
    return Boolean(hello.setName || hello.msg === 'isdbgrid');
  } finally {
    await c.close().catch(() => {});
  }
}

function deploymentSupportsSessions(kind: 'standalone' | 'replicaset' | 'sharded'): boolean {
  return kind !== 'standalone';
}

import { MongoCompatibilityError } from 'mongodb';
try {
  const s = client.startSession();
  await collection.findOne({}, { session: s });
} catch (e) {
  if (e instanceof MongoCompatibilityError && /sessions/.test(e.message)) {
    // retry without an explicit session, or point at a replica set
  }
  throw e;
}

When it happens

Trigger: Calling collection.find(..., { session }) or any operation with an explicit session against a standalone mongod. Fires in Connection.prepareCommand (src/cmap/connection.ts:386-399) when this.hasSessionSupport is false and the passed session has explicit === true.

Common situations: Connecting with directConnection=true to a standalone mongod and trying to use transactions or explicit sessions; running tests against a local standalone while app code assumes replica set; server version < 3.6 which predates sessions; topology not yet discovered as replica set / sharded cluster at the moment of the command.

Related errors

• Driver attempted to initialize in load balancing mode, but t
• This MongoDB deployment does not support retryable writes. P
• Transactions are not supported in snapshot sessions
• SRV URI does not support directConnection
• Cannot use srvMaxHosts option with replicaSet

AI-assisted analysis of mongodb/node-mongodb-native@3366c21a63 (2026-08-04). Data as JSON: /data/errors/416b80ca1f78c471.json. Report an issue: GitHub.