On a sharded cluster (with the coll collection unsharded), the following command fails with the message "Use of undefined variable: var": db.coll.insert({a: 1}) db.coll.aggregate({$lookup: {from: "coll", as: "coll", let: {var: "$a"}, pipeline: [ {$unionWith: {coll: "coll", pipeline: [ {$match: {$expr: {$eq: ["$a", "$$var"]}}}]}}]}}) However, running this against a mongod returns: { "_id" : ObjectId("620bc09e443b85085d27efd5"), "a" : 1, "coll" : [ { "_id" : ObjectId("620bc09e443b85085d27efd5"), "a" : 1 }, { "_id" : ObjectId("620bc09e443b85085d27efd5"), "a" : 1 } ] }