Adding @linda.qin's repro:  [direct: mongos] test> db.coll1.find() [ { _id: 1 } ] [direct: mongos] test> sh.status() shardingVersion { _id: 1, clusterId: ObjectId( '683f872d28620650923ed0c1' ) } --- shards [ { _id: 'shard01' , host: 'shard01/localhost:30001' , state: 1, topologyTime: Timestamp({ t: 1748993840, i: 12 }), replSetConfigVersion: Long ( '-1' ) }, { _id: 'shard02' , host: 'shard02/localhost:30002' , state: 1, topologyTime: Timestamp({ t: 1748993840, i: 31 }), replSetConfigVersion: Long ( '-1' ) }, { _id: 'shard03' , host: 'shard03/localhost:30003' , state: 1, topologyTime: Timestamp({ t: 1748993840, i: 55 }), replSetConfigVersion: Long ( '-1' ) } ] --- active mongoses [ { '8.0.5' : 1 } ] --- autosplit { 'Currently enabled' : 'yes' } --- balancer { 'Currently running' : 'no' , 'Currently enabled' : 'yes' , 'Failed balancer rounds in last 5 attempts' : 0, 'Migration Results for the last 24 hours' : { '1' : 'Success' } } --- shardedDataDistribution [ { ns: 'test.coll1' , shards: [ { shardName: 'shard01' , numOrphanedDocs: 0, numOwnedDocuments: 1, ownedSizeBytes: 14, orphanedSizeBytes: 0 }, { shardName: 'shard02' , numOrphanedDocs: 0, numOwnedDocuments: 0, ownedSizeBytes: 0, orphanedSizeBytes: 0 } ] } ] --- databases [ { database: { _id: 'config' , primary: 'config' , partitioned: true }, collections: {} }, { database: { _id: 'test' , primary: 'shard01' , version: { uuid: UUID( '15b94f0d-c704-465b-ace5-8f0c42c44747' ), timestamp: Timestamp({ t: 1748993856, i: 2 }), lastMod: 1 } }, collections: { 'test.coll1' : { shardKey: { a: 1, b: 1 }, unique: false , balancing: true , chunkMetadata: [ { shard: 'shard01' , nChunks: 1 }, { shard: 'shard02' , nChunks: 1 } ], chunks: [ { min: { a: MinKey(), b: MinKey() }, max: { a: 100, b: 100 }, 'on shard' : 'shard01' , 'last modified' : Timestamp({ t: 2, i: 1 }) }, { min: { a: 100, b: 100 }, max: { a: MaxKey(), b: MaxKey() }, 'on shard' : 'shard02' , 'last modified' : Timestamp({ t: 2, i: 0 }) } ], tags: [] } } } ] [direct: mongos] test> db.coll1.updateOne({_id:1}, {$set:{a:100, b:100}},{upsert: true }) { acknowledged: true , insertedId: null , matchedCount: 1, modifiedCount: 1, upsertedCount: 0 } [direct: mongos] test> db.coll1.find() [ { _id: 1, a: 100, b: 100 } ]7:41[direct: mongos] test> db.coll1.updateOne({_id:1}, {$set:{a:10, b:10}},{upsert: true }) { acknowledged: true , insertedId: null , matchedCount: 1, modifiedCount: 1, upsertedCount: 0 } if I move the chunk back to shard01, so only shard01 has the chunks, the update fails: [direct: mongos] test> db.adminCommand({moveChunk:"test.coll1",find: {a:100, b:100} , to:"shard01", _waitForDelete:true}) { millis: 258, ok: 1, '$clusterTime': { clusterTime: Timestamp( { t: 1748995234, i: 30 } ), signature: { hash: Binary.createFromBase64('AAAAAAAAAAAAAAAAAAAAAAAAAAA=', 0), keyId: Long('0') } }, operationTime: Timestamp({ t: 1748995234, i: 30 }) } [direct: mongos] test> db.coll1.find() [ { _id: 1, a: 10, b: 10 } ] [direct: mongos] test> db.coll1.updateOne({_id:1}, {$set:{a:10, b:20}},{upsert:true}) MongoServerError: Shard key update is not allowed without specifying the full shard key in the query sh.status()   collections: { 'test.coll1': { shardKey: { a: 1, b: 1 } , unique: false, balancing: true, chunkMetadata: [ { shard: 'shard01', nChunks: 2 } ], chunks: [ { min: { a: MinKey(), b: MinKey() } , max: { a: 100, b: 100 }, 'on shard': 'shard01', 'last modified': Timestamp({ t: 2, i: 1 }) }, { min: { a: 100, b: 100 } , max: { a: MaxKey(), b: MaxKey() }, 'on shard': 'shard01', 'last modified': Timestamp({ t: 3, i: 0 }) } ], tags: [] } } For v7.0, the update always fails. So the above behaviour that the update can succeed when two or more shards have the chunks on v8.0, I guess it's related to the change for updates on missing shard key value that @Murat Akca mentioned above