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Assigned Teams:

Query Execution
Backwards Compatibility:
Fully Compatible
Operating System:
ALL
Backport Requested:

v8.2, v8.0
Steps To Reproduce:
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Insert any one document in a collection. Run aggregation:

db.test.aggregate([{$lookup: {as: "out", from: "test", pipeline: [{$project: {_id: 0, now: "$$NOW"}}]}}, {$addFields: {now: "$$NOW"}}])

Observe the result:

[ { _id: ObjectId('69259026e301e10a0d726631'), a: 1, out: [ { now: ISODate('2025-11-25T11:18:31.095Z') } ], now: ISODate('2025-11-25T11:18:31.094Z') } ]
Show
Insert any one document in a collection. Run aggregation: db.test.aggregate([{$lookup: {as: "out" , from: "test" , pipeline: [{$project: {_id: 0, now: "$$NOW" }}]}}, {$addFields: {now: "$$NOW" }}]) Observe the result: [ { _id: ObjectId( '69259026e301e10a0d726631' ), a: 1, out: [ { now: ISODate( '2025-11-25T11:18:31.095Z' ) } ], now: ISODate( '2025-11-25T11:18:31.094Z' ) } ]
Sprint:
QE 2026-01-19, QE 2025-12-08
CAR Domain/s:
None

In an aggregation pipeline, if $$NOW is referenced after a sub-pipeline stage, it can have a different value inside and outside of a sub-pipeline.

is caused by

SERVER-87159 Avoid generating unused runtime constants in ExpressionContext

is related to

SERVER-114196 $$NOW unavailable after $unionWith in aggregation pipeline

SERVER-116235 When saving variables from sub-pipeline, use correct ExpressionContext

SERVER-107976 Split ExpressionContext class to simplify OperationContext propagation

related to

SERVER-116293 Make ExpressionContext::variables private and provide getters