168 InvalidPipelineOperator: InvalidPipelineOperator

What does it mean?

An unrecognized or invalid operator/stage was used in an aggregation pipeline. MongoDB could not resolve the specified operator name and rejected the pipeline.

Why does it happen?

A misspelled operator name (e.g. '$groups' instead of '$group'), a new operator not available in the MongoDB version in use, or an expression used in the wrong context. A missing $ prefix or an operator used in an invalid position also triggers it.

How to fix it

Verify the spelling of operator/stage names and the $ prefix. Make sure the operator is supported by the server's MongoDB version (new operators do not exist in older versions). Compare the pipeline with the documented syntax; use the expression inside the correct stage (e.g. $expr, $project). Isolate the stage and test it with a small sample.