9.4.0 / 2026-04-03

• perf(document+model): avoid parallel save error instantiation, simplify resetting atomics, streamline validation and collection handling
• feat(document): add $getChanges() alias, deprecate getChanges() #15959 techcodie
• fix(schema): support toJSONSchema on unions #16179
• fix(schema): implement validation for Union schemas and subdocuments techcodie
• fix(connection): snapshot Date in heartbeat handler and flush queue on recovery #16183 andreialecu
• fix(model): use duck-typing with version check to validate the argument to useConnection() is actually a connection #16098
• fix(setDefaultsOnInsert): run setters on default values during upsert #16051 mahmoodhamdi
• fix(utils): properly compare Set objects in deepEqual KhanjarSingh
• fix(utils): wrap discriminator merge check in parentheses to fix precedence Necro-Rohan
• fix(schema): correct template literal in encryptionType error message Mridul012
• fix(schema): correct error when unsupported query operator with number #16062
• fix(types): make MergeType and UnpackedIntersection distributive over union types techcodie
• types: add id to HydratedDocument virtuals by default unless explicitly set #16178
• types(populate): use marker type to track populated vs depopulated type for perf
• types(populate): retain populated paths in toObject() and toJSON() unless depopulate: true set #16085
• types(query): make TypeScript error on $and with unrecognized query operator
• chore: use TSTyche assertions mrazauskas
• docs(connection): remove references to useUnifiedTopology and fix backtick
• docs: fix typo 'retreiving' -> 'retrieving' in SchemaType getter JSDoc yogesh968
• docs: fix typos around 'retrieve' in schemaType and tests ayushshukla1807
• docs: fix typos in code comments Goldyvaiiii