In the with_mongot_extension_* suites (featureFlagVectorSearchExtension ON, featureFlagExtensionsInsideHybridSearch OFF), a $vectorSearch inside $unionWith fails with IFRFlagRetry exhaustion instead of retrying with the legacy stage:

Exhausted max retries (10) for ClusterAggregate::runAggregate :: caused by :: The $vectorSearch extension stage is not supported in a $unionWith (code 479, disabledFlagName: featureFlagVectorSearchExtension)

Since the named flag is enabled, each retry resolves $vectorSearch to the extension stage again and re-emits the same kickback, looping until the retry cap. Adjacent to SERVER-130918 but distinct: that ticket covers the flag-off kickback path.

Fails jstests/with_mongot/e2e/sharding_no_passthrough/vector_search_shard_filtering.js ("filters orphans in $unionWith subpipeline") on the mongot-integration-extensions-secure variants in 11 of the last 14 mainline runs. Excluded from the with_mongot_extension suites pending this fix (SERVER-131182).