When dispatchShardPipeline dispatches to a specific mergeShardId, it continues to use a CollectionRoutingInfo (CRI) from the originally targeted namespace, which may not match the actual shard the pipeline is sent to.

This causes a violation in the database version protocol, specifically this tassert in buildDatabaseVersionedRequest:

    tassert(
                10162102,
                fmt::format("Expected exactly one shard matching the database primary shard when no shard "
                                        "version is required. Found shard: {}, expected: {}, for namespace: {}.",
                                        shardId.toString(),
                                        cri.getDbPrimaryShardId().toString(),
                                        nss.toStringForErrorMsg()),
                shardId == cri.getDbPrimaryShardId());

Here is an example of a failing patch

This happens when:

• Any aggregation runs against a user database (like $listClusterCatalog) 
• The pipeline sets mergeShardId = "config"
• But the CRI passed to dispatchShardPipeline corresponds to the original user database, whose primary shard does not match "config"
• This results in an assertion failure because the shardId used to dispatch does not match cri.getDbPrimaryShardId().

Another example in this log:

• The aggregation ran on the test database, which has primary shard shard-rs1.
• The $merge stage wrote to a collection (outColl) that was already created on shard-rs0.
• As a result, the aggregation was dispatched to shard-rs0 for execution.
• However, the CollectionRoutingInfo used was still for the test database (primary: shard-rs1), causing a mismatch with the target shard and triggering the tassert above.

NOTE: We have currently added an exception to skip the tassert, but the intention is to remove it and enable it to prevent future misusages when versioning a request.

To summarize: If you're dispatching a request to a specific shard for a database, you must use a CollectionRoutingInfo (CRI) whose dbPrimaryShardId matches the target shard.