HNSW index is not supported for M30 cluster tier

Question

HNSW index is not supported for M30 cluster tier

Anonymous

Hi everyone,

We have a Cosmos DB MongoDB (vcore) M30 instance running for over a year. We were using the HNSW indexes for vector searches. However, recently vector search queries started failing without any change on our side, causing our product feature to fail. Additionally, when I try to create a new HNSW index, I get the error "hnsw index is not supported for this cluster tier". According to Microsofts Azures own documentation, HNSW indexes should be available for M30 clusters and higher (https://learn.microsoft.com/en-us/azure/cosmos-db/mongodb/vcore/vector-search?tabs=hnsw).

Does anybody have a similar issue or know what may have caused this behavior to occur?

Thanks in advance,

Pascal

Khelan Modi 0 Reputation points Microsoft Employee

2026-04-28T18:50:27.21+00:00

hey! i'm on the product team and led vector search for Cosmos DB MongoDB (vcore)/ azure documentdb.

you're right that M30 should support HNSW. we actually lowered the entry tier to M30 for both HNSW and DiskANN last year, so the doc is accurate. something sounds off on the backend side if existing indexes stopped working and new ones are throwing that tier error.

can you shoot an email to mongodb-feedback@microsoft.com with your cluster details? i'll loop in engineering to dig into what changed. this shouldn't be happening.

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Khelan Modi 0 Reputation points Microsoft Employee

2026-04-28T18:50:27.21+00:00

hey! i'm on the product team and led vector search for Cosmos DB MongoDB (vcore)/ azure documentdb.

you're right that M30 should support HNSW. we actually lowered the entry tier to M30 for both HNSW and DiskANN last year, so the doc is accurate. something sounds off on the backend side if existing indexes stopped working and new ones are throwing that tier error.

can you shoot an email to mongodb-feedback@microsoft.com with your cluster details? i'll loop in engineering to dig into what changed. this shouldn't be happening.

Answer 1

Mahesh Kurva 10,520 Microsoft External Staff Moderator

Hi Pascal Pieters,

Greetings!!

HNSW indexes are not supported on M30 cluster tiers.

According to the updated service limits documentation, HNSW and DiskANN indexes are now only supported on M40 and higher cluster tiers. This appears to be a recent change.

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If vector search is critical to your product, consider upgrading your cluster tier to M40 or above to restore HNSW support.

Hope this helps. Do let us know if you any further queries.

If this answers your query, do click Accept Answer and Yes for was this answer helpful. And, if you have any further query do let us know.

Anonymous

2025-06-13T11:26:44.5333333+00:00

Hi Mahesh,

Thank you for your response. It is very concerning to me though that Microsoft would just break our existing deployment and also not update the documentation properly. In addition, I now have to test whether this works on a new instance, since I cannot downgrade to M30 if it doesn't work. Thank you for your answer, I'm just not happy with the course of events.

Best Pascal

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