NIP-45 defines how clients ask relays to count events matching a filter without transferring the matching events themselves. It reuses NIP-01 filter syntax, so a client can often turn an existing REQ into a COUNT request with the same filters.

How It Works

A client sends a COUNT request with a subscription ID and filter:

["COUNT", "<subscription_id>", {"kinds": [7], "#e": ["<event_id>"]}]

The relay responds with a count:

["COUNT", "<subscription_id>", {"count": 238}]

This avoids downloading hundreds or thousands of events just to display a number. If a client sends multiple filters in one COUNT request, the relay aggregates them into a single result, just as multiple REQ filters are ORed together.

HyperLogLog Approximate Counting

As of February 2026, NIP-45 supports HyperLogLog (HLL) approximate counting (PR #1561). Relays can mark a result as approximate, and for cross-relay deduplication they can return 256 HLL registers alongside the count:

["COUNT", "<subscription_id>", {"count": 4527, "hll": "<512-char hex string>"}]

HLL solves a fundamental problem: counting distinct events across multiple relays. If relay A reports 50 reactions and relay B reports 40, the total is not 90 because many events exist on both relays. Clients merge HLL values by taking the maximum value at each register position, which gives a network-wide estimate without downloading the raw events.

The spec fixes the register count at 256 for interoperability. That keeps the payload small and makes relay-side caching practical because every relay computes the same register layout for the same eligible filter.

Interop Notes

HLL is only defined for filters with a tag attribute, because the offset used to build the registers is derived from the first tagged value in the filter. The spec also calls out a small set of canonical queries, including reactions, reposts, quotes, replies, comments, and follower counts. Those are the easiest counts for relays to precompute or cache.

Why It Matters

Follower counts, reaction counts, and reply counts are the main use cases. Without NIP-45, clients must either trust a single relay’s local view or download all matching events and deduplicate them locally. NIP-45 keeps counting inside the relay, and HLL makes multi-relay counts practical without turning one relay into the authority.

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