Core subscriptions expose bitcoind’s ZMQ event stream over WebSocket. blockd receives raw ZMQ messages from bitcoind and re-shapes them into JSON events. These are lower-level events intended for applications that need direct chain data.

---

List of core subscriptions

Subscription	Description
core.hashblock	New block hash from bitcoind
core.hashtx	New transaction hash from bitcoind
core.rawblock	Raw block bytes from bitcoind
core.rawtx	Raw transaction bytes from bitcoind
core.sequence	Mempool and chain sequence events

core.hashblock

Fires when bitcoind broadcasts a new block hash.

{
  "subscriptions": [
    {
      "subscribe": "core.hashblock"
    }
  ]
}

Event:

{
  "event": "bitcoind_hashblock",
  "hash": "00000000590f69588b42f409796be4049ad8c4528e04f26f2fdabebbab7e0e18",
  "height": 909745,
  "time": 1755032952,
  "sequence": 0
}

blockd automatically calls getblockheader RPC to fetch height and time and adds them to the event. If the getblockheader RPC call has not yet completed at the time of broadcast, height and time will be absent from the event.

---

core.hashtx

Fires when bitcoind broadcasts a new transaction hash.

{
  "subscriptions": [
    {
      "subscribe": "core.hashtx"
    }
  ]
}

Event:

{
  "event": "bitcoind_hashtx",
  "hash": "82cb1fc54a4d891d687c3898a7ebcd54da996e7f26e1fd6f2a21dc2f655194fa",
  "sequence": 0
}

---

core.rawblock

Fires when bitcoind broadcasts a new block.

{
  "subscriptions": [
    {
      "subscribe": "core.rawblock"
    }
  ]
}

Event:

{
  "event": "bitcoind_rawblock",
  "data": "<hex>",
  "sequence": 0
}

data is the hex-encoded raw block bytes.

---

core.rawtx

Fires when bitcoind broadcasts a new transaction.

{
  "subscriptions": [
    {
      "subscribe": "core.rawtx"
    }
  ]
}

Event:

{
  "event": "bitcoind_rawtx",
  "data": "<hex>",
  "sequence": 0
}

data is the hex-encoded raw transaction bytes.

---

core.sequence

Fires on bitcoind mempool and chain sequence events.

{
  "subscriptions": [
    {
      "subscribe": "core.sequence"
    }
  ]
}

No args subscribes to all ops. To filter by op:

{
  "subscriptions": [
    {
      "subscribe": "core.sequence",
      "args": {
        "op": [
          "block_connected",
          "block_disconnected",
          "tx_added",
          "tx_removed"
        ]
      }
    }
  ]
}

Event:

{
  "event": "bitcoind_sequence",
  "op": "block_connected",
  "hash": "00000000590f69588b42f409796be4049ad8c4528e04f26f2fdabebbab7e0e18",
  "height": 909745,
  "time": 1755032952,
  "sequence": 0
}

For block_connected and block_disconnected events, blockd automatically calls getblockheader RPC to fetch height and time and adds them to the event. These fields are not present in the raw ZMQ message — blockd enriches them for you.

Fields present by op:

Op	hash	height	time	mempool_sequence
block_connected	block hash	✓	✓	—
block_disconnected	block hash	✓	✓	—
tx_added	tx hash	—	—	✓
tx_removed	tx hash	—	—	✓

---

sequence field

All core events include a sequence field — a monotonic counter from bitcoind’s ZMQ publisher. Gaps in the sequence indicate missed messages, which may occur under high load or during ZMQ disruptions. See zmq_outages under ZMQ monitoring.

In production, monitor for gaps and treat them as a signal that your application may have missed events. If a gap is detected, re-execute relevant workflows and verify the current block tip to ensure your local state is current.

mempool_sequence is a separate monotonic counter tracking mempool state changes, present only on tx_added and tx_removed ops.