eth_newBlockFilter | Cronos RPC Docs

Create a filter for new block notifications on Cronos. Essential for block monitoring, chain progression tracking, and reorg detection for DeFi protocols, NFT marketplaces, and Crypto.com ecosystem integrations.

Creates a filter on Cronos that notifies when new blocks arrive. Once created, poll the filter with eth_getFilterChanges to receive an array of block hashes for each new block added to the chain since your last poll.

Why Cronos? Build on the EVM-compatible Crypto.com blockchain with fast finality and deep Crypto.com ecosystem integration.

When to Use This Method

eth_newBlockFilter is essential for Cronos developers building DeFi and payment applications:

Block Monitoring — Detect new blocks on Cronos as they are mined or finalized, without repeatedly calling eth_blockNumber
Chain Progression Tracking — Build dashboards or alerting systems that track block production rate and timing
Reorg Detection — Identify chain reorganizations by monitoring for blocks that are replaced or missing
Event-Driven Architectures — Trigger downstream processing (indexing, notifications, settlement) whenever a new block appears

Request Parameters

Request

This method accepts no parameters.

Response Body

Response

resultQUANTITY

A hex-encoded filter ID used to poll for new blocks via eth_getFilterChanges

resultArray<DATA>

Array of 32-byte block hashes for each new block since the last poll

Error Responses

Errors

Error Response-32603

{  "jsonrpc": "2.0",  "method": "eth_newBlockFilter",  "params": [],  "id": 1}

{  "jsonrpc": "2.0",  "id": 1,  "result": "0x1a2b3c"}

{  "jsonrpc": "2.0",  "id": 1,  "error": {    "code": -32603,    "message": "Internal error"  }}

Code Examples

Bash

# Create the block filter
FILTER_ID=$(curl -s -X POST https://api-cronos-mainnet-archive.n.dwellir.com/YOUR_API_KEY \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "method": "eth_newBlockFilter",
    "params": [],
    "id": 1
  }' | jq -r '.result')

echo "Filter ID: $FILTER_ID"

# Poll for new blocks
curl -X POST https://api-cronos-mainnet-archive.n.dwellir.com/YOUR_API_KEY \
  -H "Content-Type: application/json" \
  -d "{
    \"jsonrpc\": \"2.0\",
    \"method\": \"eth_getFilterChanges\",
    \"params\": [\"$FILTER_ID\"],
    \"id\": 2
  }"

Common Use Cases

1. Block Production Rate Monitor

Track block times and detect slowdowns on Cronos:

JavaScript

async function monitorBlockRate(provider) {
  const filterId = await provider.send('eth_newBlockFilter', []);
  let lastBlockTime = null;
  const blockTimes = [];

  setInterval(async () => {
    const blockHashes = await provider.send('eth_getFilterChanges', [filterId]);

    for (const hash of blockHashes) {
      const block = await provider.getBlock(hash);
      const blockTime = block.timestamp;

      if (lastBlockTime) {
        const interval = blockTime - lastBlockTime;
        blockTimes.push(interval);

        // Keep last 100 block times
        if (blockTimes.length > 100) blockTimes.shift();

        const avgBlockTime = blockTimes.reduce((a, b) => a + b, 0) / blockTimes.length;
        console.log(`Block #${block.number} | interval: ${interval}s | avg: ${avgBlockTime.toFixed(1)}s`);

        if (interval > avgBlockTime * 3) {
          console.warn(`Slow block detected — ${interval}s vs ${avgBlockTime.toFixed(1)}s average`);
        }
      }
      lastBlockTime = blockTime;
    }
  }, 2000);
}

2. Reorg-Aware Block Tracker

Detect chain reorganizations by tracking block hashes:

JavaScript

async function trackBlocksWithReorgDetection(provider) {
  const filterId = await provider.send('eth_newBlockFilter', []);
  const blockHistory = new Map(); // blockNumber -> blockHash

  setInterval(async () => {
    const newBlockHashes = await provider.send('eth_getFilterChanges', [filterId]);

    for (const hash of newBlockHashes) {
      const block = await provider.getBlock(hash);
      const existingHash = blockHistory.get(block.number);

      if (existingHash && existingHash !== hash) {
        console.warn(`Reorg detected at block #${block.number}!`);
        console.warn(`  Old hash: ${existingHash}`);
        console.warn(`  New hash: ${hash}`);
        // Trigger reorg handling logic
      }

      blockHistory.set(block.number, hash);

      // Prune old entries
      if (blockHistory.size > 1000) {
        const minBlock = block.number - 500;
        for (const [num] of blockHistory) {
          if (num < minBlock) blockHistory.delete(num);
        }
      }
    }
  }, 2000);
}

3. Block-Triggered Task Scheduler

Execute tasks whenever a new block is produced:

JavaScript

async function onNewBlock(provider, callback) {
  const filterId = await provider.send('eth_newBlockFilter', []);

  const poll = async () => {
    try {
      const hashes = await provider.send('eth_getFilterChanges', [filterId]);
      for (const hash of hashes) {
        await callback(hash);
      }
    } catch (error) {
      if (error.message.includes('filter not found')) {
        return provider.send('eth_newBlockFilter', []).then(newId => {
          console.log('Block filter recreated');
          return newId;
        });
      }
      throw error;
    }
    return filterId;
  };

  let currentFilterId = filterId;
  setInterval(async () => {
    currentFilterId = await poll() || currentFilterId;
  }, 2000);
}

// Usage
onNewBlock(provider, async (blockHash) => {
  console.log(`Processing block: ${blockHash}`);
  // Run indexer, check conditions, send notifications, etc.
});

Error Handling

Common errors and solutions:

Error Code	Description	Solution
-32603	Internal error	Node may not support filters — verify node capabilities
-32000	Filter not found	Filter expired (~5 min inactivity) — recreate with `eth_newBlockFilter`
-32005	Rate limit exceeded	Reduce polling frequency or implement client-side rate limiting
-32601	Method not found	Node does not support filter methods — use `eth_blockNumber` polling instead

JavaScript

async function createBlockFilterWithFallback(provider) {
  try {
    const filterId = await provider.send('eth_newBlockFilter', []);
    return { type: 'filter', id: filterId };
  } catch (error) {
    console.warn('Block filter not supported — falling back to polling');
    return { type: 'polling', lastBlock: await provider.getBlockNumber() };
  }
}

eth_getFilterChanges — Poll this filter for new block hashes
eth_uninstallFilter — Remove the block filter when no longer needed
eth_blockNumber — Get the current block number (alternative to filter-based monitoring)
eth_getBlockByHash — Fetch full block details for hashes returned by the filter

eth_newBlockFilter - Cronos RPC Method

When to Use This Method

Request Parameters

Response Body

Error Responses

Code Examples

Common Use Cases

1. Block Production Rate Monitor

2. Reorg-Aware Block Tracker

3. Block-Triggered Task Scheduler

Error Handling

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