diff --git a/backend/.eslintrc.js b/backend/.eslintrc.js
index 15bc17cd9..b53232ee0 100644
--- a/backend/.eslintrc.js
+++ b/backend/.eslintrc.js
@@ -15,7 +15,7 @@ module.exports = {
     "plugin:@typescript-eslint/recommended",
     "prettier"
   ],
-  "ignorePatterns": ["dist", "eslint-local-rules", ".eslintrc.js", "testSetup*.ts", "jest.integration.*.ts", "__tests__", "*.config.ts"],
+  "ignorePatterns": ["dist", "eslint-local-rules", ".eslintrc.js", "testSetup*.ts", "jest.integration.*.ts", "__tests__", "__e2e__", "*.config.ts"],
   "overrides": [
     {
       "files": ["src/__integration_tests__/**/*"],
diff --git a/backend/jest.config.ts b/backend/jest.config.ts
index 7989fca81..fd6e5abf3 100644
--- a/backend/jest.config.ts
+++ b/backend/jest.config.ts
@@ -19,6 +19,7 @@ const config: Config.InitialOptions = {
   testPathIgnorePatterns: [
     '/node_modules/',
     '/__integration_tests__/',
+    'test-utils\\.ts$',
   ],
 };
 export default config;
diff --git a/backend/src/__e2e__/cluster-mempool/harness/rpc-client.ts b/backend/src/__e2e__/cluster-mempool/harness/rpc-client.ts
new file mode 100644
index 000000000..6ef621028
--- /dev/null
+++ b/backend/src/__e2e__/cluster-mempool/harness/rpc-client.ts
@@ -0,0 +1,137 @@
+import http from 'http';
+
+export interface RpcConfig {
+  host: string;
+  port: number;
+  user: string;
+  pass: string;
+}
+
+interface RpcResponse {
+  result: any;
+  error: { code: number; message: string } | null;
+  id: string;
+}
+
+export class RpcClient {
+  private config: RpcConfig;
+  private idCounter = 0;
+
+  constructor(config: RpcConfig) {
+    this.config = config;
+  }
+
+  private async call(method: string, params: any[] = []): Promise<any> {
+    const id = String(++this.idCounter);
+    const body = JSON.stringify({ jsonrpc: '2.0', id, method, params });
+    const auth = Buffer.from(`${this.config.user}:${this.config.pass}`).toString('base64');
+
+    return new Promise((resolve, reject) => {
+      const req = http.request(
+        {
+          hostname: this.config.host,
+          port: this.config.port,
+          method: 'POST',
+          headers: {
+            'Content-Type': 'application/json',
+            'Authorization': `Basic ${auth}`,
+            'Content-Length': Buffer.byteLength(body),
+          },
+        },
+        (res) => {
+          let data = '';
+          res.on('data', (chunk) => (data += chunk));
+          res.on('end', () => {
+            try {
+              const parsed: RpcResponse = JSON.parse(data);
+              if (parsed.error) {
+                reject(new Error(`RPC error ${parsed.error.code}: ${parsed.error.message}`));
+              } else {
+                resolve(parsed.result);
+              }
+            } catch (e) {
+              reject(new Error(`Failed to parse RPC response: ${data.slice(0, 500)}`));
+            }
+          });
+        },
+      );
+      req.on('error', reject);
+      req.write(body);
+      req.end();
+    });
+  }
+
+  async getRawMempool(): Promise<string[]> {
+    return this.call('getrawmempool');
+  }
+
+  async getRawTransaction(txid: string, verbose = true): Promise<any> {
+    return this.call('getrawtransaction', [txid, verbose]);
+  }
+
+  async getBlockTemplate(rules: string[] = ['segwit']): Promise<any> {
+    return this.call('getblocktemplate', [{ rules }]);
+  }
+
+  async getMempoolEntry(txid: string): Promise<any> {
+    return this.call('getmempoolentry', [txid]);
+  }
+
+  async getMempoolCluster(txid: string): Promise<any> {
+    return this.call('getmempoolcluster', [txid]);
+  }
+
+  async getBlockCount(): Promise<number> {
+    return this.call('getblockcount');
+  }
+
+  async batch(calls: { method: string; params: any[] }[]): Promise<any[]> {
+    if (calls.length === 0) {
+      return [];
+    }
+    const bodies = calls.map((c) => ({
+      jsonrpc: '2.0',
+      id: String(++this.idCounter),
+      method: c.method,
+      params: c.params,
+    }));
+    const body = JSON.stringify(bodies);
+    const auth = Buffer.from(`${this.config.user}:${this.config.pass}`).toString('base64');
+
+    return new Promise((resolve, reject) => {
+      const req = http.request(
+        {
+          hostname: this.config.host,
+          port: this.config.port,
+          method: 'POST',
+          headers: {
+            'Content-Type': 'application/json',
+            'Authorization': `Basic ${auth}`,
+            'Content-Length': Buffer.byteLength(body),
+          },
+        },
+        (res) => {
+          let data = '';
+          res.on('data', (chunk) => (data += chunk));
+          res.on('end', () => {
+            try {
+              const parsed: RpcResponse[] = JSON.parse(data);
+              const results = parsed.map((r) => {
+                if (r.error) {
+                  return { error: r.error };
+                }
+                return r.result;
+              });
+              resolve(results);
+            } catch (e) {
+              reject(new Error(`Failed to parse batch RPC response: ${data.slice(0, 500)}`));
+            }
+          });
+        },
+      );
+      req.on('error', reject);
+      req.write(body);
+      req.end();
+    });
+  }
+}
diff --git a/backend/src/__e2e__/cluster-mempool/harness/run-harness.ts b/backend/src/__e2e__/cluster-mempool/harness/run-harness.ts
new file mode 100644
index 000000000..3e252122b
--- /dev/null
+++ b/backend/src/__e2e__/cluster-mempool/harness/run-harness.ts
@@ -0,0 +1,461 @@
+/**
+ * Cluster Mempool Test Harness
+ *
+ * Standalone script that compares our ClusterMempool block template ordering
+ * against Bitcoin Core's getblocktemplate output.
+ *
+ * Uses the backend's own transactionUtils to fetch and convert transactions,
+ * ensuring fields (sigops, adjustedVsize, etc.) match exactly.
+ *
+ * Usage:
+ *   npx ts-node src/__tests__/cluster-mempool/harness/run-harness.ts [options]
+ *
+ * Options:
+ *   --host <host>       Override CORE_RPC host
+ *   --port <port>       Override CORE_RPC port
+ *   --user <user>       Override CORE_RPC username
+ *   --pass <pass>       Override CORE_RPC password
+ *   --interval <ms>     Comparison interval in ms (default: 30000)
+ *   --poll <ms>         Mempool poll interval in ms (default: 1000)
+ *   --max <n>           Max comparisons before exit (0 = unlimited, default: 0)
+ */
+
+import { RpcClient, RpcConfig } from './rpc-client';
+
+// ─── CLI Parsing (must happen before backend imports) ───────────────────────
+
+interface CliOptions {
+  rpcOverrides: Partial<RpcConfig>;
+  comparisonInterval: number;
+  pollInterval: number;
+  maxComparisons: number;
+}
+
+function parseArgs(): CliOptions {
+  const args = process.argv.slice(2);
+  const overrides: Partial<RpcConfig> = {};
+  let comparisonInterval = 30_000;
+  let pollInterval = 1000;
+  let maxComparisons = 0;
+
+  for (let i = 0; i < args.length; i++) {
+    switch (args[i]) {
+      case '--host':
+        overrides.host = args[++i];
+        break;
+      case '--port':
+        overrides.port = parseInt(args[++i], 10);
+        break;
+      case '--user':
+        overrides.user = args[++i];
+        break;
+      case '--pass':
+        overrides.pass = args[++i];
+        break;
+      case '--interval':
+        comparisonInterval = parseInt(args[++i], 10);
+        break;
+      case '--poll':
+        pollInterval = parseInt(args[++i], 10);
+        break;
+      case '--max':
+        maxComparisons = parseInt(args[++i], 10);
+        break;
+    }
+  }
+
+  return { rpcOverrides: overrides, comparisonInterval, pollInterval, maxComparisons };
+}
+
+const cliOptions = parseArgs();
+
+// ─── Backend Module Loading ─────────────────────────────────────────────────
+// Import config first, apply CLI overrides, then import modules that depend on it.
+// This works because Node's require() caches modules — when transactionUtils
+// (via bitcoinClient) reads config, it sees our modified values.
+
+const config = require('../../../config').default;
+
+if (cliOptions.rpcOverrides.host) { config.CORE_RPC.HOST = cliOptions.rpcOverrides.host; }
+if (cliOptions.rpcOverrides.port) { config.CORE_RPC.PORT = cliOptions.rpcOverrides.port; }
+if (cliOptions.rpcOverrides.user) { config.CORE_RPC.USERNAME = cliOptions.rpcOverrides.user; }
+if (cliOptions.rpcOverrides.pass) { config.CORE_RPC.PASSWORD = cliOptions.rpcOverrides.pass; }
+
+// Now load modules that depend on config
+const bitcoinApi = require('../../../api/bitcoin/bitcoin-api-factory').default;
+const transactionUtils = require('../../../api/transaction-utils').default;
+const { ClusterMempool } = require('../../../cluster-mempool/cluster-mempool');
+
+import { MempoolTransactionExtended } from '../../../mempool.interfaces';
+import { MempoolDiff } from '../../../cluster-mempool/cluster-mempool';
+
+// ─── Main Harness ───────────────────────────────────────────────────────────
+
+const TX_FETCH_BATCH_SIZE = 1000;
+
+class Harness {
+  private rpc: RpcClient;
+  private clusterMempool: InstanceType<typeof ClusterMempool> | null = null;
+  private mempool: { [txid: string]: MempoolTransactionExtended } = {};
+  private knownTxids = new Set<string>();
+  private lastBlockHeight = -1;
+  private comparisonInterval: number;
+  private pollInterval: number;
+  private maxComparisons: number;
+  private pollTimer: ReturnType<typeof setInterval> | null = null;
+  private polling = false;
+  private nextComparisonTime = 0;
+  private stats = {
+    templateMatches: 0,
+    templateMismatches: 0,
+    comparisons: 0,
+  };
+
+  constructor(rpcConfig: RpcConfig, opts: CliOptions) {
+    this.rpc = new RpcClient(rpcConfig);
+    this.comparisonInterval = opts.comparisonInterval;
+    this.pollInterval = opts.pollInterval;
+    this.maxComparisons = opts.maxComparisons;
+  }
+
+  async run(): Promise<void> {
+    console.log('=== Cluster Mempool Test Harness ===');
+    console.log(`RPC: ${config.CORE_RPC.HOST}:${config.CORE_RPC.PORT}`);
+    console.log(`Backend: ${config.MEMPOOL.BACKEND}`);
+    console.log(`Poll interval: ${this.pollInterval}ms`);
+    console.log(`Comparison interval: ${this.comparisonInterval}ms`);
+    console.log();
+
+    // Step 1: Fetch full mempool
+    console.log('Fetching full mempool...');
+    const txids: string[] = await bitcoinApi.$getRawMempool();
+    console.log(`Got ${txids.length} txids from getrawmempool`);
+
+    const fetchStart = Date.now();
+    await this.fetchTransactions(txids);
+    const fetchTime = Date.now() - fetchStart;
+    console.log(`Fetched ${Object.keys(this.mempool).length} transactions in ${fetchTime}ms`);
+
+    // Step 2: Initialize ClusterMempool
+    console.log('Building cluster mempool...');
+    const buildStart = Date.now();
+    this.clusterMempool = new ClusterMempool(this.mempool);
+    const buildTime = Date.now() - buildStart;
+    console.log(`Cluster mempool built in ${buildTime}ms`);
+    console.log(`  Clusters: ${this.clusterMempool.getClusterCount()}`);
+    console.log(`  Transactions: ${this.clusterMempool.getTxCount()}`);
+    console.log();
+
+    // Record initial block height
+    this.lastBlockHeight = await this.rpc.getBlockCount();
+
+    // Run initial comparison, then start polling
+    await this.runComparison();
+    this.nextComparisonTime = Date.now() + this.comparisonInterval;
+    this.pollTimer = setInterval(() => this.pollMempool(), this.pollInterval);
+
+    console.log('\nHarness running. Press Ctrl+C to stop.\n');
+
+    process.on('SIGINT', () => this.shutdown());
+    process.on('SIGTERM', () => this.shutdown());
+  }
+
+  private async fetchTransactions(txids: string[]): Promise<MempoolTransactionExtended[]> {
+    const added: MempoolTransactionExtended[] = [];
+
+    for (let offset = 0; offset < txids.length; offset += TX_FETCH_BATCH_SIZE) {
+      let batch = txids.slice(offset, offset + TX_FETCH_BATCH_SIZE);
+      let txs: MempoolTransactionExtended[] = [];
+      let tries = 0;
+      while (batch.length && tries < 20) {
+        try {
+          tries++;
+          txs = txs.concat(await transactionUtils.$getMempoolTransactionsExtended(batch, false, false, false));
+          let missing: string[] = [];
+          console.log(`txs: ${txs.length} of ${batch.length} fetched`);
+          for (const txid of batch) {
+            if (!txs.some(tx => tx.txid === txid)) {
+              console.log(`missing ${txid} at offset ${offset}, retrying`);
+              missing.push(txid);
+            }
+          }
+          batch = missing;
+          if (batch.length) {
+            await new Promise(resolve => setTimeout(resolve, 500));
+          }
+        } catch (err: any) {
+          console.log(`  Fetch batch failed at offset ${offset}: ${err.message}, retrying`);
+        }
+      }
+
+      for (const tx of txs) {
+        this.mempool[tx.txid] = tx;
+        this.knownTxids.add(tx.txid);
+        added.push(tx);
+      }
+
+      if (txids.length > TX_FETCH_BATCH_SIZE && (offset + batch.length) % 5000 < TX_FETCH_BATCH_SIZE) {
+        console.log(`  ${offset + batch.length} / ${txids.length} fetched...`);
+      }
+    }
+
+    return added;
+  }
+
+  private async pollMempool(): Promise<void> {
+    if (this.polling || !this.clusterMempool) {
+      return;
+    }
+    this.polling = true;
+
+    try {
+      // Check for new block
+      const height = await this.rpc.getBlockCount();
+      const newBlock = height > this.lastBlockHeight;
+      if (newBlock) {
+        console.log(`\n--- New block detected (height ${height}) ---`);
+        this.lastBlockHeight = height;
+      }
+
+      // Get current mempool
+      const currentTxids = await bitcoinApi.$getRawMempool();
+      const currentSet = new Set(currentTxids);
+
+      // Find added and removed
+      const addedTxids: string[] = [];
+      for (const txid of currentTxids) {
+        if (!this.knownTxids.has(txid)) {
+          addedTxids.push(txid);
+        }
+      }
+      const removedTxids: string[] = [];
+      for (const txid of this.knownTxids) {
+        if (!currentSet.has(txid)) {
+          removedTxids.push(txid);
+        }
+      }
+
+      if (addedTxids.length === 0 && removedTxids.length === 0) {
+        if (newBlock) {
+          await this.runComparison();
+        }
+        return;
+      }
+
+      const added = await this.fetchTransactions(addedTxids);
+
+      // Apply diff before deleting from cache — processRemovals needs the tx data
+      const diff: MempoolDiff = { added, removed: removedTxids, accelerations: {} };
+      const t0 = Date.now();
+      this.clusterMempool.applyMempoolChange(diff);
+      const dt = Date.now() - t0;
+      for (const txid of removedTxids) {
+        delete this.mempool[txid];
+        this.knownTxids.delete(txid);
+      }
+      console.log(
+        `Applied${newBlock ? ' block' : ''} diff: +${added.length} -${removedTxids.length} txs in ${dt}ms` +
+        ` (${this.clusterMempool.getTxCount()} txs, ${this.clusterMempool.getClusterCount()} clusters)`
+      );
+
+      if (newBlock) {
+        await this.runComparison();
+      } else if (Date.now() >= this.nextComparisonTime) {
+        await this.runComparison();
+        this.nextComparisonTime = Date.now() + this.comparisonInterval;
+      }
+    } catch (err: any) {
+      console.error(`Poll error: ${err.message}`);
+    } finally {
+      this.polling = false;
+    }
+  }
+
+  private async runComparison(): Promise<void> {
+    if (!this.clusterMempool) {
+      return;
+    }
+    this.stats.comparisons++;
+    console.log(`\n=== Comparison #${this.stats.comparisons} ===`);
+
+    // Step 1: Get Core's block template
+    let template: any;
+    try {
+      template = await this.rpc.getBlockTemplate(['segwit']);
+    } catch (err: any) {
+      console.log(`getblocktemplate failed: ${err.message}`);
+      return;
+    }
+    const coreTxids: string[] = template.transactions.map((t: any) => t.txid);
+    const coreSet = new Set(coreTxids);
+
+    const missingTxids: string[] = [];
+    for (const txid of coreSet) {
+      if (!this.mempool[txid]) {
+        missingTxids.push(txid);
+      }
+    }
+    if (missingTxids.length) {
+      const added = await this.fetchTransactions(missingTxids);
+      if (added.length) {
+        console.log(`Reconciled: +${added.length} missing txs`);
+        this.clusterMempool.applyMempoolChange({ added, removed: [], accelerations: {} });
+      }
+    }
+
+    const ourBlocks = this.clusterMempool.getBlocks(1, true);
+    const ourTxids: string[] = ourBlocks[0]?.txids || [];
+
+    await this.compareTemplateOrdering(coreTxids, ourTxids);
+
+    this.printStats();
+
+    if (this.maxComparisons > 0 && this.stats.comparisons >= this.maxComparisons) {
+      console.log(`\nReached ${this.maxComparisons} comparisons, stopping.`);
+      this.shutdown();
+    }
+  }
+
+  private async compareTemplateOrdering(coreTxids: string[], ourTxids: string[]): Promise<void> {
+    const coreSet = new Set(coreTxids);
+    const ourSet = new Set(ourTxids);
+
+    let inBoth = 0;
+    let onlyCore = 0;
+    let onlyOurs = 0;
+    for (const txid of coreSet) {
+      if (ourSet.has(txid)) {
+        inBoth++;
+      } else {
+        onlyCore++;
+      }
+    }
+    for (const txid of ourSet) {
+      if (!coreSet.has(txid)) {
+        onlyOurs++;
+      }
+    }
+
+    console.log(`Template sets: Core ${coreTxids.length} txs, Ours ${ourTxids.length} txs`);
+    console.log(`  In both: ${inBoth}, Only Core: ${onlyCore}, Only ours: ${onlyOurs}`);
+
+    // Build position maps for the intersection
+    const corePos = new Map<string, number>();
+    for (let i = 0; i < coreTxids.length; i++) {
+      if (ourSet.has(coreTxids[i])) {
+        corePos.set(coreTxids[i], i);
+      }
+    }
+
+    // Filter to shared txids in each order
+    const sharedInCoreOrder = coreTxids.filter(t => ourSet.has(t));
+    const sharedInOurOrder = ourTxids.filter(t => coreSet.has(t));
+
+    let exactMatches = 0;
+    let firstDivergence = -1;
+    for (let i = 0; i < sharedInCoreOrder.length; i++) {
+      if (sharedInCoreOrder[i] === sharedInOurOrder[i]) {
+        exactMatches++;
+      } else if (firstDivergence === -1) {
+        firstDivergence = i;
+      }
+    }
+
+    const orderMatchRate = sharedInCoreOrder.length > 0
+      ? (exactMatches / sharedInCoreOrder.length * 100).toFixed(1)
+      : 'N/A';
+    console.log(`  Ordering: ${orderMatchRate}% exact position match (${exactMatches}/${sharedInCoreOrder.length} shared txs)`);
+
+    if (firstDivergence >= 0) {
+      const coreTx = sharedInCoreOrder[firstDivergence];
+      const ourTx = sharedInOurOrder[firstDivergence];
+      console.log(`\n  First divergence at position ${firstDivergence}:`);
+      console.log(`    Core wants: ${coreTx}`);
+      console.log(`    We placed:  ${ourTx} (Core has this at position ${corePos.get(ourTx)})`);
+
+      for (const [label, txid] of [['Core tx', coreTx], ['Our tx', ourTx]] as const) {
+        const inOurMempool = !!this.mempool[txid];
+        const info = this.clusterMempool?.getClusterInfo(txid);
+        console.log(`\n    --- ${label}: ${txid} ---`);
+        console.log(`    In our mempool: ${inOurMempool}`);
+        if (inOurMempool) {
+          const tx = this.mempool[txid];
+          console.log(`    fee=${tx.fee} weight=${tx.weight} vsize=${tx.vsize} sigops=${tx.sigops} adjustedVsize=${tx.adjustedVsize}`);
+        }
+        if (info) {
+          console.log(`    clusterId=${info.clusterId} chunkIndex=${info.chunkIndex} chunkFeerate=${info.chunkFeerate.toFixed(6)}`);
+          const cluster = this.clusterMempool?.getCluster(info.clusterId);
+          if (cluster) {
+            console.log(`    Cluster has ${cluster.chunks.length} chunk(s):`);
+            for (let ci = 0; ci < cluster.chunks.length; ci++) {
+              const c = cluster.chunks[ci];
+              console.log(`      chunk[${ci}]: ${c.txs.length} txs, feerate=${c.feerate.toFixed(6)}`);
+              if (ci === info.chunkIndex) {
+                const chunkTxids = c.txs.map((idx: number) => cluster.txs[idx]?.txid).filter(Boolean);
+                for (const tid of chunkTxids) {
+                  const t = this.mempool[tid];
+                  const parents = t?.vin
+                    ?.filter(v => !v.is_coinbase && this.mempool[v.txid] && chunkTxids.includes(v.txid))
+                    .map(v => v.txid.substring(0, 12)) || [];
+                  console.log(`        ${tid.substring(0, 12)}  fee=${t?.fee} size=${(t?.weight || 0) / 4} sigops=${t?.sigops} parents=[${parents.join(', ')}]`);
+                }
+              }
+            }
+          }
+        } else {
+          console.log(`    NOT in our cluster mempool`);
+        }
+      }
+
+      for (const [label, txid] of [['Core', coreTx], ['Our', ourTx]] as const) {
+        try {
+          const coreCluster = await this.rpc.getMempoolCluster(txid);
+          console.log(`\n    --- Core cluster for ${label} tx ${txid.substring(0, 12)} ---`);
+          console.log(`    Raw response: ${JSON.stringify(coreCluster).substring(0, 2000)}`);
+        } catch (e: any) {
+          console.log(`    Failed to get Core cluster for ${label} tx: ${e.message}`);
+        }
+      }
+
+      this.stats.templateMismatches++;
+    } else if (sharedInCoreOrder.length === coreTxids.length && sharedInCoreOrder.length === ourTxids.length) {
+      console.log(`  PERFECT MATCH`);
+      this.stats.templateMatches++;
+    } else {
+      console.log(`  Ordering matches for shared txs, but sets differ`);
+      this.stats.templateMatches++;
+    }
+  }
+
+  private printStats(): void {
+    console.log('\n--- Cumulative Stats ---');
+    const total = this.stats.templateMatches + this.stats.templateMismatches;
+    console.log(`Template comparisons: ${total} (${this.stats.templateMatches} perfect, ${this.stats.templateMismatches} divergent)`);
+  }
+
+  private shutdown(): void {
+    console.log('\nShutting down...');
+    if (this.pollTimer) {
+      clearInterval(this.pollTimer);
+    }
+    this.printStats();
+    process.exit(0);
+  }
+}
+
+// ─── Entry Point ────────────────────────────────────────────────────────────
+
+const rpcConfig: RpcConfig = {
+  host: config.CORE_RPC.HOST,
+  port: config.CORE_RPC.PORT,
+  user: config.CORE_RPC.USERNAME,
+  pass: config.CORE_RPC.PASSWORD,
+};
+
+console.log(`Connecting to Bitcoin Core at ${rpcConfig.host}:${rpcConfig.port}`);
+
+const harness = new Harness(rpcConfig, cliOptions);
+harness.run().catch((err) => {
+  console.error('Harness failed:', err);
+  process.exit(1);
+});