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Run Hardfork examples in the interactive playground
Conceptual Guide - For API reference and method documentation, see Hardfork API.
Ethereum hardforks are backwards-incompatible protocol upgrades that introduce new features, fix bugs, and improve performance across the network. This guide teaches hardfork fundamentals using Tevm.

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What Are Hardforks?

Hardforks are coordinated protocol changes that require all nodes to upgrade. Unlike soft forks (backward-compatible), hardforks break compatibility with older node software - nodes that don’t upgrade cannot validate new blocks. Key characteristics:
  • Backwards-incompatible - Old nodes reject new blocks
  • Network-wide activation - All validators must upgrade
  • Block number or timestamp - Activation at specific height or time
  • EIP bundling - Multiple Ethereum Improvement Proposals (EIPs) per fork

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Why Hardforks Are Needed

Hardforks enable protocol evolution without fragmenting the network:
  1. Bug fixes - Fix security vulnerabilities (DAO fork, Spurious Dragon)
  2. Performance improvements - Optimize gas costs, add new opcodes
  3. Feature additions - New transaction types (EIP-1559, EIP-4844), consensus changes (The Merge)
  4. Security hardening - Remove exploitable edge cases, strengthen validation

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Timeline of Major Hardforks

import { Hardfork } from 'tevm';

// Early era (2015-2017)
Hardfork.FRONTIER          // July 2015 - Genesis block
Hardfork.HOMESTEAD         // March 2016 - Gas cost adjustments
Hardfork.BYZANTIUM         // October 2017 - EIP-140 REVERT, EIP-211 RETURNDATASIZE

// Stability era (2019)
Hardfork.CONSTANTINOPLE    // February 2019 - EIP-1283 SSTORE gas
Hardfork.ISTANBUL          // December 2019 - EIP-1344 CHAINID, EIP-2200 gas

// Modern era (2021-2024)
Hardfork.BERLIN            // April 2021 - EIP-2929 access list gas costs
Hardfork.LONDON            // August 2021 - EIP-1559 base fee mechanism
Hardfork.MERGE             // September 2022 - Proof of Stake consensus
Hardfork.SHANGHAI          // April 2023 - EIP-3855 PUSH0 opcode
Hardfork.CANCUN            // March 2024 - EIP-4844 blob transactions

// Compare versions
console.log(Hardfork.compare(Hardfork.LONDON, Hardfork.CANCUN)); // -1 (London is earlier)

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Feature Activation

Hardforks activate at predetermined block numbers (pre-Merge) or timestamps (post-Merge): 
import { Hardfork } from 'tevm';

// Block-based activation (pre-Merge)
const berlinBlock = 12244000;   // Mainnet Berlin activation
const londonBlock = 12965000;   // Mainnet London activation

// Timestamp-based activation (post-Merge)
const shanghaiTimestamp = 1681338455;  // April 13, 2023
const cancunTimestamp = 1710338135;    // March 13, 2024

// Version comparison for feature gating
import { InvalidFormatError } from 'tevm/errors'

function getGasCostMultiplier(hardfork: string, blockNumber: number): number {
  const fork = Hardfork(hardfork);
  if (!fork) {
    throw new InvalidFormatError("Invalid hardfork", {
      value: hardfork,
      expected: "Valid hardfork name",
      code: "INVALID_HARDFORK"
    })
  }

  // Berlin changed access list gas pricing
  if (Hardfork.isAtLeast(fork, Hardfork.BERLIN)) {
    return 2600; // EIP-2929 cold SLOAD cost
  }

  return 800; // Pre-Berlin SLOAD cost
}

console.log(getGasCostMultiplier("istanbul", 11000000)); // 800
console.log(getGasCostMultiplier("berlin", 13000000));   // 2600

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Checking Hardfork Support

Use version comparison to gate features based on hardfork: 
import { Hardfork } from 'tevm';
import { InvalidFormatError, ValidationError } from 'tevm/errors'

function validateTransaction(tx: any, networkHardfork: string) {
  const fork = Hardfork(networkHardfork);
  if (!fork) {
    throw new InvalidFormatError("Invalid hardfork", {
      value: networkHardfork,
      expected: "Valid hardfork name",
      code: "INVALID_HARDFORK"
    })
  }

  // Type 0/1: All hardforks
  if (tx.type === 0 || tx.type === 1) {
    return true;
  }

  // Type 2 (EIP-1559): London+
  if (tx.type === 2) {
    if (!Hardfork.hasEIP1559(fork)) {
      throw new ValidationError("EIP-1559 transactions require London hardfork or later", {
        value: tx.type,
        expected: "London or later hardfork",
        code: "UNSUPPORTED_TX_TYPE"
      })
    }
    return true;
  }

  // Type 3 (EIP-4844): Cancun+
  if (tx.type === 3) {
    if (!Hardfork.hasEIP4844(fork)) {
      throw new ValidationError("Blob transactions require Cancun hardfork or later", {
        value: tx.type,
        expected: "Cancun or later hardfork",
        code: "UNSUPPORTED_TX_TYPE"
      })
    }
    return true;
  }

  throw new ValidationError("Unknown transaction type", {
    value: tx.type,
    expected: "Transaction type 0, 1, 2, or 3",
    code: "UNKNOWN_TX_TYPE"
  })
}

// Valid - Cancun supports all transaction types
validateTransaction({ type: 3 }, "cancun");

// Throws - London doesn't support blob transactions
try {
  validateTransaction({ type: 3 }, "london");
} catch (e) {
  if (e instanceof ValidationError) {
    console.error(e.name);    // "ValidationError"
    console.error(e.message); // "Blob transactions require Cancun hardfork or later"
    console.error(e.code);    // "UNSUPPORTED_TX_TYPE"
  }
}

​
Comparing Hardfork Features

Compare feature availability across hardforks: 
import { Hardfork } from 'tevm';
import { InvalidFormatError } from 'tevm/errors'

function compareFeatures(forkA: string, forkB: string) {
  const a = Hardfork(forkA);
  const b = Hardfork(forkB);
  if (!a || !b) {
    throw new InvalidFormatError("Invalid hardfork", {
      value: !a ? forkA : forkB,
      expected: "Valid hardfork name",
      code: "INVALID_HARDFORK"
    })
  }

  return {
    comparison: Hardfork.compare(a, b),
    features: {
      eip1559: {
        [forkA]: Hardfork.hasEIP1559(a),
        [forkB]: Hardfork.hasEIP1559(b)
      },
      push0: {
        [forkA]: Hardfork.hasEIP3855(a),
        [forkB]: Hardfork.hasEIP3855(b)
      },
      blobs: {
        [forkA]: Hardfork.hasEIP4844(a),
        [forkB]: Hardfork.hasEIP4844(b)
      },
      transientStorage: {
        [forkA]: Hardfork.hasEIP1153(a),
        [forkB]: Hardfork.hasEIP1153(b)
      }
    }
  };
}

const comparison = compareFeatures("london", "cancun");
console.log(comparison);
// {
//   comparison: -1,  // London < Cancun
//   features: {
//     eip1559: { london: true, cancun: true },
//     push0: { london: false, cancun: true },
//     blobs: { london: false, cancun: true },
//     transientStorage: { london: false, cancun: true }
//   }
// }

​
Hardfork-Dependent Features

Common features tied to specific hardforks:

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EIP-1559 Base Fee (London)

import { Hardfork } from 'tevm';
import { InvalidFormatError, ValidationError } from 'tevm/errors'

function calculateMaxFee(hardfork: string, baseFee: bigint, priorityFee: bigint): bigint {
  const fork = Hardfork(hardfork);
  if (!fork) {
    throw new InvalidFormatError("Invalid hardfork", {
      value: hardfork,
      expected: "Valid hardfork name",
      code: "INVALID_HARDFORK"
    })
  }

  if (Hardfork.hasEIP1559(fork)) {
    // London+: maxFeePerGas = baseFee + maxPriorityFeePerGas
    return baseFee + priorityFee;
  } else {
    // Pre-London: Only gas price (no base fee)
    throw new ValidationError("EIP-1559 not supported before London", {
      value: hardfork,
      expected: "London or later hardfork",
      code: "EIP1559_NOT_SUPPORTED"
    })
  }
}

console.log(calculateMaxFee("london", 20n, 2n));   // 22n
console.log(calculateMaxFee("cancun", 15n, 3n));   // 18n

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EIP-3855 PUSH0 Opcode (Shanghai)

import { Hardfork } from 'tevm';
import { InvalidFormatError } from 'tevm/errors'

function supportsOptimizedBytecode(hardfork: string): boolean {
  const fork = Hardfork(hardfork);
  if (!fork) {
    throw new InvalidFormatError("Invalid hardfork", {
      value: hardfork,
      expected: "Valid hardfork name",
      code: "INVALID_HARDFORK"
    })
  }

  // PUSH0 (0x5f) allows cheaper zero-pushing
  return Hardfork.hasEIP3855(fork);
}

console.log(supportsOptimizedBytecode("london"));   // false
console.log(supportsOptimizedBytecode("shanghai")); // true
console.log(supportsOptimizedBytecode("cancun"));   // true

​
EIP-4844 Blob Transactions (Cancun)

import { Hardfork } from 'tevm';
import { InvalidFormatError } from 'tevm/errors'

function supportsBlobTransactions(hardfork: string): boolean {
  const fork = Hardfork(hardfork);
  if (!fork) {
    throw new InvalidFormatError("Invalid hardfork", {
      value: hardfork,
      expected: "Valid hardfork name",
      code: "INVALID_HARDFORK"
    })
  }

  // EIP-4844 blob transactions for L2 data availability
  return Hardfork.hasEIP4844(fork);
}

console.log(supportsBlobTransactions("shanghai")); // false
console.log(supportsBlobTransactions("cancun"));   // true

​
EIP-1153 Transient Storage (Cancun)

import { Hardfork } from 'tevm';
import { InvalidFormatError } from 'tevm/errors'

function supportsTransientStorage(hardfork: string): boolean {
  const fork = Hardfork(hardfork);
  if (!fork) {
    throw new InvalidFormatError("Invalid hardfork", {
      value: hardfork,
      expected: "Valid hardfork name",
      code: "INVALID_HARDFORK"
    })
  }

  // TLOAD/TSTORE opcodes for cheaper temporary storage
  return Hardfork.hasEIP1153(fork);
}

console.log(supportsTransientStorage("london"));  // false
console.log(supportsTransientStorage("cancun"));  // true

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Resources

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Next Steps