Dry Run

A dry run (or simulation) is a critical development and testing technique where a blockchain transaction is executed locally in a controlled environment, such as a developer's node or a test network, without being broadcast to the main network. This process uses the same execution logic as a real transaction—checking the smart contract code, state changes, and gas consumption—but does not result in any permanent changes to the blockchain state or require the payment of gas fees. It is the primary method for validating the logic and safety of a transaction before committing real funds.

The primary use cases for a dry run are pre-transaction validation and debugging. Developers use it to estimate the exact gas cost of a complex operation, verify that a transaction will succeed under current network conditions, and check for potential errors like reverts or out-of-gas exceptions. Tools like eth_call on Ethereum or similar RPC methods on other chains are specifically designed for this purpose. For users, wallet interfaces often perform a background dry run to provide an accurate gas estimate and warn if a transaction is likely to fail, preventing wasted fees.

Executing a dry run requires specifying all standard transaction parameters—such as the sender address, recipient, calldata, and a simulated block state—but the sender's signature is often not required. The simulation returns the potential outcome: the return data from the contract call, the precise gas used, and any logs emitted. This allows for detailed analysis. A key limitation is the "simulation gap," where the state of the network at the exact moment of the real transaction may differ from the simulated state, potentially leading to different results, though this risk is minimal for most well-designed queries.

Beyond basic validation, dry runs are foundational for advanced blockchain interactions. They enable gas optimization by allowing developers to test different contract paths to find the most efficient one. In DeFi, they are used to simulate complex multi-step transactions, like swaps or loans, to preview slippage or liquidation risks. Security auditors rely heavily on dry runs to fuzz-test contracts with myriad inputs, and meta-transaction systems use them to validate signed messages before relaying. This makes the dry run an indispensable tool for safe and efficient blockchain development and interaction.

A dry run (also known as a simulation or preflight check) is a critical development and debugging tool in blockchain ecosystems. It involves executing a transaction's code—including smart contract calls, state changes, and signature verification—locally or on a test node, as if it were being processed by the network's consensus mechanism. The key distinction is that a dry run does not sign the transaction with a private key, does not broadcast it, and therefore has zero on-chain effect or cost. This allows developers to inspect the exact outcome, including any potential errors, gas consumption, or state modifications, before committing real funds or making permanent changes.

The technical process typically involves submitting a transaction object to a node's RPC endpoint (e.g., eth_call on Ethereum, simulateTransaction on Solana) with a specified execution context. This context includes the sender address, the current block state, and sometimes a specific block height. The node's execution environment processes the transaction against this frozen state and returns a detailed result. This result reveals whether the transaction would succeed or revert, the precise amount of gas or compute units it would consume, any logs it would emit, and the final state changes to storage. This is invaluable for estimating fees and catching logic errors in require(), assert(), or revert() statements before deployment.

Beyond basic validation, dry runs are essential for complex operations like gas estimation and bundle simulation. Wallets and dApp interfaces use them to provide users with accurate fee quotes. In advanced trading or DeFi scenarios, they are used to simulate the outcome of multi-step transactions, such as arbitrage routes or flash loans, to verify profitability and safety. On networks like Solana, the simulation response includes a list of all accounts that would be touched, which is used to build and sign the final transaction. This pre-execution analysis prevents failed transactions, saving users from paying fees for reverted operations and protecting them from unexpected outcomes.

Implementing a dry run requires careful configuration to mirror mainnet conditions. The simulated state must be recent and accurate, often requiring access to an archive node. For transactions dependent on precise timing (e.g., those involving oracle prices or time-locks), the simulation must be run against the correct block timestamp. Furthermore, some edge cases, like certain MEV-related transactions or those relying on in-block context, may behave differently in simulation versus actual execution. Therefore, while dry runs are highly reliable for logic and gas testing, they are a simulation and cannot guarantee an identical outcome in a live, contested block space environment.

In blockchain development, a dry run (also known as a simulation or preflight check) is the process of executing a transaction's logic locally against a node's current state. This is done without signing the transaction or submitting it to the network's mempool. The primary outputs of a dry run are the predicted gas cost, the final state changes, and any potential execution errors or reverts. This allows developers to answer critical questions before committing real assets: Will the transaction succeed? How much will it cost? What will the resulting account balances be?

The technical mechanism involves the node's execution client (e.g., Geth, Erigon) processing the transaction as if it were the next block, using the same EVM or VM runtime but in an isolated, read-only context. Key inputs for a dry run include the from and to addresses, the exact calldata, and the proposed gasLimit. Advanced tools like Tenderly and Foundry's cast command provide rich dry run capabilities, offering detailed traces that show every opcode step, storage read/write, and internal call, which is invaluable for debugging complex smart contract interactions.

Dry runs are a fundamental practice for security and user experience. They enable gas estimation tools in wallets like MetaMask to provide users with cost predictions, and they are used by dApps to prevent failed transactions by validating user inputs beforehand. In the context of meta-transactions or account abstraction, relayers perform dry runs to ensure they will be reimbursed for gas before sponsoring a transaction. This pre-execution validation is a critical line of defense against costly on-chain errors and failed transactions that waste gas.