simulateTransaction

The simulateTransaction method simulates sending a transaction without actually submitting it to the cluster. This is an invaluable testing and debugging tool that allows developers to see exactly what would happen if a transaction were executed, including all state changes, logs, and potential errors, without spending any fees or modifying on-chain state.

Transaction simulation executes the transaction against the current blockchain state in a sandboxed environment. It processes all instructions, invokes programs, modifies accounts, and generates logs exactly as would happen in real execution, but then discards all changes. This enables developers to preview transaction results, debug complex program interactions, estimate compute units, and catch errors before spending real SOL on failed transactions.

This method is essential for wallet UIs showing transaction previews, debugging program behavior, validating transaction construction before submission, estimating fees and compute units, and testing complex multi-instruction transactions. It significantly improves development speed by catching errors early and provides transparency about what transactions will do before users approve them.

Parameters

parameter	type	description
transaction	string	Transaction as an encoded string. The transaction must have a valid blockhash but does not need valid signatures
config	object	Optional configuration object
config.encoding	string	Encoding for the transaction: 'base58' (slow) or 'base64' (default: 'base58')
config.commitment	string	Commitment level for simulation: 'processed', 'confirmed', or 'finalized' (default: 'finalized')
config.sigVerify	boolean	If true, verify transaction signatures (default: false)
config.replaceRecentBlockhash	boolean	If true, replace the transaction's recent blockhash with the latest blockhash (default: false)
config.accounts	object	Optional. Accounts configuration with encoding and addresses to return
config.minContextSlot	number	Optional. Minimum slot that the request can be evaluated at

Return Object

field	type	description
context	object	RPC response context with slot information
context.slot	number	The slot at which simulation was performed
value	object	Simulation result object
value.err	object\|null	Error if transaction would fail, null if successful
value.logs	array	Array of log messages generated during simulation
value.accounts	array\|null	Array of accounts with post-transaction state if requested in config
value.unitsConsumed	number	Number of compute units consumed by the transaction
value.returnData	object\|null	Return data from the transaction's instructions, if any

Request Example

Request

Response Example

Response

Tip: Set replaceRecentBlockhash: true to avoid blockhash expiration issues during testing. The transaction doesn't need valid signatures unless sigVerify: true. Check logs array for detailed program output - invaluable for debugging. unitsConsumed helps estimate compute budget needs. Simulation uses current state, so results may differ if state changes before actual submission.