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The simulation builder splits its inputs across three surfaces:
  • Session defaults in the strip at the top apply to every step: Network, Block, and the default From address. Click Configure to edit them.
  • Step fields define the call itself: sender, target contract, function, and arguments.
  • Cheatcodes below the step fields modify the execution environment immediately before the step runs.
For the overall builder layout (session strip, bundle rail, focused step view), see the Simulator UI overview.

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Session defaults

Click Configure in the session strip to open the defaults panel. Values set here apply to every new step in the bundle.
Configure panel showing Network details with Mainnet, a pinned block number, Tx Index set to At start of block, Gas, Gas Price, Value, and the Send transaction from address

The Configure panel: network, block number, transaction index, gas defaults, and the default From address.

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Pending vs historical block

Leave Block number empty to simulate on the pending block. Tenderly executes the bundle against the latest available network state, as if it were included in the next block. Type a block number to simulate on a historical block. The bundle then executes against the chain state as it was at that block, on any block going back to the start of the network. With a block pinned, Tx Index controls where in the block the simulation executes:
Configure panel with block number 25250000 pinned and the Tx Index menu open showing At start of block, At tx, and At end of block options

A pinned block unlocks the Tx Index control with At start of block, At tx, and At end of block options.

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From addresses

Simulations need no private keys or signatures, so any address can act as the sender. The From dropdown offers:
  • The zero address (0x0000…0000), the default sender.
  • Your project wallets on the selected network, grouped under “Selected network”. Changing the network changes which wallets appear.
  • Any address you paste or search in the input at the top of the dropdown.
Send transaction from dropdown open, showing a search input, the zero address, and two project wallets grouped under Selected network

The From dropdown: search or paste an address, or pick the zero address or a project wallet on the selected network.

The address set in the Configure panel is the default for new steps. Each step also has its own From field in the step editor, so different steps in a bundle can use different senders.

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Step fields

Each step is built in Decoded or Raw mode, toggled at the top of the step editor.
Step editor showing the Dai contract selected, the transfer function with dst address and wad uint256 inputs, and the Override gas & value, Fund address, and Increase block cheatcodes

A step in Decoded mode: From, Contract (with Edit source and Edit ABI), Function, one typed input per argument, and the cheatcode sections below.

Decoded mode builds the call from the contract’s ABI: Raw mode takes a target address and a 0x-prefixed hex payload directly. Use it when you already have calldata from an existing transaction or a manual encode.

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Cheatcodes

Each step carries six cheatcode sections. They run immediately before the step executes and are scoped to that step only.

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Override gas & value

Set a custom Gas limit, Gas price, and Value for this step, overriding the session defaults.

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Fund address

Credit any address with token balances before the step runs. Each entry takes a Target address, a token (searchable by symbol, name, or address), and an amount. Click Add more tokens to fund multiple tokens, and add further entries for other addresses. To set an address’s native balance instead, use the custom balance switch in State overrides.

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Increase block

Advance the block number by an offset before the step executes. The section shows the current block and the resulting block. Contracts that read block.number see the increased value.

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Override timestamp

Set the block timestamp the step executes with, in one of two modes:
  • Increase time: advance the clock by an amount and unit (seconds, minutes, and so on).
  • Set UNIX timestamp: jump to an absolute timestamp.
The section previews the resulting date and time. Contracts that read block.timestamp see the overridden value.

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State overrides

Override contract state before the step runs. Each override targets one contract (defaulting to the step’s target) and supports:
  • Use custom balance: set the contract’s native balance.
  • Storage variables: key and value pairs for individual storage slots, with Add storage override for more slots.
Click Add State Override to override additional contracts in the same step. For a walkthrough including storage slot calculation, see Simulation with State Overrides.

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Optional Access Lists

Attach an EIP-2930 access list to the step. Announcing the contracts and storage keys the transaction touches can lower its gas cost on calls spanning many contracts. Add addresses one by one or import the list as JSON.

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Saving a starting point

Simulations are stateless: each run starts from the configured block. To keep a modified state (overrides, funded accounts, advanced timestamps) as a persistent environment you can keep transacting against, use Virtual Environments.