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Simulation

Dry-run the transaction for a LayerHop quote with POST /v1/simulate before the user signs, to catch reverts, low allowances and failing Solana transactions.

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To check whether a LayerHop swap would succeed before the user signs, call POST /v1/simulate with the quoteId. LayerHop builds the same transaction as POST /v1/orders and dry-runs it over RPC (eth_call and eth_estimateGas on EVM, simulateTransaction on Solana) without recording an order or sending anything.

What does POST /v1/simulate return?#

HTTP
POST /v1/simulate
{ "quoteId": "q_…" }
JSON
{
  "quoteId": "q_…",
  "provider": "relay",
  "ok": true,
  "vm": "evm",
  "needsApproval": true,
  "simulated": "tx",
  "gasEstimate": "184211",
  "tx": { "vm": "evm", "chainId": 8453, "to": "0x…", "data": "0x…", "value": "0", "approval": { "...": "..." } }
}
Field Meaning
ok true if the simulated transaction would succeed
vm evm or svm
gasEstimate EVM gas units for what was simulated (decimal string), when the RPC returns one
computeUnits Solana compute units consumed
needsApproval EVM: the sender's allowance is below tx.approval.amount; send the approval first
simulated tx (the swap itself was simulated) or approval (only the approval could be)
error { code, message, logs? } when ok is false
tx The built transaction, the same shape as from POST /v1/orders

A failing simulation is still HTTP 200 with ok: false. Errors such as an unknown or expired quote use the normal error shape.

How is an EVM transaction simulated?#

  1. If the transaction has an approval, LayerHop reads the sender's current allowance.
  2. When the allowance is too low, it sets the allowance through an RPC state override (trying the common ERC-20 storage layouts at once) and confirms it with an allowance() call. The swap is then simulated as if the approval were already mined, and needsApproval is true.
  3. If the RPC does not support state overrides, only the approval is simulated (simulated: "approval").
  4. The swap runs as eth_call from the sender, followed by eth_estimateGas. Reverts are decoded into a readable message where possible.

How is a Solana transaction simulated?#

With simulateTransaction, signature verification off and the recent blockhash replaced, so an unsigned transaction can be checked. On failure, error.logs holds the last program log lines.

What do the error codes mean?#

error.code Meaning What to do
SIMULATION_FAILED The transaction would revert or fail Show the message; get a new quote, or check balance and recipient
RPC_ERROR The simulation could not run (RPC unreachable, rate-limited, method unsupported) Retry, or skip simulation and rely on the wallet's own preview

What does simulation not cover?#

  • It runs against the current chain state on a public RPC, not a fork; the state can change before the user signs.
  • It covers the source-chain transaction only. Delivery on the destination chain is up to the provider.
  • There is no call trace yet.

Simulation is optional. Wallets also preview transactions, and the order flow works without it. From the SDK: await lh.simulate(quoteId). See Simulate the transaction.

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