Simulates an unsigned EVM transaction against live chain state using eth_call and eth_estimateGas, returning whether the call would succeed or revert (with revert reason), estimated gas units, approximate gas cost in USD, and native token balance changes from the transaction value. Use this to in...
Part of the Syenite server.
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AI agents call tx.simulate to retrieve information from Syenite without modifying any data. This is common in research, monitoring, and reporting workflows where the agent needs context before taking action. Because read operations don't change state, they are generally safe to allow without restrictions -- but you may still want rate limits to control API costs.
Even though tx.simulate only reads data, uncontrolled read access can leak sensitive information or rack up API costs. An agent caught in a retry loop could make thousands of calls per minute. A rate limit gives you a safety net without blocking legitimate use.
Read-only tools are safe to allow by default. No rate limit needed unless you want to control costs.
{
"version": "1",
"default": "deny",
"tools": {
"tx.simulate": {}
}
}See the full Syenite policy for all 46 tools.
These attack patterns abuse exactly the kind of access tx.simulate gives an agent. Each links to the full case and the policy that stops it:
Other read tools across the catalogue. The same approach applies to each: allow, with a rate cap to control cost.
Simulates an unsigned EVM transaction against live chain state using eth_call and eth_estimateGas, returning whether the call would succeed or revert (with revert reason), estimated gas units, approximate gas cost in USD, and native token balance changes from the transaction value. Use this to independently verify what a transaction will do before signing — particularly useful when the transaction was constructed by a third party. Requires transaction.to, transaction.from, transaction.data, and optionally transaction.value and transaction.chainId; pass chain or chainId to target a specific network (ethereum, arbitrum, base, bsc supported). Does not submit the transaction; ERC-20 balance diffs require trace-level simulation (not available on public RPCs) and are noted but not fully computed.. It is categorised as a Read tool in the Syenite MCP Server, which means it retrieves data without modifying state.
Register the Syenite MCP server in PolicyLayer and add a rule for tx.simulate: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Syenite. Nothing to install.
tx.simulate is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the tx.simulate rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for tx.simulate. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
tx.simulate is provided by the Syenite MCP server (@syenite/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 46 Syenite tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
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