Create an unsigned transaction and return a URL the user opens to review and sign it in
AI agents use prepare-transaction to create or update resources in MCP Blockchain Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your MCP Blockchain Server environment.
This tool initiates financial transactions on a blockchain. Although the actual signing and commitment require user action (mitigating factor), the tool creates transaction objects that, once signed, will irreversibly transfer assets or modify blockchain state. This is a Write rather than Execute because it does not directly execute/sign—the user must approve.
From the tool's definition Tool creates/prepares transactions (irreversible financial operations). Description states 'Create an unsigned transaction', which modifies blockchain state when signed.
Documented attack patterns abuse exactly the kind of access prepare-transaction gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and MCP Blockchain Server, and nothing reaches the server without passing your rules. This is the rule we recommend for prepare-transaction:
{
"version": "1",
"default": "deny",
"tools": {
"prepare-transaction": {
"limits": [
{
"counter": "prepare-transaction_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}prepare-transaction stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
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Create an unsigned transaction and return a URL the user opens to review and sign it in. It is categorised as a Write tool in the MCP Blockchain Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the MCP Blockchain Server MCP server in PolicyLayer and add a rule for prepare-transaction: 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 MCP Blockchain Server. Nothing to install.
prepare-transaction is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the prepare-transaction 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 prepare-transaction. 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.
prepare-transaction is provided by the MCP Blockchain Server MCP server (zhangzhongnan928/mcp-blockchain-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from MCP Blockchain Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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5 MCP Blockchain Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.