AI agents call query_balance to retrieve information from Substrate MCP Server without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This tool queries account balance information from the Substrate blockchain without modifying, executing arbitrary code, or destroying any data. It is a read-only operation that retrieves existing state information.
From the tool's definition Tool name 'query_balance' and description 'Fetch the balance of an account' both indicate a retrieval operation with no side effects.
Documented attack patterns abuse exactly the kind of access query_balance gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Substrate MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for query_balance:
{
"version": "1",
"default": "deny",
"tools": {
"query_balance": {}
}
}query_balance is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.
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Fetch the balance of an account. It is categorised as a Read tool in the Substrate MCP Server MCP Server, which means it retrieves data without modifying state.
Register the Substrate MCP Server MCP server in PolicyLayer and add a rule for query_balance: 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 Substrate MCP Server. Nothing to install.
query_balance 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 query_balance 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 query_balance. 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.
query_balance is provided by the Substrate MCP Server MCP server (thomasmarches/substrate-mcp-rs). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Substrate MCP 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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13 Substrate MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.