Analyze historic best/worst FX performance for a currency pair over a Guard's duration. Uses Bank of England historic rates. Returns a 'performances' array with 6 entries — BEST and WORST for each of three lookback periods (5, 10, 25 years). Each entry contains: type, lookbackYears, startDate/end...
Part of the CurrencyGuard Guard Pricing server.
Free to start. No card required.
AI agents call get_historic_best_worst to retrieve information from CurrencyGuard Guard Pricing 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 get_historic_best_worst 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": {
"get_historic_best_worst": {}
}
}See the full CurrencyGuard Guard Pricing policy for all 8 tools.
These attack patterns abuse exactly the kind of access get_historic_best_worst 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.
Analyze historic best/worst FX performance for a currency pair over a Guard's duration. Uses Bank of England historic rates. Returns a 'performances' array with 6 entries — BEST and WORST for each of three lookback periods (5, 10, 25 years). Each entry contains: type, lookbackYears, startDate/endDate, startRate/endRate, performancePercentage, startAmount, finalAmount, deltaAmount. For Guard-Pay: negative delta = cost decreased = BEST; positive delta = cost increased = WORST. For Guard-Receive: positive delta = receipt increased = BEST; negative delta = receipt decreased = WORST. Parameters must match the values used in the preceding price_guard call, including payReceive (PAY or RECEIVE).. It is categorised as a Read tool in the CurrencyGuard Guard Pricing MCP Server, which means it retrieves data without modifying state.
Register the CurrencyGuard Guard Pricing MCP server in PolicyLayer and add a rule for get_historic_best_worst: 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 CurrencyGuard Guard Pricing. Nothing to install.
get_historic_best_worst 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 get_historic_best_worst 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 get_historic_best_worst. 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.
get_historic_best_worst is provided by the CurrencyGuard Guard Pricing MCP server (https://api.currencyguard.com/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 8 CurrencyGuard Guard Pricing tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
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