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fix(mcp_impl): validate JSON amounts as exact integers before big.Int conversion #143

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fix(mcp_impl): validate JSON amounts as exact integers before big.Int conversion #143

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20 changes: 17 additions & 3 deletions internal/mcp_impl/handlers.go
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,11 @@ import (
"github.com/mark3labs/mcp-go/server"
)

// maxExactJSONInt is the largest integer that float64 can represent
// without loss (2^53 - 1). JSON-decoded amounts past this magnitude have
// already lost precision before reaching the handler, so we reject them.
const maxExactJSONInt = float64(9_007_199_254_740_991)

func parseBalancesJson(balancesRaw any) (interpreter.Balances, *mcp.CallToolResult) {
balances, ok := balancesRaw.(map[string]any)
if !ok {
Expand All @@ -33,11 +38,20 @@ func parseBalancesJson(balancesRaw any) (interpreter.Balances, *mcp.CallToolResu
for asset, amountRaw := range assets {
amount, ok := amountRaw.(float64)
if !ok {
return interpreter.Balances{}, mcp.NewToolResultError(fmt.Sprintf("Expected float for amount: %v", amountRaw))
return interpreter.Balances{}, mcp.NewToolResultError(fmt.Sprintf("Expected number for amount on %s/%s, got: <%#v>", account, asset, amountRaw))
}

// JSON numbers arrive here as float64. Reject anything that
// cannot be losslessly represented as an integer in float64
// precision: fractional values and magnitudes past 2^53 - 1.
// Silent truncation / rounding on a balance is not acceptable
// for a financial DSL — the caller should switch to a safer
// encoding when they need values outside the safe range.
if amount < -maxExactJSONInt || amount > maxExactJSONInt || amount != float64(int64(amount)) {
return interpreter.Balances{}, mcp.NewToolResultError(fmt.Sprintf("amount for %s/%s must be an exact integer in [-(2^53-1), 2^53-1], got: %v", account, asset, amount))
}

n, _ := big.NewFloat(amount).Int(new(big.Int))
iBalances[account][asset] = n
iBalances[account][asset] = big.NewInt(int64(amount))
}
}
return iBalances, nil
Expand Down