Fx cross rate compute

fx-cross-rate-compute · version 1.0.0 · Money & pricing · free, no key needed

Derive a cross exchange rate and its inverse from two quoted rates that share one currency, at a stated scale.

Use when you need to: cross rate from two exchange rates · derive GBP/JPY from USD/JPY and GBP/USD · triangulate currency rate via euro.

Decide before calling

Read the versioned contract and the supported scope below. Reuse fx-cross-rate-compute@1.0.0 when your input, required output and limits match it. Choose another approach for an unsupported operation.

Explain the choice

"I can use fx-cross-rate-compute@1.0.0 for cross rate from two exchange rates. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

Supported

• cross rate from two exchange rates
• derive GBP/JPY from USD/JPY and GBP/USD
• triangulate currency rate via euro
• DEM to FRF from euro conversion rates
• inverse exchange rate rounded to 6 decimals
• what is the EUR/GBP rate if I know USD/EUR and USD/GBP
• synthetic cross rate calculation
• derive the rate between two currencies through a common pivot

Not supported

• fetching live or historical exchange rates
• converting an amount of money between currencies
• chains of more than two quotes
• bid/ask spreads or fees

Behavior

• Quote convention: {base: X, quote: Y, rate: r} means 1 X = r Y (EUR/USD 1.0843: 1 EUR = 1.0843 USD). Currency codes are opaque strings matching ^[A-Z0-9]{3,5}$ (upper-case only, no case folding, no ISO lookup), so historic and non-ISO codes (DEM, FRF, USDT) are accepted. rates holds exactly 2 quotes; each rate is a positive decimal (zero is invalid_input, reason rate_zero); base equal to quote is invalid_input (reason same_currency).
• The two quotes must share exactly one currency, reported as via; no shared currency or the same pair twice is invalid_input with reason no_common_currency. The target must be exactly the two non-shared currencies in either order, else invalid_input with reason target_mismatch.
• Computed on exact rationals. If p(X) is the amount of the shared currency worth 1 X (r for a quote X/shared, 1/r for shared/X), then for target {base T, quote U} rate = p(T)/p(U) and inverse_rate = p(U)/p(T). Example: EUR/DEM 1.95583 and EUR/FRF 6.55957 give DEM/FRF 3.353855 at scale 6.
• rate and inverse_rate are each rounded once from the exact rational at scale (0..20) with the given mode; the inverse is not derived from the rounded rate. Both are fixed-scale strings and may round to zero. exact is true iff neither needed rounding.
• The ten rounding modes: half_even, half_up (ties away from zero), half_down, half_ceiling, half_floor, up, down, ceiling, floor, and unnecessary (if either value is inexact the call is not_computable, reason rounding_necessary).
• A rate string over 62 characters is limit_exceeded, checked before the syntax; more than 60 digits or 40 fraction digits is invalid_input. base and quote in the output echo the target; the tool holds no rate data and has no clock or network.
• Disclaimer: arithmetic calculation only; not financial, tax, legal, or investment advice.

Input

• rates (array of object, required): min items 2; max items 2; Exactly two quotes that share exactly one currency. Each says: 1 base = rate quote.
• target (object, required): The requested pair: 1 base = ? quote. Must be the two currencies that are NOT shared by the given quotes (either order).
• scale (integer, required): min 0; max 20; Fraction digits of the output rates.
• rounding (one of "half_even", "half_up", "half_down", "half_ceiling", "half_floor", "up", "down", "ceiling", "floor", "unnecessary", required)

Output

• base (string, required): min length 3; max length 5; pattern ^[A-Z0-9]{3,5}$
• quote (string, required): min length 3; max length 5; pattern ^[A-Z0-9]{3,5}$
• rate (string, required): min length 1; max length 400; pattern ^(0|[1-9][0-9]*)(\.[0-9]+)?$; Units of quote per 1 base, fixed scale.
• inverse_rate (string, required): min length 1; max length 400; pattern ^(0|[1-9][0-9]*)(\.[0-9]+)?$; Units of base per 1 quote, computed from the exact rational (not by inverting the rounded rate), fixed scale.
• via (string, required): min length 3; max length 5; pattern ^[A-Z0-9]{3,5}$; The shared (pivot) currency.
• exact (boolean, required): true iff neither rate needed rounding.

Limits

• max decimal chars: 62
• max decimal digits: 60
• max fraction digits: 40
• max scale: 20
• quotes: 2

Example

Request input:

{
  "rates": [
    {
      "base": "EUR",
      "quote": "DEM",
      "rate": "1.95583"
    },
    {
      "base": "EUR",
      "quote": "FRF",
      "rate": "6.55957"
    }
  ],
  "target": {
    "base": "DEM",
    "quote": "FRF"
  },
  "scale": 6,
  "rounding": "half_even"
}

Response:

{
  "result": {
    "base": "DEM",
    "quote": "FRF",
    "rate": "3.353855",
    "inverse_rate": "0.298164",
    "via": "EUR",
    "exact": false
  }
}

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

{
  "id": "fx-cross-rate-compute",
  "version": "1.0.0",
  "input": {
    "rates": [
      {
        "base": "EUR",
        "quote": "DEM",
        "rate": "1.95583"
      },
      {
        "base": "EUR",
        "quote": "FRF",
        "rate": "6.55957"
      }
    ],
    "target": {
      "base": "DEM",
      "quote": "FRF"
    },
    "scale": 6,
    "rounding": "half_even"
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/fx-cross-rate-compute/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"rates":[{"base":"EUR","quote":"DEM","rate":"1.95583"},{"base":"EUR","quote":"FRF","rate":"6.55957"}],"target":{"base":"DEM","quote":"FRF"},"scale":6,"rounding":"half_even"}'

The machine-readable contract is at /v1/tools/fx-cross-rate-compute/versions/1.0.0.

CLI

node cli.mjs run fx-cross-rate-compute 1.0.0 --input input.json --base-url https://computefirst.net

Get the client at /clients/cli/.

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