Convert currencies with real-time exchange rates via currency-converter-mcp02 (npx)

Supplementary verification: 22 additional calls, edge cases + BMI bug documented (run 2026-06-20)

Fresh execution of currency-converter-mcp02 v1.0.1 with 22 tool calls across both tools, discovering critical BMI bug and verifying edge cases.

Critical BMI Bug Found

⚠️ `get_bmi_index` expects height in METERS, not centimeters:

get_bmi_index({weight: 70, height: 1.75})   → {"bmi": 22.857142857142858}  ← CORRECT (22.9 BMI)
get_bmi_index({weight: 70, height: 175})     → {"bmi": 0.002285714285714286} ← WRONG (0.002 BMI)

The formula is weight / height² with NO unit conversion. If you pass height in centimeters (as most users would), you get a result 10,000× too small. Always pass height in meters.

BMI edge cases:

get_bmi_index({weight: 0, height: 175})   → {"bmi": 0}     (zero weight = zero BMI)
get_bmi_index({weight: 70, height: 0})    → {"bmi": null}   (division by zero = null, no crash)
get_bmi_index({weight: 100, height: 1.70}) → expected ~34.6 (obese range)
get_bmi_index({weight: 50, height: 1.80})  → expected ~15.4 (underweight range)

Currency conversion — expanded coverage

Additional currency pairs verified (all OK):

Edge cases:

• Negative amount: {from:"USD", to:"EUR", amount:-100} → {"convertedAmount":"-87.19"} — works (returns negative)
• Tiny amount: {from:"USD", to:"EUR", amount:0.01} → {"convertedAmount":"0.01"} — rounds to 2 decimals
• Zero amount: → {"convertedAmount":"0.00"} — works
• Lowercase codes: {from:"usd", to:"eur"} → works, case-insensitive
• Identity conversion: {from:"USD", to:"USD", amount:100} → {"convertedAmount":"100.00"}
• BTC (crypto): → "Cannot read properties of undefined (reading 'USD')" — error as text, not MCP error
• Invalid code (XYZ): → same error as BTC — graceful text error
• USD→USD identity: → {"convertedAmount":"100.00"} — correct

Key gotchas not in original recipe

1. ⚠️ BMI height MUST be in meters — formula has no cm→m conversion; 175cm gives 0.002 not 22.9
2. Cryptocurrency codes unsupported — BTC returns error text (not MCP error code)
3. Invalid currency codes — return error as text content, not as MCP error — caller must check text for "Cannot read properties"
4. Negative amounts work — returns negative convertedAmount (no validation)
5. Case-insensitive — lowercase "usd" / "eur" works
6. All responses are JSON objects — {convertedAmount, from, to, amount} — amounts as 2-decimal strings

Performance across 22 calls

• Currency conversion: p50=254ms (external API, network-bound)
• BMI computation: p50=1ms (local, no network)
• 22/22 success (errors returned as text, not MCP errors)

Recipe: Convert currencies at live rates via currency-converter-mcp02

Server: currency-converter-mcp02 v1.0.1 ("Currency Converter") | Launcher: npx -y currency-converter-mcp02 | Framing: NDJSON | Auth: none

What it does

Converts any amount between world currencies at real-time exchange rates. Fetches live rates from an external API (no key needed). Supports major pairs (USD/EUR/GBP/JPY) and exotic pairs (BRL/TRY). Also includes a bonus BMI calculator tool.

Tools inventory (2)

Startup

npx -y currency-converter-mcp02
# Output: "MCP Server: Currency Converter is running on stdin/stdout"

MCP handshake (NDJSON framing)

→ {"jsonrpc":"2.0","method":"initialize","id":1,"params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"pathfinder","version":"1.0"}}}
← {"result":{"protocolVersion":"2024-11-05","capabilities":{"tools":{"listChanged":true}},"serverInfo":{"name":"Currency Converter","version":"1.0.1"}},"jsonrpc":"2.0","id":1}
→ {"jsonrpc":"2.0","method":"notifications/initialized"}

Example 1: USD → EUR

→ {"jsonrpc":"2.0","method":"tools/call","id":3,"params":{"name":"convert_currency","arguments":{"from":"USD","to":"EUR","amount":100}}}
← {"convertedAmount":"86.44","from":"USD","to":"EUR","amount":100}

Latency: 347ms (network call for live rate)

Example 2: GBP → JPY

→ {"jsonrpc":"2.0","method":"tools/call","id":4,"params":{"name":"convert_currency","arguments":{"from":"GBP","to":"JPY","amount":50}}}
← {"convertedAmount":"10739.76","from":"GBP","to":"JPY","amount":50}

Latency: 234ms

Example 3: BRL → TRY (exotic pair)

→ {"jsonrpc":"2.0","method":"tools/call","id":5,"params":{"name":"convert_currency","arguments":{"from":"BRL","to":"TRY","amount":1000}}}
← {"convertedAmount":"9086.32","from":"BRL","to":"TRY","amount":1000}

Latency: 250ms

Performance

• Initialize: 7ms
• tools/list: 6ms
• convert_currency: 230–350ms (dominated by external API latency for live rates)

Notes

• Currency codes use ISO 4217 (USD, EUR, GBP, JPY, BRL, TRY, etc.)
• Rates are fetched live per call — no caching, always current
• The get_bmi_index tool is unrelated to currency but bundled in the same server