MCP Server Quickstart

Connect the Digital Calculator MCP server to your AI tool in under 5 minutes. The server is hosted at https://mcp.digitalcalculator.info/mcp (Streamable HTTP, POST, no auth) — there is nothing to install for the recommended path.

What you'll get

13 calculator tools usable directly from claude.ai, Claude Desktop, MCP Inspector, any Streamable HTTP MCP client, or plain curl. Free — no signup, no API key. The hosted endpoint is governed by its hosted-API terms (LICENSE-API.md — rate limits + acceptable use); the npm shim package is MIT-licensed (see the npm package).

Option 1 — Claude Custom Connectors (recommended)

Time: ~1 minute. Works in both claude.ai (web) and Claude Desktop. No install required.

Step 1: Open Connectors settings

In claude.ai or Claude Desktop, open Settings and go to the Connectors section.

Step 2: Add the custom connector

Click "Add custom connector" and paste the server URL:

https://mcp.digitalcalculator.info/mcp

No authentication is required — leave any auth fields blank and save.

Step 3: Confirm the tools are available

Start a new conversation and ask "what MCP tools do you have access to?" — Claude should list the 13 calculator tools (mortgage_monthly_payment, compound_interest_future_value, retirement_401k_projection, and more). If they don't appear, check that the connector is enabled in the conversation's tools menu.

Step 4: Try it

Ask Claude something like:

Claude will invoke mortgage_monthly_payment and return a structured answer with the math + the YMYL disclaimer.

Option 2 — MCP Inspector (for debugging)

Time: ~1 minute.

The official MCP Inspector is a browser-based debugging UI for MCP servers.

npx @modelcontextprotocol/inspector

In the Inspector UI, select the Streamable HTTP transport, enter https://mcp.digitalcalculator.info/mcp, and connect. You'll see:

• All 13 tools listed in the Tools tab
• The YMYL disclaimer (dc://disclaimers/ymyl) and methodology manifest (dc://methodologies/manifest) in the Resources tab
• The MCP protocol handshake logged in the bottom panel

Click any tool, fill the input form, and hit "Call Tool" to invoke it. Useful for verifying tool schemas before integrating in your own client.

Option 3 — npm shim for stdio-only clients

Some MCP clients and agent frameworks only speak MCP over stdio. The @markcolabs/mcp package (v0.4+) is a thin stdio→HTTPS shim: it runs locally, accepts stdio JSON-RPC, and forwards every call to the hosted endpoint.

npx @markcolabs/mcp

For frameworks like LangChain's MCP adapter or the OpenAI Agents SDK, configure the server with command: "npx" and args: ["@markcolabs/mcp"]. If your client supports Streamable HTTP natively (as Claude's Custom Connectors do), prefer Option 1 — it skips the local process entirely.

Option 4 — Plain HTTP (no MCP client needed)

Time: ~30 seconds, just curl.

The endpoint speaks JSON-RPC 2.0 over plain HTTP POST, so any HTTP client can call it. Use this for server-side scripts, CI checks, or backend fetch calls.

curl -X POST https://mcp.digitalcalculator.info/mcp \
  -H 'Content-Type: application/json' \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": {
      "name": "mortgage_monthly_payment",
      "arguments": {
        "principal": 400000,
        "annualRatePercent": 6.5,
        "termYears": 30
      }
    }
  }'

Response (HTTP 200) — a JSON-RPC envelope whose result.content[0].text contains the standardized tool result:

{
  "result": {
    "monthlyPayment": 2528.27,
    "totalInterest": 510178.83,
    "totalPaid": 910178.83
  },
  "disclaimer": "Estimates only — for educational purposes...",
  "methodology": {
    "url": "https://www.digitalcalculator.info/mortgage-calculator/",
    "version": "2026-05-09"
  },
  "calculatedAt": "2026-06-02T18:42:11.123Z",
  "engineVersion": "1.0.0"
}

Example tool calls

All examples use the same JSON-RPC tools/call wrapper — only params.name and params.arguments change. A selection across the 13 tools:

Compound interest with monthly contributions

curl -X POST https://mcp.digitalcalculator.info/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"compound_interest_future_value","arguments":{"principal":10000,"annualRatePercent":7,"years":30,"compoundingFrequency":"monthly","monthlyContribution":500}}}'

401(k) projection through retirement

curl -X POST https://mcp.digitalcalculator.info/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"retirement_401k_projection","arguments":{"currentBalance":25000,"annualSalary":80000,"contributionPercent":6,"employerMatchPercent":50,"employerMatchLimitPercent":6,"annualSalaryGrowthPercent":3,"annualReturnPercent":7,"currentAge":35,"retirementAge":65}}}'

Social Security estimate

curl -X POST https://mcp.digitalcalculator.info/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"social_security_estimated_benefit","arguments":{"birthYear":1965,"currentEarnings":80000,"claimAge":67}}}'

IRA contribution limit

curl -X POST https://mcp.digitalcalculator.info/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"ira_contribution_limit","arguments":{"age":52,"filingStatus":"single","magi":120000,"type":"roth"}}}'

Emergency fund recommendation

curl -X POST https://mcp.digitalcalculator.info/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"emergency_fund_recommendation","arguments":{"monthlyExpenses":4500,"currentSavings":8000,"monthlySavings":600,"employmentType":"self-employed","incomeEarners":"single","dependents":2,"savingsAPY":4.3}}}'

Loan monthly payment

curl -X POST https://mcp.digitalcalculator.info/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"loan_monthly_payment","arguments":{"loanAmount":25000,"annualRatePercent":8.5,"termYears":5}}}'

For all 13 tools with full input schemas, output payload shapes, and per-tool detail, see the API Reference.

Common errors

Unknown tool: <name>

You're calling a tool name that doesn't exist. Tool names are snake_case (e.g., mortgage_monthly_payment). The legacy dot-notation names from the v0.2.x stdio package (e.g., dc.calculator.mortgage.monthlyPayment) were retired at the hosted-server cutover — if you integrated against them, switch to the snake_case names listed in the API Reference. The npm version history has full migration notes.

INPUT_VALIDATION with field: "<name>"

Input failed schema validation. The message field is human-readable ("principal must be between 0 and 100000000") and the field identifies which input. Check the API Reference for valid ranges.

BUSINESS_RULE error

Inputs are individually valid but the combination doesn't produce a meaningful result (e.g., 401(k) retirementAge not greater than currentAge). Fix the cross-field issue.

RATE_LIMIT (HTTP 429)

The hosted endpoint is throttled. Back off and retry — retriable: true in the error envelope. If a legitimate use case needs sustained higher volume, email us.

Next steps

• API Reference — full schemas + output shapes for all 13 tools
• FAQ — accuracy, YMYL posture, support, change policy
• npm package — stdio shim + CHANGELOG
• Contact & support — questions, feedback, integration help

Building something interesting with the MCP server? We'd love to hear about it — email admin@markcolabs.com.