macalc

calculate_compound_interest

Read-onlyIdempotent

Calculate the future value of a lump sum investment with compound interest. Input principal, annual rate, and duration to get final amount, total interest, and yearly breakdown.

Instructions

Compute compound growth of a lump sum with A = P·(1+r/n)^(n·t). Use for savings, investment or retirement projections without periodic contributions. Inputs: principal (initial amount), annual_rate (annual rate in percent), years (duration), optional compounds_per_year (compounding frequency, default annual). Returns final amount, total interest earned and a yearly breakdown. Read-only, deterministic. For loans/repayments use calculate_loan_payment or calculate_mortgage instead.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "principal": "<principal>",
      "annual_rate": "<annual_rate>",
      "years": "<years>"
    },
    "name": "calculate_compound_interest"
  }
}

Input Schema

Name	Required	Description
`years`	Yes	Investment duration in years
`principal`	Yes	Initial amount
`annual_rate`	Yes	Annual interest rate in %
`compounds_per_year`	No	Compounding frequency per year

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedv0.4.0Jul 6, 2026

changedOutput schema / (root)

Previous value: -{
-  "$schema": "http://json-schema.org/draft-07/schema#",
-  "additionalProperties": false,
-  "properties": {
-    "formula": {
-      "description": "Human-readable formula or method used (e.g. \"I=P·r·t\", \"Magnus formula\").",
-      "type": "string"
-    },
-    "reference_url": {
-      "description": "Link to a calcul2 page documenting the calculation in detail.",
-      "type": "string"
-    },
-    "result": {
-      "description": "Computed result. Object whose fields depend on the tool (e.g. {tax, marginal_rate, brackets} for tax tools, {volume_l, gallons} for volume tools)."
-    },
-    "source": {
-      "description": "Authoritative source for the rule or formula (e.g. \"Article 197 CGI\", \"NF DTU 21\").",
-      "type": "string"
-    }
-  },
-  "type": "object"
-}New value: +null

Changed1 schema field changedv0.1.4Apr 25, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": false,
+  "properties": {
+    "formula": {
+      "description": "Human-readable formula or method used (e.g. \"I=P·r·t\", \"Magnus formula\").",
+      "type": "string"
+    },
+    "reference_url": {
+      "description": "Link to a calcul2 page documenting the calculation in detail.",
+      "type": "string"
+    },
+    "result": {
+      "description": "Computed result. Object whose fields depend on the tool (e.g. {tax, marginal_rate, brackets} for tax tools, {volume_l, gallons} for volume tools)."
+    },
+    "source": {
+      "description": "Authoritative source for the rule or formula (e.g. \"Article 197 CGI\", \"NF DTU 21\").",
+      "type": "string"
+    }
+  },
+  "type": "object"
+}

AddedApr 24, 2026

TDQS

A4.6/5.0

Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

The description states 'Read-only, deterministic', which aligns with all annotations (readOnlyHint, idempotentHint, etc.). It also discloses returns (final amount, total interest, yearly breakdown), adding value beyond annotations. No contradictions.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Three sentences front-loaded with formula, then usage, inputs, returns, and alternatives. No wasted words; every sentence provides distinct information. Highly efficient.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the complexity of compound interest and absence of output schema, the description covers the formula, usage scope, all inputs (with clarification of optional parameter), and what is returned. It is complete for an agent to select and invoke this tool correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters2/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

While schema coverage is 100%, the description adds a misleading default value for 'compounds_per_year': it says default annual, but the schema clearly sets default 12 (monthly). This introduces confusion and undermines the otherwise clear parameter listing. The description does not add significant new meaning beyond the schema's own descriptions.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool computes compound growth using a specific formula, and explicitly distinguishes from alternatives like loan payment calculators. The verb 'Compute' and resource 'compound growth' are specific, and it mentions the tool is for projections without periodic contributions, differentiating it from siblings.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description explicitly advises use for savings, investment, or retirement projections without periodic contributions, and directly names alternative tools (calculate_loan_payment, calculate_mortgage) for loans/repayments. This provides clear when-to-use and when-not-to-use guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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