EveryCalc

calculate_compound_interest

Read-onlyIdempotent

Future value of an investment with monthly contributions and monthly compounding. Use when the user asks how savings or an investment grows over time with regular contributions. For a 401k with employer match use calculate_401k; for a retirement balance use calculate_retirement_savings_planner. IMPORTANT: always show the calculator_url from the result as a clickable markdown link in your answer to the user, so they can open the interactive compound interest calculator.

Input Schema

Name	Required	Description
`years`	No	Projection horizon; default 30, max 100.
`principal`	No	Starting balance in USD.
`annual_rate_pct`	Yes	Annual return percent.
`include_schedule`	No	Include the yearly growth schedule.
`monthly_contribution`	No	Added at the end of each month in USD.

Output Schema

Name	Required	Description
`ok`	Yes	True when the calculation succeeded.
`note`	Yes	Disclaimer that accompanies every result.
`tool`	Yes	Name of the tool that ran.
`inputs`	No	The normalized inputs the calculation used.
`results`	Yes	The computed figures for this calculator.
`calculator_url`	Yes	Deep link to the matching interactive calculator on everycalc.xyz.

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedOct 21, 2026

addedOutput schema / properties / note

Added value: +{
+  "description": "Disclaimer that accompanies every result.",
+  "type": "string"
+}

changedOutput schema / required

Previous value: -[
-  "ok",
-  "tool",
-  "results",
-  "calculator_url"
-]New value: +[
+  "ok",
+  "tool",
+  "results",
+  "calculator_url",
+  "note"
+]

Changed1 schema field changedOct 17, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "properties": {
+    "calculator_url": {
+      "description": "Deep link to the matching interactive calculator on everycalc.xyz.",
+      "type": "string"
+    },
+    "inputs": {
+      "description": "The normalized inputs the calculation used.",
+      "type": "object"
+    },
+    "ok": {
+      "description": "True when the calculation succeeded.",
+      "type": "boolean"
+    },
+    "results": {
+      "description": "The computed figures for this calculator.",
+      "type": "object"
+    },
+    "tool": {
+      "description": "Name of the tool that ran.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "ok",
+    "tool",
+    "results",
+    "calculator_url"
+  ],
+  "type": "object"
+}

First observedOct 15, 2026

TDQS

A4.5/5.0

Behavior4/5

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

Annotations already declare a safe, read-only, idempotent operation, so the safety profile is covered. The description adds genuinely non-obvious behavioral guidance: the result carries a calculator_url that MUST be surfaced as a clickable markdown link, which is output-handling context not present in the schema. Return shape details are omitted, but the output schema covers those.

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?

Front-loaded purpose, then usage trigger, then disambiguation, then the critical output instruction — four tight sentences with no redundancy. Every sentence earns its place.

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?

With a full output schema present, the description need not explain return values, and it instead supplies the pieces an agent actually needs: purpose, when to invoke, sibling routing, and the mandatory calculator_url display behavior. Nothing material is missing.

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

Parameters3/5

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

Schema description coverage is 100%, so all five parameters are already documented and the baseline is 3. The description only echoes 'monthly contributions and monthly compounding' and adds no syntax, units, or default detail beyond what the schema already states.

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?

States a specific verb+resource ('Future value of an investment with monthly contributions and monthly compounding'), which cleanly separates it from the many other calculate_* finance tools. It also explicitly names the two nearest siblings (calculate_401k, calculate_retirement_savings_planner) it should not be confused with.

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?

Gives a concrete trigger ('when the user asks how savings or an investment grows over time with regular contributions') and routes two adjacent scenarios to their correct alternatives with the distinguishing condition (employer match → calculate_401k; retirement balance → calculate_retirement_savings_planner). Explicit when-to-use plus alternatives.

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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