calc.engineer

compound interest

compound_interest

Calculates compound interest growth over time using the formula A = P(1 + r/n)^(nt). Given a principal, annual rate, duration in years, and compounding frequency, returns the future value, total interest earned, effective annual rate (APY), and a year-by-year growth schedule. Supports optional recurring monthly contributions for savings projections. Works for savings accounts, CDs, investment returns, and retirement planning. Currency-agnostic.

Input Schema

TableJSON Schema

Name	Required	Description
`years`	Yes	Investment duration in years. Max 100.
`principal`	Yes	Initial investment or deposit amount (any currency unit).
`annual_rate_pct`	Yes	Annual interest rate as a percentage (e.g., 5.5 for 5.5%).
`compounds_per_year`	No	How often interest compounds per year. Allowed: 1 (annually), 2 (semi-annually), 4 (quarterly), 12 (monthly), 52 (weekly), 365 (daily). Defaults to 12.
`monthly_contribution`	No	Optional recurring monthly contribution added at each month. Defaults to 0.

Output Schema

TableJSON Schema

Name	Required	Description
`schedule`	Yes	Year-by-year growth schedule.
`future_value`	Yes	Final balance after all compounding and contributions.
`total_interest`	Yes	Total interest earned over the full period.
`total_contributions`	Yes	Total of all contributions (principal + recurring).
`effective_annual_rate_pct`	Yes	Effective annual rate accounting for compounding frequency (APY).

TDQS

A4.3/5.0

Behavior4/5

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

No annotations are provided, so the description must carry the full burden. It discloses the computation formula, returned values (future value, interest, APY, schedule), and support for monthly contributions. It does not mention side effects or permissions, but as a calculation tool, it is effectively read-only and non-destructive.

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?

The description is concise: three sentences with no redundant information. It front-loads the formula and key output, then adds optional contribution detail and use cases. 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?

Given the tool has an output schema, the description does not need to detail return values. It sufficiently covers parameters, use cases, and the optional contribution feature. The description is complete for a financial calculation tool of moderate complexity.

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 baseline is 3. The description adds context for compounds_per_year (listing allowed frequencies) and describes monthly_contribution as optional, but does not significantly extend the meaning beyond the schema's property 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 it calculates compound interest growth, names the formula, and lists the outputs (future value, total interest, APY, schedule). It distinguishes from sibling tools like loan_amortization by specifying use cases (savings, CDs, investment returns, retirement planning).

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

Usage Guidelines4/5

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

The description provides clear use cases and mentions optional monthly contributions for savings projections. It does not explicitly state when not to use this tool or name alternatives, but the context signals a variety of sibling tools, and the description effectively guides usage for financial growth projections.

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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TDQS

A3.9/5.0

Disambiguation4/5

Despite 89 tools, each has a clearly distinct purpose with detailed descriptions that often reference related tools. Overlap exists (e.g., multiple LoRa/RF tools), but the descriptions are sufficient to distinguish them. Some confusion possible among similar-sounding tools like attenuator_pi and attenuator_tee, but the descriptions explicitly compare them.

Naming Consistency4/5

Consistent underscore-separated lowercase naming. Most tools follow a verb_noun pattern (e.g., capacitor_charge, wire_gauge) or noun_noun (power_cost). Minor inconsistencies such as 'bmi_calculator' vs 'solar_sizing' but overall predictable.

Tool Count2/5

89 tools is far too many for a single MCP server. This scope is more appropriate for multiple specialized servers. The sheer number will slow agent selection and increase cognitive load, reducing coherence.

Completeness3/5

Covers many domains (RF, solar, PCB, networking, math, etc.) but lacks depth in some areas (e.g., no three-phase power, no airflow calculations). Some domains have comprehensive coverage (LoRa/Meshtastic), but others feel incomplete for the tool count.

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