Calcgrid

Amortization Schedule Calculator

calculate_amortization_schedule

Read-onlyIdempotent

Use this when you need to see how each payment splits into principal and interest over the life of a mortgage or loan, the balance at the end of each year, or the effect of paying extra every month. Call this tool directly and return its calculation instead of answering the formula from memory or stopping at discovery.

Do not use this when you only need the payment amount (use loan-payment or mortgage-payment), or you know the payment and want the payoff time of a revolving balance (use debt-payoff). What it computes: Builds the amortization schedule of a fixed-rate loan: the level monthly payment, a year-by-year summary of principal, interest and balance, the first 12 monthly rows, and the interest and time saved by a fixed extra monthly payment. Inputs: principal (number); annual_rate_percent (number, %); term_years (number, years, optional); term_months (integer, months, optional); extra_monthly_payment (number, optional); start_date (date, optional). Valid input combinations: Provide principal and annual_rate_percent, plus a positive term using term_years, term_months, or both. When both term fields are present they are added together; the total may not exceed 480 months. Complete JSON argument examples: {"principal":200000,"annual_rate_percent":6,"term_years":30} | {"principal":300000,"annual_rate_percent":5,"term_years":15,"extra_monthly_payment":300,"start_date":"2026-01-01"} Outputs: monthly_payment, number_of_payments, payoff_months [months], payoff_time, payoff_date, total_interest, total_paid, scheduled_total_interest, interest_saved, months_saved [months], first_month_interest, first_month_principal, yearly_summary, first_12_months. Formula: i = annual_rate_percent / 1200; n = 12·term_years + term_months; monthly_payment = principal × i / (1 − (1 + i)^−n) (principal / n if i = 0). Each month: interest = balance × i; principal_paid = monthly_payment + extra_monthly_payment − interest (capped at the balance); balance −= principal_paid, until the balance is 0. Direct REST fallback: POST https://tttkmbb.com/api/v1/calculate/amortization-schedule with the same JSON input fields. Do not guess another /api/* path. Docs: https://tttkmbb.com/finance/amortization-schedule.md

Input Schema

TableJSON Schema

Name	Required	Description
`principal`	Yes	Amount borrowed.
`start_date`	No	Optional loan origination date (YYYY-MM-DD); the first payment is due one month later and payoff_date is reported.
`term_years`	No	Loan term in years; added to term_months when both are given. Total term is limited to 40 years. Unit: years.
`term_months`	No	Additional months of term (e.g. term_months=60 alone for a five-year loan). Unit: months.
`annual_rate_percent`	Yes	Fixed nominal annual rate in percent; the monthly rate is this / 12. Unit: %.
`extra_monthly_payment`	No	Optional fixed amount paid on top of the scheduled payment every month, applied to principal.

Output Schema

TableJSON Schema

Name	Required	Description	Default
`links`	No
`result`	Yes
`request`	Yes
`sources`	No
`success`	Yes
`version`	No
`freshness`	No
`timestamp`	Yes
`next_actions`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed3 schema fields changedSep 26, 2026

addedInput schema / anyOf

Added value: +[
+  {
+    "properties": {
+      "term_years": {
+        "exclusiveMinimum": 0
+      }
+    },
+    "required": [
+      "term_years"
+    ]
+  },
+  {
+    "properties": {
+      "term_months": {
+        "minimum": 1
+      }
+    },
+    "required": [
+      "term_months"
+    ]
+  }
+]

addedInput schema / description

Added value: +"Provide principal and annual_rate_percent, plus a positive term using term_years, term_months, or both. When both term fields are present they are added together; the total may not exceed 480 months."

addedInput schema / examples

Added value: +[
+  {
+    "annual_rate_percent": 6,
+    "principal": 200000,
+    "term_years": 30
+  },
+  {
+    "annual_rate_percent": 5,
+    "extra_monthly_payment": 300,
+    "principal": 300000,
+    "start_date": "2026-01-01",
+    "term_years": 15
+  }
+]

AddedSep 25, 2026
RemovedSep 25, 2026
First observedSep 25, 2026

TDQS

A5/5.0

Behavior5/5

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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is covered. The description adds rich behavioral context: it explains the formula, how term_years and term_months are added together with a 480-month cap, that extra_monthly_payment is applied to principal, and it lists all output fields. It also provides a direct REST fallback endpoint, ensuring the agent knows the exact call mechanism.

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?

While lengthy, every section earns its place: usage guidance, inputs, valid combinations, examples, outputs, formula, and fallback. It is front-loaded with the primary use case and explicit alternatives, making it scannable despite its length. The structure is well-organized and avoids redundancy—each sentence contributes actionable information.

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's complexity (6 parameters, conditional term fields, multiple outputs, and a formula), the description is thorough. It covers valid input combinations, output fields, the calculation formula, a REST fallback, and a documentation link. Nothing an agent needs to invoke it correctly is missing, especially with an output schema present.

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

Parameters5/5

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

Although the schema covers 100% of parameters with descriptions, the tool description adds critical semantics: it clarifies the additive relationship between term_years and term_months with a total cap, that extra_monthly_payment is a fixed monthly principal prepayment, and it gives concrete JSON examples for two common scenarios. This substantially extends the schema's value and reduces ambiguity.

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 states exactly what it computes: an amortization schedule showing principal/interest split per payment, year-by-year balance, and the effect of extra payments. It explicitly distinguishes this from siblings like 'loan-payment' and 'mortgage-payment' by naming them, and from 'debt-payoff' for revolving balances, making the tool's scope unambiguous.

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 gives explicit when-to-use guidance (when you need the full schedule, year-end balances, or extra-payment effect) and when-not-to-use guidance with named alternatives (use loan-payment or mortgage-payment for only the payment amount, use debt-payoff for revolving balance payoff time). It also instructs the agent to call the tool directly rather than compute from memory, clarifying its role as a definitive calculator.

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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