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HomeCalc — Canadian Mortgage & Tax Calculators

by ca.homecalc
OverviewDiscussionsAdmin

Mortgage Prepayment Penalty Calculator (IRD)

mortgage_penalty
Read-only

Estimate the penalty to break a Canadian mortgage: three months' interest vs the interest rate differential (IRD), incl. the big-bank posted-rate method. Fixed = greater of the two; variable = usually three months' interest. The three optional rate inputs are lender-specific: if omitted, illustrative site defaults are used (the answer can be several times off) and listed in assumedInputs — show them to the user and have them confirm with their lender.

Input Schema

NameRequiredDescriptionDefault
balanceYesOutstanding mortgage balance in CAD
rateTypeNoFixed or variable mortgagefixed
lenderMethodNoBig banks use the posted-rate IRD, monolines actual rates. Either can be larger, depending on the original discount vs today's posted rate.big-bank
contractRatePctYesYour contract rate as a percent, e.g. 5.25
monthsRemainingYesMonths remaining in the term
postedAtOriginationPctNoBig-bank method: posted (not discounted) rate when signed, from the mortgage documents. Can change the penalty several-fold. Omit if unknown (assumption is flagged).
todaysPostedForClosestTermPctNoBig-bank method: the lender's posted rate today for the closest term. Omit if unknown (assumption is flagged).
currentRateForRemainingTermPctNoMonoline method: today's rate for the remaining term (percent). Omit if unknown (assumption is flagged).

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed7 schema fields changed
    • removedInput schema / $schema
      Removed value: -"https://json-schema.org/draft/2020-12/schema"
    • addedInput schema / examples
      Added value: +[
+  {
+    "balance": 400000,
+    "contractRatePct": 4.29,
+    "monthsRemaining": 30,
+    "postedAtOriginationPct": 6.49,
+    "todaysPostedForClosestTermPct": 6.09
+  }
+]
    • changedInput schema / properties / currentRateForRemainingTermPct / description
      Previous value: -"Monoline method: today's rate for the remaining term (percent). Lender-specific — omit if unknown and the reply will say it was assumed."New value: +"Monoline method: today's rate for the remaining term (percent). Omit if unknown (assumption is flagged)."
    • changedInput schema / properties / lenderMethod / description
      Previous value: -"Big banks use the posted-rate IRD, monolines use actual rates. Which is larger depends on the gap between the discount originally received and today's posted rate — it is NOT always the big bank."New value: +"Big banks use the posted-rate IRD, monolines actual rates. Either can be larger, depending on the original discount vs today's posted rate."
    • changedInput schema / properties / postedAtOriginationPct / description
      Previous value: -"Big-bank method: posted (not discounted) rate when you signed, from your mortgage documents. HIGH IMPACT — this single value can change the penalty several-fold. Omit if unknown and the reply will say it was assumed."New value: +"Big-bank method: posted (not discounted) rate when signed, from the mortgage documents. Can change the penalty several-fold. Omit if unknown (assumption is flagged)."
    • addedInput schema / properties / rateType / description
      Added value: +"Fixed or variable mortgage"
    • changedInput schema / properties / todaysPostedForClosestTermPct / description
      Previous value: -"Big-bank method: your lender's posted rate today for the closest term. Omit if unknown and the reply will say it was assumed."New value: +"Big-bank method: the lender's posted rate today for the closest term. Omit if unknown (assumption is flagged)."
  2. First observed

TDQS

A4.7/5.0
Behavior5/5

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

Annotations cover read-only safety, and the description adds the crucial non-obvious behavior: omitted rate inputs fall back to site defaults that 'can be several times off', they are surfaced in `assumedInputs`, and the agent should show them to the user. This is exactly the kind of caveat an agent cannot infer from structured fields.

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 the core definition, then the fixed/variable rule, then the fallback caveat. Every clause carries information; nothing is redundant with the title or schema.

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?

For an 8-parameter read-only calculator with no output schema, the description covers method selection, input-omission behavior, the `assumedInputs` return field, and the accuracy caveat. An agent has everything needed to call it and communicate results responsibly.

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

Parameters4/5

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

Schema coverage is 100%, so baseline is 3, but the description adds meaning beyond the schema by grouping the three optional rate parameters as 'lender-specific' and explaining the consequence of omission. It doesn't restate individual param formats, which the schema already handles well.

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 and resource ('Estimate the penalty to break a Canadian mortgage') plus the two calculation methods (three months' interest vs IRD). It is clearly distinguishable from siblings like mortgage_payment or mortgage_affordability, which compute different quantities.

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?

Gives explicit conditional logic for which method applies: 'Fixed = greater of the two; variable = usually three months' interest', and notes the three rate inputs are lender-specific and may be omitted. It does not explicitly route to a sibling tool, but the siblings are unrelated calculations, so that omission is minor.

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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