DMD Costs legal costs tools

Interest on costs calculator (Judgments Act 1838, 8%)

calculate_interest_on_costs

Read-onlyIdempotent

Works out simple interest at 8% a year on an order for costs in England and Wales: from the date of the order (or acceptance of a Part 36 offer, or the date the court set) to the date you choose, allowing for payments on account, the County Court £5,000 rule and its exceptions, and leap years. It does not decide rates where the court has a discretion.

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": {
      "court": "<court>",
      "orderKind": "<orderKind>",
      "orderDate": "<orderDate>",
      "costs": "<costs>"
    },
    "name": "calculate_interest_on_costs"
  }
}

Input Schema

Name	Required	Description
`costs`	Yes	The costs the interest is on: the assessed or agreed sum, in pounds (e.g. "12500.50").
`court`	Yes	High Court or County Court.
`crown`	No	County Court: the costs are payable by the Crown, such as the MoD.
`endDate`	No	The date to calculate interest to (defaults to today).
`payments`	No	Payments on account, each with a date and an amount.
`orderDate`	Yes	The date of that order or event.
`orderKind`	Yes	What gave the right to costs: "assessed" (Order for costs to be assessed if not agreed), "stated" (Order for costs of a stated sum (summary assessment)), "p36-in-time" (Part 36 offer accepted within the relevant period, after the claim was issued), "p36-late" (Part 36 offer accepted late, and the court decided costs), "approval" (Settlement approved by the court (child or protected party)), "costs-only" (Order in costs-only proceedings (Part 8)), "other" (Another order or event).
`otherSums`	No	County Court: any other sums, such as damages, ordered in the same judgment, for the £5,000 test, in pounds (e.g. "12500.50").
`startDate`	No	Where customStart is true: the date interest runs from.
`customStart`	No	The court ordered interest to run from another date.

Schema Changelog

Changes observed during successful MCP inspections.

Changed7 schema fields changedNov 1, 2026

changedInput schema / properties / costs / description

Previous value: -"The costs the interest is on: the assessed or agreed sum, in pounds, e.g. \"12500\" or \"12,500.50\"."New value: +"The costs the interest is on: the assessed or agreed sum, in pounds (e.g. \"12500.50\")."

changedInput schema / properties / endDate / description

Previous value: -"The date to calculate interest to (defaults to today) (YYYY-MM-DD)."New value: +"The date to calculate interest to (defaults to today)."

changedInput schema / properties / orderDate / description

Previous value: -"The date of that order or event (YYYY-MM-DD)."New value: +"The date of that order or event."

changedInput schema / properties / otherSums / description

Previous value: -"County Court: any other sums, such as damages, ordered in the same judgment, for the £5,000 test, in pounds, e.g. \"12500\" or \"12,500.50\"."New value: +"County Court: any other sums, such as damages, ordered in the same judgment, for the £5,000 test, in pounds (e.g. \"12500.50\")."

changedInput schema / properties / payments / items / properties / amount / description

Previous value: -"Amount paid, in pounds, e.g. \"12500\" or \"12,500.50\"."New value: +"Amount paid, in pounds (e.g. \"12500.50\")."

changedInput schema / properties / payments / items / properties / date / description
```
Previous value: -"Date paid (YYYY-MM-DD)."New value: +"Date paid."
```

changedInput schema / properties / startDate / description

Previous value: -"Where customStart is true: the date interest runs from (YYYY-MM-DD)."New value: +"Where customStart is true: the date interest runs from."

First observedOct 20, 2026

TDQS

A4.1/5.0

Behavior4/5

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

Annotations already declare readOnly/idempotent/non-destructive, so the description's added value is domain behaviour: it accounts for payments on account, the County Court £5,000 rule and its exceptions, and leap years, and it flags the discretionary-rate limitation. It never says what the result contains (total interest, daily rate, breakdown), a real gap given there is no output schema.

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

Conciseness4/5

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

Two sentences, front-loaded with scope before limitations, and no padding. The first sentence is long with stacked parentheticals, which costs a little readability for an agent parsing the constraints.

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

Completeness4/5

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

For a 10-parameter statutory calculation with 100% schema coverage and no output schema, the description covers scope, inclusions and exclusions adequately. The remaining shortfall is the unexplained return shape and how the customStart/startDate pair interacts, which only the schema hints at.

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 coverage is 100%, so all ten parameters are already documented, including the detailed orderKind enum. The description reinforces the logic of orderKind (order date, Part 36 acceptance, court-set date) and the role of payments and the £5,000 test, but adds little that the schema does not already say.

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 ('works out simple interest at 8% a year on an order for costs in England and Wales'), plus the statutory basis. The 'order for costs' framing distinguishes it from the sibling calculate_interest_on_damages and calculate_judgment_interest without needing their schemas.

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 a clear applicability boundary (England and Wales costs orders, Judgments Act 1838 8%) and an explicit negative case: it 'does not decide rates where the court has a discretion.' It stops short of naming the sibling tool to use for discretionary or non-costs interest, so the routing is inferred rather than stated.

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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