macalc

Calculate Percentage

calculate_percentage

Read-onlyIdempotent

Calculate percentages: value of total, percentage change, what percent. See list_bundles for related 'math' calculators.

Input Schema

Name	Required	Description
`a`	Yes	First value
`b`	Yes	Second value
`operation`	Yes	of: X% of Y; change: from A to B; what_pct: X is what % of Y

Output Schema

Name	Required	Description
`result`	No	Computed result. Object whose fields depend on the tool (e.g. {tax, marginal_rate, brackets} for tax tools, {volume_l, gallons} for volume tools).
`source`	No	Authoritative source for the rule or formula (e.g. "Article 197 CGI", "NF DTU 21").
`formula`	No	Human-readable formula or method used (e.g. "I=P·r·t", "Magnus formula").
`reference_url`	No	Link to a calcul2 page documenting the calculation in detail.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedApr 25, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": false,
+  "properties": {
+    "formula": {
+      "description": "Human-readable formula or method used (e.g. \"I=P·r·t\", \"Magnus formula\").",
+      "type": "string"
+    },
+    "reference_url": {
+      "description": "Link to a calcul2 page documenting the calculation in detail.",
+      "type": "string"
+    },
+    "result": {
+      "description": "Computed result. Object whose fields depend on the tool (e.g. {tax, marginal_rate, brackets} for tax tools, {volume_l, gallons} for volume tools)."
+    },
+    "source": {
+      "description": "Authoritative source for the rule or formula (e.g. \"Article 197 CGI\", \"NF DTU 21\").",
+      "type": "string"
+    }
+  },
+  "type": "object"
+}

First observedApr 8, 2026

TDQS

B3.2/5.0

Behavior3/5

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

Annotations already declare this as a read-only, idempotent, non-destructive operation, so the safety profile is covered. The description adds no extra behavioral context such as error handling or output format, which is acceptable given the annotations but not enriching.

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?

The description is a compact sentence that front-loads the core operations and includes a brief pointer to bundles. It wastes little space, though the bundle reference is somewhat tangential.

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 simple three-parameter calculator with full schema coverage and an output schema, the description provides enough to call it correctly. The only missing piece is sibling differentiation, which is more of a usage concern.

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 parameters are fully documented in the input schema. The description adds no additional meaning beyond what the schema provides, which is the expected baseline.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description names the resource (percentages) and lists three specific calculation modes, making the tool's function clear. However, it does not distinguish this tool from the sibling calculate_percentage_change, which likely covers similar ground.

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

Usage Guidelines2/5

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

The only guidance is a pointer to list_bundles for related calculators, which is not a when-to-use statement. There is no direction on when to choose this tool versus the overlapping calculate_percentage_change sibling, leaving the agent to infer usage.

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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