Hopi

Pregnancy due date calculator

hopi_due_date_calculator

Read-onlyIdempotent

Estimate a pregnancy due date from the first day of the last menstrual period (LMP) and cycle length. Method: adjust the LMP by (cycle - 28) days, then add 280 days for the due date. Also returns the dates the second and third trimesters begin (LMP-adjusted + 84 and + 182 days). Dates are ISO 'YYYY-MM-DD'. Source: https://hopi.co.uk/due-date-calculator/

Input Schema

TableJSON Schema

Name	Required	Description	Default
`lmp`	Yes	First day of the last menstrual period, ISO date 'YYYY-MM-DD'
`cycleLength`	No	Cycle length in days, 20 to 45 (default 28)

Output Schema

TableJSON Schema

Name	Required	Description
`lmp`	No
`dueDate`	No
`summary`	Yes
`source_url`	Yes	The tool's page on hopi.co.uk (cite or open to check the figure)
`cycleLength`	No
`trimester2Starts`	No
`trimester3Starts`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedSep 20, 2026
- addedOutput schema / required
```
Added value: +[
+  "source_url",
+  "summary"
+]
```
First observedSep 20, 2026

TDQS

A4.7/5.0

Behavior5/5

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

The description goes well beyond the readOnly/idempotent annotations by disclosing the exact calculation method: adjust LMP by (cycle - 28), add 280 days, and add +84 and +182 days for trimester starts. It also states the ISO date format, giving the agent a precise behavioral model.

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?

Every sentence earns its place: purpose, algorithm, trimester offsets, date format, and source. The description is informative yet compact, with no redundant or vague phrasing.

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?

The schema fully documents parameters, annotations cover safety, and the description explains the algorithm and return values. An agent has everything needed to invoke the tool correctly without guessing at behavior or output semantics.

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 the schema already documents lmp and cycleLength. The description adds value by explaining how the parameters interact in the formula, particularly the cycle-length adjustment, which is not obvious from the schema alone.

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 names the exact operation ('Estimate a pregnancy due date'), specifies the inputs (LMP and cycle length), and adds the distinct scope of returning trimester start dates. This clearly separates it from generic date or age calculators.

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 intended use case is clearly stated: pregnancy due date estimation from LMP and cycle length. It does not explicitly name alternative calculators or exclusion cases, but the domain-specific purpose is unambiguous among the sibling set.

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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