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

Annotations already cover the read-only, idempotent, open-world, non-destructive profile, so the description correctly spends its words elsewhere: it discloses the credit cost, the date behavior, and a precise output limitation (largest party's margin as simple arithmetic, not the official working majority). The one gap is staleness/timing of the underlying API data, which is only implied by 'after defections and by-elections'.

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

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

Three tight sentences, front-loaded with what the tool returns, then usage, then the limitation and cost. Every clause carries information an agent needs; nothing is restated from the schema or annotations.

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

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

For a one-optional-parameter read tool with no output schema, the description supplies exactly what's missing: the shape of the result (per-party seats, totals, women MPs, largest-party margin), the data source, and the cost. Nothing an agent needs to call it correctly is absent.

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

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

Schema coverage is 100% and the single optional 'date' parameter is fully documented there (format, not-in-future constraint, default today UTC). The description reinforces that the date selects a historical state of the parties, which adds mild meaning, but nothing about format or defaults beyond the schema — the baseline 3 is appropriate.

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

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

The description opens with a precise verb+resource+scope: seat counts per party in the House of Commons for a given date, sourced from the UK Parliament Members API 'state of the parties'. It also states the return content (totals and number of women MPs), which lets an agent tell it apart from siblings like lords_party_composition or constituency_election_result without opening a schema.

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

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

It gives a concrete use case ("how many MPs does each party have?" after defections and by-elections) and an explicit exclusion (it does not compute the official working majority). It doesn't name a sibling tool as an alternative for the Lords or for working-majority calculations, so routing is clear by scope rather than by explicit pointer.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.