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

With no annotations, the description carries the full burden and does real work: it discloses cost (free), authentication (no key), geographic coverage limits, and the nature of the response (headroom, RAG status, siting signal). Not disclosed: behaviour for coordinates outside GB, coordinate reference system, or data freshness — the kind of edge-case guidance a location-based lookup would benefit from.

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 sentences, each carrying distinct payload: capability + returned metrics first, then geographic scope, then cost/auth. Nothing is repeated from the schema and no sentence is filler — 'Free, no key' and the non-England coverage note both earn their place.

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?

An output schema exists, so the description need not explain return values, and it wisely only summarizes them. For a two-parameter read-only lookup this is nearly sufficient; the sole substantive gap is the coordinate format/CRS, which leaves the calling agent exposed on input construction.

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 description coverage is 0% — neither lat nor lon has any documentation in the schema — so the description must compensate, and it does not. It only says 'for a location' without specifying decimal degrees vs. other notation, WGS84/EPSG:4326, acceptable ranges, or precision. An agent must guess the coordinate convention for a tool that fails silently on wrong input.

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?

States a specific verb (check), a specific resource (nearest DNO grid connection headroom) and enumerates the payload: generation/demand headroom in MW/MVA, RAG status, voltage, and a combined BNG+grid siting signal. The domain (electricity grid capacity) is unmistakably distinct from the sibling tools covering planning history and BNG cost/income, so an agent can route without opening any 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?

'Covers all of Great Britain (England/Scotland/Wales), not just England' gives an explicit scope boundary that tells the agent when this tool applies geographically, and 'Free, no key' removes a common pre-call blocker. It stops short of naming a sibling or stating a when-not-to-use condition, but the domain separation from the BNG and planning siblings is self-evident.

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