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

Annotations already establish readOnly/idempotent/openWorld, so the bar is lower, yet the description still adds real context: the counted population is limited to open, rolling and forthcoming schemes, the counting rules mirror search_grants, and with no output schema it discloses the return shape (a number plus a website search link). It does not discuss rate limits or empty-result behavior, but this is solid added value.

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 short sentences, front-loaded with the action and scope, followed by return value and the sibling hand-off. No filler and nothing buried.

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?

With a 5-parameter filter surface but full schema coverage and no output schema, the description supplies what structure can't: the counted population, the delegate for counting rules, and the return format. It is complete enough to call correctly; only marginally more detail on what 'matching' means for deadline-less rolling schemes would improve it.

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 100%, so all five filters including enums and the date format are already documented in the schema. The description only says 'optional filters' and defers to search_grants' rules, adding no per-parameter meaning beyond structured data. Baseline 3 applies.

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?

Specific verb ('count') plus a precisely scoped resource ('open, rolling and forthcoming UK grant schemes matching optional filters'), and it explicitly separates itself from search_grants. An agent can distinguish it from every sibling 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 names the alternative tool and the condition that selects it: 'Use search_grants to see the schemes themselves.' Combined with the explicit scope restriction (only open/rolling/forthcoming schemes), the agent knows both when this is appropriate and when to route elsewhere.

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