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

Annotations cover the safety profile (readOnly, idempotent, non-destructive), so the bar is lower; the description still adds real context by disclosing what the tool computes (days remaining/overdue, penalty band, next-band cost) and constraining applicability to UK private companies. It does not contradict any annotation, though it says nothing about rate limits or where penalty-band rules come 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?

Two sentences, front-loaded with the outcome and ending with the scope restriction. Slightly padded by the promotional phrase 'the number that makes someone act today,' which adds flavor rather than information.

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 single-parameter, read-only calculation with no output schema, the description covers what is computed, the shape of the result, and the jurisdictional scope, which is enough for correct invocation. It stops short of stating that figures are statutory estimates or covering edge cases such as already-filed accounts.

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% and the schema already documents the single accountsDue parameter, including its source (lookup_company or a Companies House letter). The description adds no format or sourcing detail beyond that, so 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?

States a specific computation (penalty cost of missing a Companies House accounts deadline) plus what it returns (days remaining/overdue, incurred band, next band cost). This is clearly distinguishable from siblings like check_filing_extension and track_company_deadlines, which handle extensions and tracking rather than cost.

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?

Only implicit guidance via 'Given the due date', and the closing 'UK private companies only' scope note. It never says when to choose this over check_filing_extension or track_company_deadlines, nor any prerequisite for obtaining the due date beyond the schema.

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