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

Annotations already declare this is a safe, closed-world read, so the bar is lower. The description still adds real behavioral value: it explains the two output modes (amount owed vs. full penalty schedule), the ISO date format, that filedOn defaults to today, and the domain rules that drive doubling and tax-geared penalties.

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 what the tool returns and followed by the workflow dependency and format notes. Every clause carries information; nothing is filler.

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 no output schema, the description does explain the return shape (amount owed plus the penalty date schedule) and the input format conventions. For a six-parameter calculation tool, this is nearly complete, though it does not clarify jurisdiction scope beyond UK or what happens when a deadline is not provided.

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 every parameter is already documented in the schema, including the pounds/currency for unpaidTax and the doubling semantics for accountsLateLastYear and ct600ThirdLateInARow. The description restates the ISO format and the filedOn default, adding little beyond the schema, 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 resource (late filing penalties for UK accounts and CT600) and a specific computed output: the amount owed for a given filing date plus the full schedule of penalty dates. It also names the complementary sibling tool, so an agent can separate it from lookup_uk_company or ct600_filing_guide.

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?

Gives explicit context: use it together with lookup_uk_company, which supplies the accounts deadline. This establishes the workflow and preconditions, but it does not state when not to use it (e.g. for non-UK entities or for self-assessment deadlines), so it stops short of full routing guidance.

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