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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds 'Returns the figure plus its source', which tells the agent that provenance is included and that values are authoritative reference data rather than computed output — useful but modest. It says nothing about how the data is versioned or whether the current year can be missing.

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, zero filler: the verb and resource lead, the category enumeration sits inline where it is needed, and the return-value note is a compact trailing clause. Nothing could be cut without losing 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?

With no output schema, the description supplies the essential fact that a figure plus its source is returned, and the category enumeration covers the required parameter's domain. Minor gaps remain — nothing on what happens when an item or tax year isn't available, and nothing distinguishing it from the tax-rule and tax-fact search siblings.

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%, so the baseline is 3, but the description earns an extra point by enumerating the eleven valid category values in the parenthetical — material value because the schema declares no enums and only gives three examples. It also reinforces the 'current' default that the tax_year parameter implies.

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 and resource — 'Look up current UK tax rates, thresholds and allowances' — and enumerates all eleven supported categories, which is far more precise than the title alone. It does not differentiate itself from overlapping siblings such as search_tax_facts, lookup_tax_rule and list_tax_rules, leaving the agent to guess which one to open.

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?

The phrasing 'Look up current UK tax rates' implies the use case (retrieving authoritative figures rather than computing them), and the category list hints at scope. But there is no explicit when-to-use versus the several tax-related siblings, and no guidance on preferring this over search_tax_facts for lookups vs. free-text search.

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