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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuinely useful behavior beyond them: the return payload is split into headline fields and a complete raw record whose shape varies by the schema version the certificate was registered under — a caveat the annotations cannot express.

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 verb and resource, then a second sentence that spends its words on the return shape. Every clause adds information; no 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?

No output schema exists, so the description carries the return-value burden itself and does so by naming the headline fields and flagging the variable raw payload. For a one-parameter lookup tool this covers what an agent needs to call and interpret it correctly.

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% and the single parameter is self-describing, so baseline is 3. The description goes further by identifying where the certificate_number value comes from (the two search tools), which is meaning the schema's format example does not provide.

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 (Fetch) and resource (one Energy Performance Certificate or Display Energy Certificate) plus the exact lookup key (certificate number). It also names the sibling tools that produce that key, so an agent can place it precisely among search_domestic_epc/search_non_domestic_epc.

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 parenthetical '(from search_domestic_epc or search_non_domestic_epc)' tells the agent this is a follow-up lookup after a search, which is real routing guidance. It stops short of stating when not to use it or what an alternative would be, so it is clear context rather than full when/when-not guidance.

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