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

Beyond readOnlyHint, it discloses null vs error behavior: 'Returns the full EPC certificate, or null only when no such certificate is lodged... If the EPC service cannot be reached the tool raises an error instead.' This is critical for not misinterpreting errors as absence of data.

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?

The description is front-loaded with the primary action and then provides usage context, parameter clarification, and return semantics. Slight redundancy: the null behavior is repeated twice ('or null only when no such certificate is lodged' and 'A null result means no such certificate is lodged'), which could be trimmed without losing meaning.

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 look-up tool, it covers the purpose, orchestration (using summaries first), parameter origin, return values, and error handling. The presence of an output schema means it need not describe the return structure in detail, so nothing essential is missing.

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 only defines lmk_key as string with no description. The description adds essential meaning: 'The parameter is named lmk_key as a compatibility alias; pass the certificate number, which is returned in every property_epc_summaries row.' This fully compensates for the 0% schema coverage.

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?

Tool states a specific verb and resource: 'Fetch a single EPC certificate by its GOV.UK certificate number.' It also distinguishes from siblings by mentioning it is faster than property_epc and used after property_epc_summaries, making its role unambiguous.

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?

Explicitly states when to use: 'Use after property_epc_summaries has listed the candidates and you have picked one.' It also names an alternative (property_epc) and warns against deprecated property_epc_search, giving clear routing guidance.

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