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

Annotations already declare readOnly/idempotent/non-destructive, so safety is covered. The description adds genuine behavioral context beyond that: the full set of returned data (registry summary, outcomes, dates, every recorded change, citing papers) and the special attribution/processing-date field for WHO ICTRP records, which matters since no output schema exists.

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?

Front-loaded with the usage trigger, then a compact inventory of returns; every sentence carries information. The final WHO ICTRP attribution sentence is somewhat niche but still actionable for display purposes, so it earns its place.

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 must cover return contents, and it does so explicitly. Input identifier scope, return shape, and the WHO attribution caveat are all present, and annotations handle the safety profile.

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 nct_id parameter is fully documented with pattern and example, so the baseline is 3. The description adds real value by enumerating the accepted registry namespaces (NCT, ISRCTN, ChiCTR, CTRI, CTIS and others), clarifying the identifier space beyond the schema's pattern.

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?

The description names a specific verb+resource (fetch one clinical trial by registry number) and enumerates what the caller gets: design, outcomes, dates, change history, and citing papers. It is clearly distinguishable from the plural sibling list_trials.

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?

"Use this when the user asks about one clinical trial by its registry number" gives an explicit trigger condition and implicitly scopes it to single-record lookups versus list_trials. It never names the alternative tool or states when not to use it, so it falls short of a 5.

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