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

Annotations already cover readOnly/idempotent/openWorld, so the description spends its space well on non-obvious behavior: the 10-country ceiling and, critically, the field-matching rules (case-insensitive, partial match, and 'Economy > Exports' pinning a section). It does not describe how results are shaped, but the annotations carry the safety profile.

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 verb, scope, and field list, then usage examples, then matching rules — a sensible order. Dense but not padded; the long enumeration of fields is slightly over-long relative to a pointer to factbook_fields, but it earns its place by showing the domain.

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?

Covers scope, inputs, matching semantics and a discovery pointer, which is enough for a two-parameter read-only comparison tool with no output schema. The one gap is that it never hints at the response shape (a comparison grid vs a flat list), which would help the agent consume the result.

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 parameter naming is already handled; the description nevertheless adds real semantics the schema lacks — case-insensitive and partial field matching plus section pinning — which prevents failed lookups. It also restates the country-count limit, which is only implicit in the schema example.

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?

Opens with a specific verb+resource ('Compare CIA World Factbook fields side by side across up to 10 countries') and enumerates the comparable field domain, so the agent immediately knows this is a multi-country, multi-field lookup rather than a single-country profile. It is clearly distinguishable from siblings like factbook_country and factbook_entities.

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?

Two concrete natural-language phrasings ('compare population and GDP per capita of Kenya, Nigeria and Ghana', 'life expectancy in Japan vs Germany') make the trigger condition unambiguous, and the 'factbook_fields lists the exact names' pointer routes the agent to a discovery step first. It stops short of naming when NOT to use it (e.g. single-country detail belongs to factbook_country).

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