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

Annotations already mark it read-only and non-destructive, and the description adds significant behavioral context beyond that: it is a pure reference endpoint, requires no birth data, and discloses the special Number 5 handling in detail — that it is never computed and that the chart returned for 5 is the Kua 2 chart. This is exactly the kind of non-obvious behavior an agent needs to know.

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 compact and front-loaded: purpose first, then usage intent, then the special Number 5 caveat. Both sentences earn their place, and the unusual behavior is explained without unnecessary padding.

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 simple reference endpoint with one required parameter, read-only annotations, and a fully documented schema, the description covers everything needed to call it correctly: what it returns, that it needs no birth data, and the special case for 5. The lack of an output schema is mitigated by the explicit list of returned fields.

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?

The schema already documents all three parameters with 100% coverage, so the baseline is 3. The description adds value beyond the schema by explaining the special meaning of number=5, which is the most confusing parameter value, though it does not need to elaborate on lang or compact because their schema descriptions are already thorough.

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 starts with a specific verb and resource: 'Look up the reference chart for one Kua number', and clearly enumerates the returned content: trigram, east/west life group, and all eight compass sectors. It also distances itself from computed endpoints by calling itself 'a pure reference endpoint with no birth data required', which differentiates it from the post_kua and post_eight_mansions siblings.

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 description states when to use the tool: 'for building a lookup table or a picker', and clarifies that no birth data is required, which implies this is not for computed Kua results. It does not explicitly name sibling alternatives or say 'do not use when...', so it falls just short of fully explicit when-not guidance.

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