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

Annotations already declare readOnly, idempotent, non-destructive, closed-world behavior, so the safety profile is fully covered. The description adds one genuinely useful behavioral fact — that cost is not in the public credit manifest, so billing is plan-dependent — but says nothing about response shape or limits.

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 core sentence is short, front-loaded, and waste-free. The bracketed Group/Cost tags are boilerplate but compact and carry real information (cost uncertainty).

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 an output schema, rich annotations, and full schema coverage, the description only needs to nail purpose and routing. Purpose is clear, but among six I Ching siblings an agent would benefit from a routing hint that this is the direct-lookup path rather than a reading/casting path.

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 description coverage is 100%, so the schema already explains number, fields, and precision in detail. The description adds no parameter meaning beyond what the schema provides, which is the baseline 3 case.

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 ('hexagram 1-64') with the keying scheme ('by King Wen number'), so the agent knows exactly what this returns. It does not explicitly distinguish itself from the I Ching siblings (throw_coins, by_question, daily, with_changing_lines), but the 'by number' lookup is inherently distinct.

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?

There is no guidance on when to use this versus the four other I Ching tools, no prerequisites, and no exclusions. The only contextual line is a cost caveat, which is not usage guidance.

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