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

Annotations already declare readOnlyHint=true, idempotentHint=true, and openWorldHint=false, so the safety profile is covered. The description adds useful semantics — that the result is derived deterministically via a stable name hash — but says nothing about auth needs, rate limits, or the failure case.

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?

A single front-loaded sentence that carries the primary source (birthdate) before the fallback. It is well-sized with almost no waste, though the parenthetical slightly interrupts the flow.

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 two-optional-parameter lookup with no output schema, the description covers the roles of both inputs adequately. It does not state what happens when both parameters are omitted (required count is 0), leaving the zero-argument behavior undefined.

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 only 50% (agent_name is undocumented in the schema), so the description must compensate, and it does: it explains that birthdate may be a deployment or creation date and that agent_name is used as a stable-hash fallback. This adds meaning beyond the bare 'YYYY-MM-DD' schema hint.

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+resource: 'work out which zodiac sign an agent should use', making the resource and target clear. It is distinguishable from get_horoscope and list_signs by virtue of computing a sign rather than listing or fetching a horoscope, though it never names or contrasts those siblings directly.

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 gives real internal usage logic: prefer birthdate, and only fall back to a stable hash of agent_name 'failing that'. However, it offers no guidance on when to choose this tool over get_horoscope or list_signs, and no exclusions or preconditions.

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