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

Annotations already declare readOnly, idempotent, and non-destructive, so safety is covered. The description adds the useful behavioral detail that the returned balance depends on auth method (credits vs. USD API balance) and that the call is free, but says nothing about output shape or failure modes for an unauthenticated/expired key.

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 naming the resource first, followed by a compact parenthetical that routes to the alternative. No wasted wording.

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 zero-param read tool with full annotation coverage and a sibling pointer, the description is nearly self-sufficient; only the exact returned fields (plan name, credit counts, currency) are left unspecified, which is acceptable given no output schema exists but keeps it shy of a 5.

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?

Zero parameters, so the baseline applies; the description correctly implies no inputs are needed. It adds only contextual framing (credits vs. API-key balance) rather than parameter meaning.

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 resource ('The user's own balance and plan') with the two concrete return modes (Wandy3D credits vs. US-dollar API balance via API key), and names the sibling it is not (get_prices). An agent can distinguish this from get_prices without opening either schema.

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?

Gives clear context (own balance/plan, free to call) and an explicit exclusion routing price lookups without sign-in to get_prices. It does not spell out prerequisites for the API-key variant, but the when-to-use signal is strong.

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