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

Annotations already declare a non-read-only, non-destructive, closed-world write. The description adds genuinely new behavioral facts beyond them: the one-per-agent quota and that starter credits are granted, with paid top-ups handled externally. It does not say whether a key is returned in-band, whether registration is idempotent, or what happens on a second attempt.

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?

Two compact sentences with the primary action front-loaded and the quota/alternative details trailing. Every clause carries information; nothing is 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 zero-parameter mutation with no output schema, the description covers purpose, quota, and the paid alternative, but leaves the agent guessing about the result (is an API key string returned? does it need to be stored?) and about failure behavior when the agent already has a key.

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 tool takes zero parameters, so there is nothing for the description to disambiguate and the baseline of 4 applies. No parameter-level detail is missing.

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?

Names a specific verb (register) and resource (free API key) plus the qualifying benefit (starter credits), so an agent knows exactly what the call produces. Sibling differentiation is only implicit — 'free' vs. the paid 'add_credits' path — and no sibling tool is named 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?

States a hard constraint ('one per agent') and indirectly routes paid needs to the operator's top-ups, which is useful selection context. However, it never says when to call this vs. add_credits/check_credits explicitly, nor whether prerequisites such as prior authentication apply.

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