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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered; the description adds non-obvious traits: no cost to call ('Free'), an authentication requirement ('Needs sign-in'), and that the result carries a link to add credits. That auth-and-cost disclosure is exactly the kind of behavior annotations cannot express.

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?

Four short fragments, front-loaded with the resource and followed by the two invocation conditions -- nothing is wasted. The telegraphic style is slightly clipped but every clause carries information.

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 read with full annotation coverage, the description covers what is returned (credits plus top-up link) and the sign-in prerequisite, which is enough to call it correctly. With no output schema, return-value detail is thin, but the essentials are present.

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, which is the baseline-4 case; there is nothing for the description to document and it correctly adds no redundant parameter talk.

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 and scope: the credit balance on the signed-in account, plus the top-up link. An agent immediately knows this is a balance/credits read, and the sibling tools (check_site, discover, run_tool, etc.) are functionally unrelated, so little differentiation is needed.

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 the two operational conditions that matter -- it is free to call and it requires sign-in -- which is real context for deciding whether to invoke it. However, it never states when an agent should reach for it versus other tools, nor what happens if the caller is not signed in.

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