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

Annotations already establish the safe-read profile (readOnlyHint=true, idempotentHint=true, destructiveHint=false), so the bar is lower. The description adds genuine value beyond them: the account-linking prerequisite and the fact that the response carries a pricing_url explainer rather than just a number.

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 short sentences, front-loaded with the primary return value (credit count) before the secondary pricing_url. The second clause is grammatically fragmented ('and pricing_url, a page explaining how credits work'), which slightly muddies the read but costs little.

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?

A no-argument, read-only tool with an output schema and full annotation coverage needs very little prose. The description covers purpose, an auth prerequisite, and the shape of the return payload, so an agent has everything required to invoke it correctly.

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 to disambiguate and no schema gap to compensate for. With no inputs required, parameter documentation is inherently complete at the baseline of 4.

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?

The description states a concrete resource and outcome: 'How many credits the user's RegTail account holds,' plus the accompanying 'pricing_url' explainer page. It is immediately clear this is a read of account credit state. No sibling (decode_plate, full_report, unlock_report, etc.) overlaps, so no differentiation is needed, but neither is it provided.

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?

Usage is implied rather than spelled out, though the description does supply one real precondition: 'Needs the user's RegTail account linked.' That tells the agent when the call will succeed. There is no statement of when not to call it or what to do if the account is unlinked.

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