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

Annotations already declare this a safe, idempotent, closed-world read. The description adds value beyond that by disclosing a behavioral trait the annotations cannot express: 'This call is free,' so invoking it never consumes the balance, plus the differing billing models for Free vs Builder/Pro accounts.

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?

Front-loaded with what the call returns, followed by billing context and the key 'this call is free' note. Three sentences, each earning its place, with no restatement of the name or title.

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?

With no input parameters and no output schema, the description carries the return-content burden, and it does so by listing the fields the agent will get. The account/billing context is sufficient for an agent to decide whether and why to call it.

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 schema defines zero properties, so there are no parameters to document. The description correctly spends no effort on parameter syntax; a baseline 4 applies for a no-parameter tool.

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 names a specific resource — the signed-in account's prepaid credit — and enumerates exactly what it returns (balance, per-call costs, credit by expiry, recent history, how to add more). This is far more specific than a tautology, though it never names or contrasts with the closest sibling, my_history, so it falls short of the top score.

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 only implied: an agent can infer you call this to check remaining credit before making paid calls. There is no explicit 'use this when' statement and no mention of alternatives such as my_history or supply_balance, so the guidance is adequate but not actionable on its own.

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