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

Annotations already establish readOnly, idempotent and non-destructive behavior, so the description's marginal value is limited to two facts: it is scoped to the caller's key (auth context) and it costs 0 credits despite the rest of the API being metered. That is useful, but nothing about refresh cadence, whether the balance reflects pending calls, or what a 'credit price' means is disclosed.

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 17-word sentence that leads with what you get and closes with the cost caveat. Zero filler, nothing repeated from the 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?

For a zero-argument, read-only, non-destructive tool with no output schema, the description tells the agent what comes back (balance plus credit price) and that the call is free – enough to call it confidently. Minor gap: no statement about response shape or units of the balance.

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 and the schema is fully closed (additionalProperties: false), so there is no parameter semantics for the description to carry. Baseline 4 applies.

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 verb ('Show') and a specific resource ('your key's credit balance for the data API'), plus the secondary output (price of a credit). None of the siblings (data_get, data_list, data_search, etc.) retrieve account/credit metadata, so the tool is unambiguous against its namespace.

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 credits before making metered calls, but the description never says when to call it, whether it should be polled, or how it relates to the metered data_* tools. No alternatives or exclusions are named.

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