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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety bar is low. The description adds genuinely useful non-annotation context: the balance is scoped to 'your API key' and the call itself is 'Free' (no credit consumption). It does not say what the returned value looks like, which keeps it from a 5.

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 sentences, front-loaded with the resource, zero redundant repetition of the title or schema. 'Free' is a terse fragment but carries real 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?

A zero-param, annotated read tool with no output schema needs little more than what is here. The one gap is that an agent cannot tell the return shape (numeric balance, currency, units), though for a simple balance check this is a minor omission.

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?

Zero parameters, so per the baseline this scores 4. Nothing about the empty schema needs further explanation, and the description correctly implies 'your API key' is supplied implicitly via auth rather than as an argument.

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 resource ('remaining credit balance for your API key'), which cleanly separates it from siblings like buy_credits, list_credit_packs, and get_subscription_status. It does not explicitly name those siblings, so it stops short of a 5.

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: check how much credit is left, presumably before making metered calls. There is no explicit when-to-use guidance, no exclusions, and no pointer to buy_credits when the balance runs low.

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