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

Annotations already establish readOnlyHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered. The description adds value beyond that by disclosing the shape of the returned data (limit, spent, left) with no output schema present, which is genuinely useful behavioral context.

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 sentence with a front-loaded subject and a colon-delimited enumeration of the returned figures. No filler, no repetition of 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 tool with annotations covering safety and no output schema, the description supplies the missing piece by naming what is returned. It stops short of explaining units, currency, or refresh behavior, but nothing essential to calling the tool correctly is absent.

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 for the description to disambiguate; the baseline for no-param tools is 4. The description correctly focuses on return content rather than inventing parameter detail.

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 specific resource (the Phoenix balance spendable by this key) and enumerates the returned figures: balance, limit, amount spent, amount left. It is clear what the tool does, though it does not explicitly distinguish itself from sibling tools (none of which appears to fetch a balance anyway).

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: 'the balance this key can spend' suggests a pre-purchase or pre-spend check, but there is no explicit when-to-use statement and no reference to related siblings such as top_up_with_usdc or create_top_up_link that an agent might pair with it. Adequate but with a clear gap.

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