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

Annotations already declare readOnly=true, idempotent=true, destructive=false, and closed-world, so the safety profile is covered. The description adds value beyond that by enumerating exactly what comes back (remaining, used, plan, live mcp_version) and by making clear the subject is the signed-in user, implicitly disclosing that no user identifier is needed or accepted.

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 zero filler, front-loaded on the primary return value (remaining and used credits) before the secondary fields (plan, mcp_version). Every clause adds 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?

With no input parameters, no output schema, and a full annotation set, the description supplies exactly the missing piece: the shape of the return value. Nothing an agent needs in order to call this 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?

There are no parameters, so the baseline is 4. The phrase 'signed-in user's' usefully communicates that identity is derived from the session rather than passed in, which is the one piece of parameter-relevant information an agent might wonder about.

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 (return) and a precise resource scope (the signed-in user's remaining and used credits, plan, and live mcp_version). It is clearly distinguishable from create_credit_checkout (a mutation) and from whoami by naming credits as the subject.

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 stated: an agent can infer this is the balance-lookup tool, but the description never says when to use it versus create_credit_checkout or whoami, nor any preconditions. Adequate for a zero-parameter read tool but with a clear guidance gap.

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