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

The description frames the tool as a read ('Read the account's credits...'), while annotations declare readOnlyHint=false, meaning the tool is not guaranteed to be read-only. That direct tension leaves the agent uncertain about whether calling it mutates state. The remaining content (entitlements-only scope, estimates aren't bills) is useful but does not resolve the conflict.

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?

Three short sentences, front-loaded with the core action and scope; the caveats about entitlements and estimates follow. Every sentence carries distinct information and there is no filler.

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 parameters and no output schema, the description should at least convey the shape of what comes back, and it only lists categories (credits, limits, estimates) without indicating structure or units. It is adequate to call the tool but leaves the returned payload largely opaque.

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 no parameter semantics to explain and the baseline is a 4. Nothing in the description over- or under-specifies the empty input object.

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 (read) and resource (account usage), and enumerates the returned content types: credits, length and file-size limits, and estimates. It doesn't explicitly differentiate itself from the only nearby sibling, account_preferences, but the domain is clear enough for selection.

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?

Includes explicit negative routing: 'Uses existing entitlements only: purchases, subscriptions, upgrades and checkout are not available here' tells the agent not to route commercial actions here, and 'Estimates are not bills' bounds interpretation. No positive alternative tool is named, which keeps it short of a 5.

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