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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description still adds real context beyond them: 'listing changes nothing', that the operation is 'Free', and that results are 'paged'. It does not describe pagination mechanics or rate limits, keeping it short of 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?

Front-loads what the resource list is, then the action guidance, then cost/paging in three clipped fragments. Dense but every clause carries information; the trailing 'Free, paged.' is terse but purposeful.

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 an output schema present, return values need no explanation, and the description still adds useful field names, cost, and pagination behavior. It also points to the mutation endpoints for release/renew, so an agent has everything needed to call and act on this tool.

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?

Schema description coverage is 100%, so cursor, status, service and end_user are already documented in the schema, and the description adds no filtering syntax or defaults beyond that. The mention of 'service, status, and expiry' describes returned columns rather than input parameters, so the baseline 3 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?

Names a specific verb (List) and a precise resource class (durable resources created by your calls that persist, expire, or cost money), reinforced with concrete examples (AgentMail inboxes, phone numbers, E2B sandboxes) and the fields returned. An agent can distinguish this from list_calls or get_balance without opening the schema.

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?

States the decision context clearly: use it to find what to release or renew, and even names the follow-up endpoints (POST /credits/resources/{id}/release or /renew). It also rules out side effects with 'listing changes nothing'. No explicit when-not-to-use or sibling alternative is given, but the routing intent is unambiguous.

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