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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description's added value is describing the return payload — pricing, features, spec table — which is genuinely useful since there is no output schema. It does not add auth, rate-limit, or caching behavior, keeping it at a solid 4 rather than 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?

A single, dense sentence that front-loads the core purpose and then enumerates the payload. No filler, no restatement of the title or annotations.

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 read-only detail-fetch with no output schema, the description supplies exactly the missing piece — what the response contains. Safety is covered by annotations and all parameters are documented in the schema, so nothing an agent needs 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?

Schema description coverage is 100% and all three parameters (plan_id, lang, currency) are fully documented with enums and defaults in the schema. The description adds nothing about parameters, so the baseline of 3 is appropriate.

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 (a single Ex2 plan) and enumerates the returned content: per-cycle prices, first-year discount, features, and the specification table. It implicitly contrasts with list_plans by saying 'one plan in detail', but it never names the siblings (compare_plans, list_plans) explicitly, so it stops short of full sibling differentiation.

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: an agent can infer this is the tool for fetching one plan's details, and the schema note 'plan id, from list_product_lines or list_plans' hints at a workflow, but that lives in the schema, not the description. There is no explicit when-to-use / when-not-to-use and no routing to compare_plans for multi-plan comparisons.

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