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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint false, lowering the bar. The description adds valuable behavioral context: the call is free, the prefix narrows results, and zero/null appears for types that only exist in legacy history. It does not cover pagination or rate limits, but for a read-only list tool this is adequate.

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 with no filler. The 'Free' cost note, the output-field enumeration, and the prefix example each add distinct information, and the most important scope appears first.

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?

Even without an output schema, the description enumerates the returned per-type coverage fields and legacy history ranges, and the schema documents the required provider and region parameters. It does not mention pagination or error cases, but the tool is low-complexity and the annotations cover the safety profile, so the definition is sufficiently complete.

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 the baseline is 3. The description adds meaning beyond the schema by giving concrete prefix examples (`c5.`, `n2-`) and clarifying that the prefix narrows the list, which goes beyond the schema's simple 'Case-sensitive prefix filter'.

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 clearly identifies the resource ('Instance types available in a region') and adds distinguishing output details such as per-type coverage fields and legacy history ranges. However, it lacks an explicit verb like 'list' and does not name sibling tools, so it does not fully differentiate from tools like catalog or term_instance.

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?

No explicit guidance is provided on when to use this tool versus siblings such as catalog or term_instance, and no prerequisites are stated. The intended scope is inferable, but the description never says 'use this when...' or names alternatives.

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