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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds real behavioral context beyond that: it is free, and it discloses that raw event fields are zero/null for types existing only in the legacy history, which tells the agent how to interpret sparse results.

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-loaded with the key fact ('Free. Instance types available in a region.') and compact overall. The middle sentence is dense with parenthetical field lists, which is functional but slightly heavy for one breath.

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?

An output schema exists, so return values need not be explained, yet the description usefully describes the shape of the coverage fields. Parameters are covered by the schema and safety by annotations; the only real gap is the absence of alternative-tool routing.

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 (provider, region, prefix) are documented in the schema, so the baseline is 3. The description's 'c5.' / 'n2-' prefix examples add a small amount of illustrative value but nothing the case-sensitive prefix schema note doesn't already cover.

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 concrete verb and resource ('Instance types available in a region') with a clear scope, and names the distinguishing payload fields (events/history ranges). It does not explicitly differentiate itself from siblings such as gpu_models or term_instance, so it falls short of a 5.

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?

The leading 'Free.' signals cost and the prefix note implies a narrowing use case, but there is no explicit when-to-use vs. alternatives guidance and no named sibling to route to. Usage is only implied.

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