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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is fully covered. The description adds the returned fields and notes the requirement for project and zone, but these are largely redundant with the schema. It hints at the tool's limited scope without explicit behavioral nuance.

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?

The description is three sentences, front-loaded with the core purpose, and every sentence provides value: what the tool does, what fields are returned, and how to use it. There is no fluff or repetitive text.

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?

Given the output schema and strong annotations, the description covers the essential purpose, returned fields, and required inputs. It does not explicitly differentiate from list_managed_instances or mention pagination, but those are largely addressed by the schema and tool naming. A minor gap exists in not clarifying zone-scoped listing beyond the required parameter.

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 coverage is 100% with descriptive entries for all four parameters. The description only restates that project and zone are required, adding no new meaning beyond what the schema already provides. Baseline 3 is appropriate given the high schema coverage.

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 states 'Lists Compute Engine virtual machine (VM) instances' and enumerates the exact fields returned, which distinguishes it from sibling list tools for disks, images, machine types, etc. The verb-resource pairing is immediate and unambiguous.

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?

'Use other tools to get more details about each instance' explicitly signals that this tool provides basic info and is not for deep investigation. However, it does not name specific alternative tools like get_instance_basic_info or list_managed_instances, so the guidance is useful but not exhaustive.

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