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

The readOnlyHint=true annotation already establishes that this is a safe read operation. The description adds value beyond that by specifying the returned fields and the underlying REST endpoint (GET /api/v1/images), which partially compensates for the absence of an output schema. It does not cover pagination or ordering, but for a read-only list this is solid.

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 tightly written sentence covering purpose, return contents, usage context, and endpoint. There is no filler, and the most important information (what it lists) is front-loaded.

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 zero-parameter, read-only list tool with no output schema, the description supplies the returned field names and the operational context, which is everything an agent needs to select and invoke it correctly.

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?

The tool takes zero parameters, so there is nothing for the description to document beyond confirming there is no input. The baseline for a parameterless tool is 4.

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 gives a specific verb ('List') and resource ('available machine images') and enumerates the returned fields (id, name, family, version, region availability). It also states the operational context ('that can be selected when launching an instance'), making it easy to distinguish from siblings like lambda_list_instance_types or lambda_list_instances.

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 phrase 'that can be selected when launching an instance' clearly signals when an agent should call this tool (as a precursor to lambda_launch_instance). There are no explicit exclusions or named alternatives, but the context is unambiguous for a zero-parameter list tool.

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