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

The description adds meaningful behavioral context beyond the annotations: it notes the $0.03 cost, and clearly describes the asynchronous flow by stating it returns a generation_id and that get_generation should be polled until status becomes 'completed'. This gives an agent a clear expectation of side effects and follow-up work. No contradiction with readOnlyHint=false exists.

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 cover purpose, cost, and async behavior with zero filler. The core action is front-loaded, and each remaining sentence adds necessary operational detail.

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 low-complexity tool with two fully described parameters, the description is nearly complete: it states the required inputs implicitly, explains the immediate return value, and tells the agent how to retrieve the final result. The absence of an output schema is largely compensated by the explicit get_generation polling instruction, though the final output format is left to get_generation.

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 schema already defines both parameters. The description does not add extra parameter meaning beyond what the input schema provides, 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 clearly states a specific action and resource: 'Change facial expressions.' It is unambiguous about what the tool does, but it does not explicitly differentiate itself from siblings like ai_image_editor or face_swap, so it falls 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?

The purpose implies when to use it: when the agent needs to modify a facial expression. However, there is no explicit guidance about alternatives or exclusions, such as when to prefer ai_image_editor or face_swap instead. The async workflow guidance is useful but is more about usage mechanics than use-case selection.

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