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

Annotations already declare destructiveHint=true, idempotentHint=false, and openWorldHint=true, so the agent knows this is a destructive, non-repeatable external write. The description adds the useful default-source detail ('current viewport by default') but does not say what gets overwritten (the prior cover), or any auth/connection prerequisites.

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 well-formed sentence with the action front-loaded and the default behavior appended after the em-dash. No wasted words, though being a lone fragment it leaves some completeness on the table.

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 destructive mutation with no output schema, the annotations and 100%-covered schema carry most of the load, and the description preserves the essential default. It still omits what the operation replaces and any precondition, which a destructive write would ideally disclose.

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 all five parameters (including the detailed 'as' identity field and the source enum) are fully documented in the schema. The description only corroborates the viewport default and adds nothing beyond the schema, so the baseline 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?

States a specific verb+resource ('Set the connected world's cover image') and adds the default source behavior, so the agent knows exactly what the tool does. It does not, however, distinguish itself from the nearby sibling 'world.set_media', leaving a possible routing ambiguity unresolved.

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 'captures the current viewport by default' implies the common usage path, and the schema's source enum elaborates the modes. But the description itself gives no explicit when-to-use vs alternatives, no prerequisites (e.g. a world must be connected), and no exclusions.

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