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

Annotations already declare destructive=true and idempotent=true, but the description adds real context beyond them: the account-token requirement, the domain precondition, and that the operation "does not charge". Auth and cost constraints are exactly the kind of non-annotated behavior an agent needs.

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?

Four tight sentences, front-loaded with the auth note and the core action, with no filler. The trailing "Does not charge" is slightly peripheral but genuinely useful for an agent deciding whether to invoke a billing-adjacent tool.

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 mutation tool with no output schema, it covers the essentials: auth, precondition, the two image kinds, the removal path, and cost. It omits what a successful set returns or how content conflicts with file/clear are resolved, but the schema-annotated surface keeps it close to complete.

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 documents all six parameters, including base64 content and the clear flag. The description restates content-as-base64 and clear-true-removes but adds no format or constraint detail beyond the schema, so the baseline 3 applies.

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 (sets or removes) and resource (favicon / share image) with the two image kinds named explicitly. An agent immediately knows this tool manipulates a site's icon/preview image, and no sibling tool overlaps that purpose.

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?

Provides a clear precondition ("Available once the site has its own domain") and the switch that selects removal ("clear true removes the current image"). It does not name an alternative tool, but none of the siblings serve this purpose, so the routing guidance is effectively complete.

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