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

Beyond the annotations it adds two valuable behaviors: passing an empty url removes the cover, and the change is stored as a revision the writer can undo. Both are consistent with idempotentHint=true and destructiveHint=false, so no contradiction. It still does not say what the call returns or whether it fails on a published post.

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?

Very short and front-loaded with the scoping constraint, with nothing padded. The telegraphic sentence fragments, however, cost parse effort — several clauses have no verb or subject, making the structure harder to read than a single plain sentence would be.

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 five-parameter mutation with no output schema, the description covers the url semantics, alt/caption intent and undo behavior, which is most of what an agent needs. The two gaps are the required post_id and any error/permission conditions, leaving it adequate but not 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 coverage is only 20%, so the description must compensate — and it partly does, explaining that url is an 'address' resolved from upload_media or the library and that an empty value means removal, plus naming alt text and caption. post_id, a required parameter, is left completely undocumented in both the schema and the description.

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 fragments identify the resource ('The cover picture, by address — from upload_media or the library'), but no verb ever appears: the reader must reconstruct 'sets' from the name/title. It does state scope ('On one site of the account, named in site'), yet never distinguishes this from siblings like edit_post or update_post_settings that could plausibly also touch a post's image.

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?

'from upload_media or the library' usefully points at the sibling that produces the url, which is real routing guidance. However, there is no statement of when to prefer this over edit_post or update_post_settings, and no prerequisite is given (e.g. whether the post must be in an editing session).

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