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

Annotations already declare destructiveHint=true, idempotentHint=true and openWorldHint=true, so the low bar is met; the description still adds real value by spelling out the destruction model — the existing 9 is replaced, and an empty list removes it entirely. It also discloses visible side effects (shown on the user's page, shareable as a poster), which is not inferable from the annotations.

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?

Two sentences, front-loaded with the verb and resource, and the destructive consequence ('an empty list removes it') is placed last where it stands out. The page/poster clause is contextual rather than essential but is short and earns its place by explaining why the 9 matters.

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 three-parameter mutation with no output schema but strong annotation coverage, the description covers the mutation model, the clearing case and the visibility of results. Gaps remain: what happens to titles that do not match anything in the user's library, and what occurs if both titles and item_ids are supplied.

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 items, ordering ('first to last'), the maxItems=9 cap and the titles-vs-item_ids choice. The description only adds the scoping notion of 'one kind' for media_type and reinforces order relevance; the mutual exclusivity of titles and item_ids is left to the schema's 'use instead of titles' note.

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 (set) and a specific resource (the user's My 9) and even defines the concept — the nine titles that made them, in order, from their own library. That is enough to distinguish it from siblings like set_page_sections or pin_to_page, though it never names a sibling explicitly.

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?

Usage is implied rather than stated: 'for one kind' plus the required media_type param signals one My 9 per media type, and 'replaces that 9' implies overwrite semantics. There is no explicit when-to-use/when-not guidance and no pointer to an alternative tool (e.g. get_my_page to read the current 9 first).

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