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

Annotations already cover the read-only/idempotent safety profile, and the description adds genuinely new behavior: the access tier split (poster frames for anyone vs. Pro API key for mp4s), the link lifetime (1-hour download links), and the media characteristics (~30 s clips). Auth tier and expiry are exactly the kind of context annotations cannot express.

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 dense sentences, front-loaded with what is returned and the access-tier distinction before the usage note and the slug sourcing. No filler or restatement of the name/title.

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?

With no output schema, the description carries the return-value burden and does so: poster frames and expiring mp4 links are explicitly named. Combined with the single fully documented parameter and supporting annotations, an agent has everything needed to invoke it correctly.

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 100% and the single parameter is described there ('Library slug such as instagram'), so the baseline is 3. The description goes one better by stating the provenance of slug (list_directory_apps) and the has_video filter that finds apps with videos, adding selection guidance the schema lacks.

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?

Names a specific verb+resource ('App Store preview videos for a library app') and immediately states what comes back: poster frames universally and mp4 download links under a Pro key. This clearly separates it from siblings like get_directory_screens and list_directory_apps, which list rather than return video assets.

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?

Gives a concrete purpose ('use as reference when making your own app preview video') and routes the agent to the sibling that supplies the required input, naming both the tool (list_directory_apps) and the filter (has_video=true). No inference needed to know how to obtain a valid slug.

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