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

Goes well beyond the readOnly/idempotent annotations: discloses that the URL requires no auth header, expires at expiresAt in about an hour, must be re-fetched rather than cached, is free with nothing persisted, and works as soon as the upload/import is stored before processing completes. These are non-obvious operational facts an agent would otherwise get wrong.

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?

Front-loads the core definition, then usage, then behavioral caveats, with every sentence carrying information. It is dense but slightly long; the parenthetical about 'raw upload or conformed transcode' and the 'read-only and free' clause could be trimmed since annotations already imply safety.

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?

An output schema exists, so return values need no explanation, and the description still flags the expiresAt field the agent will encounter. For a two-parameter read tool, the expiry, auth, and sibling-routing information covers 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 only 50%, so the description must compensate — and it partly does, clarifying that mediaItemId selects the media item whose source file is returned and tying the result to expiresAt. However, projectId is never mentioned in either the schema or the description, leaving one of two parameters undocumented.

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 (presigned download URL for a media item) and narrows it precisely to the stored SOURCE file, explicitly contrasting it with a render's MP4. An agent can distinguish it from refresh_render_url and list_media without opening any schema.

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 explicit when-to-use context ('this is how you copy media OUT of a library'), names the concrete downstream flows (fetch and PUT with create_upload, or import_media on another account), and names the alternative for a different case ('For a render's MP4 use refresh_render_url instead'). Nothing is left to inference.

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