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

Annotations already declare readOnlyHint=false, destructiveHint=false and, importantly, openWorldHint=true. The description adds value beyond that: it confirms the source is fetched from a public URL, gives a concrete 100 MB ceiling, enumerates per-platform attachment caps, and states the immutable-state restriction. It omits failure behavior (unsupported type, URL fetch failure) and whether re-attaching replaces or appends.

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 the action, then supported inputs/limits, then the state restriction. No filler or repetition of the name.

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?

There is no output schema, and the description never says what comes back (an attachment ID, a media object, plain confirmation) or how to learn about existing attachments. For a two-parameter mutation tool this is a minor but real gap; everything else an agent needs to call it is present.

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%, so the baseline is 3. The description still adds real meaning about the `url` parameter's acceptable inputs (public http(s) source, specific media types, 100 MB cap) and clarifies that `postId` must be a draft, going beyond the schema's bare "The draft post ID".

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 pair ("Download ... and attach") plus the resource (image or video) and the exact target object ("a Postel draft"). This clearly separates it from create-post/update-post sibling tools, which modify post content rather than attaching media.

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 clear operating context (drafts only) and an explicit when-not clause: "Scheduled and published posts cannot be changed." It does not point to an alternative tool for editing already-scheduled posts, which is the one gap.

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