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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered by structured data. The description adds real value beyond that by disclosing the returned metrics and, notably, the cost signal '(1 credit)', which an agent cannot derive from annotations. It omits auth requirements, rate limits, and error behavior for private or deleted posts, so it is not exhaustive.

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?

A single telegraphic line with zero filler; the resource and scope are front-loaded and the field list and credit cost follow. It is appropriately sized for a one-parameter tool, though the fragment style and parenthetical cost note read slightly less cleanly than a full sentence.

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 one-parameter, low-complexity tool with no output schema, enumerating the returned fields (plays, likes, comments, caption, media URLs) does the job an output schema would otherwise do. The remaining gap is failure behavior for inaccessible URLs, which is minor given the annotation coverage and simplicity.

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?

There is a single parameter with 100% schema description coverage ('Instagram post or reel URL.'), so the schema already carries the semantics and the baseline is 3. The description restates the same information in 'by URL' and 'post or reel' with no added syntax, format, or shortcode guidance.

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 description states a specific resource (a single Instagram post or reel) and a specific access mode (by URL), plus the field set returned (plays, likes, comments, caption, media URLs). That clearly separates it from siblings like instagram_user_posts or instagram_search_posts, which operate on users or queries rather than a known URL. The verb 'fetch/retrieve' is implied rather than stated, which keeps it just short of a 5.

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: you must already have a post or reel URL, which naturally excludes search-style siblings. However, there is no explicit when-to-use guidance, no mention of what to do if you only have a username or shortcode, and no stated alternatives for the same data (e.g., instagram_comments for comment threads).

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