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

With no annotations, the description must carry full behavioral disclosure, and it does so by stating the 2-credit cost, the no-charge policy for empty/failed results, the free 24h cache behavior versus default fresh fetch, and the caveat that some fields appear only 'when Facebook exposes them.' It doesn't discuss rate limits or auth, but the public-URL constraint is in the schema, and the disclosed items go well beyond a bare description.

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?

Four short sentences lead with the resource and returned fields, then cover cost, charge policy, and caching. No filler or repetition; each sentence adds information. The only slight cryptic element is 'video{} playback', but it conveys the object shape without bloat.

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?

Without an output schema, the description supplies the key return categories and the variability caveat ('when Facebook exposes them'), plus cost and caching behavior. It doesn't cover every edge case (e.g., private posts, redirects), but for a 2-parameter tool with a well-documented schema this is sufficient for an agent 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%, so the schema already documents both url and cache. The description's cache statement ('Pass cache=true for a free 24h cache hit (default always fresh)') restates the schema's default-false semantics, adding no new meaning. Baseline of 3 is appropriate because the description doesn't need to compensate.

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 identifies the target resource ('Facebook post or Reel') and enumerates returned content (caption, engagement, author id, video{} playback, music), distinguishing it from sibling Facebook tools like facebook_comments or facebook_page_details. However, there is no explicit verb like 'retrieves' or 'fetches', so the action is inferred rather than stated.

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?

The description implies the use case — call when you need details of a single Facebook post or Reel — but it never names alternative tools or conditions for choosing between them (e.g., facebook_comments for comments, facebook_profile_posts for a user's posts). The only guidance present is about not passing cross-platform URLs, which is a parameter constraint in the schema rather than tool-selection advice.

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