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

Annotations already cover the safety profile (readOnlyHint, openWorldHint), and the description adds genuinely useful context beyond that: the full set of possible states (ok/not_found/private/deactivated/memorialized/age_restricted/restricted_minor) and the 1-credit cost. Missing auth/permission notes, but this is a strong disclosure layer over the annotations.

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-loaded with the purpose and result states, and the cost line is a useful trailing note. The second sentence ("Does this handle exist, and is it public?") is a near-verbatim restatement of the first and does not earn its place, costing it a point.

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 by enumerating the possible states, plus cost. For a simple one-parameter read tool this is close to complete; it lacks only notes on auth requirements or rate limits.

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 description coverage is 100% and the single username parameter is fully documented in the schema (handle with or without @, URL accepted). The description adds only the "exact match" constraint, so it sits at the baseline rather than adding real parameter detail.

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 and resource ("Check if an Instagram handle exists and is public") and the enumerated result states make the scope unambiguous versus data-fetching siblings like instagram_profile or instagram_media. It does not explicitly name the sibling tools it differs from (e.g. instagram_search, which does fuzzy matching), so it stops 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?

"Exact match only, so a typo never resolves to someone else" implicitly tells the agent this is not the tool for discovery/lookup-by-partial-name, and the credit cost signals a lightweight validation call. However, no explicit when-to-use or when-not-to-use guidance is given relative to instagram_username or instagram_search.

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