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

Annotations only cover readOnly/openWorld; the description adds real value beyond them by enumerating the possible result states (ok, not_found, private, deactivated, memorialized, age_restricted, restricted_minor), the exact-match matching semantics, and a 1-credit cost. This is meaningful disclosure with no output schema to carry it, though auth/rate-limit behavior is unstated.

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 action and tightly sized. The sentence 'Does this handle exist, and is it public?' restates the opening line almost verbatim and is the one piece of filler.

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 and only one parameter, the description carries the key missing piece — the state enum — plus cost and matching rules, which is close to complete for a simple lookup. It could go slightly further by noting that private/not_found results are still billable or by naming the sibling to use for full profile data.

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% and the single parameter already documents the handle format (with/without @, URL accepted). The description's only added meaning is the exact-match constraint, which is more of a result-semantics note than parameter detail, so baseline 3 applies.

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 enumerates the outcome states, so an agent knows exactly what this returns. It does not explicitly differentiate itself from near-neighbors like instagram_profile, instagram_username, or instagram_user_id, which is the only gap.

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 (a cheap existence/publicity probe, reinforced by 'Cost: 1 credit') but never stated as when-to-use vs. instagram_profile or instagram_user_id. The 'Exact match only, so a typo never resolves to someone else' line is a useful caveat but not routing guidance.

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