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

With no annotations supplied, the description carries the full burden and does well: it discloses the per-call price (€0.0005), the auth mechanism (API key in Authorization: Bearer), the failure mode (422 on dimension mismatch), and an important semantic caveat (temperature offsets handled correctly). It does not explicitly state that the operation is side-effect-free/pure, which is the one behavioral trait left implicit.

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?

Two dense sentences plus a parameter gloss, front-loaded with purpose, then validity/errors, then cost and auth, then parameters. Nothing is padded and the ordering matches how an agent needs the information.

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?

An output schema exists, so return values need no explanation, and cost, auth, and error semantics are all covered for a simple 3-parameter conversion tool. The remaining gaps are the number-versus-string conflict for value and the unstated fact that the schema marks every parameter optional.

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 0% and the schema properties are bare anyOf string/null, so the description must supply meaning — and it does, naming all three parameters plus concrete unit examples ('celsius', 'km/h'). The one flaw is a type mismatch: it calls value a number while the schema declares string-or-null, and it never mentions that all three parameters are optional-by-schema though practically required.

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 ('Converts a value between physical units') and enumerates domains (length, mass, speed, temperature) with the notable 'correct offset handling' qualifier. This implicitly separates it from neighbours like tz_convert or fx, though it never names an alternative explicitly.

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?

Tells the agent what inputs are valid ('any pint-recognized unit name or symbol') and what happens on failure ('incompatible dimensions return 422'), which is useful invocation guidance. It gives no when-to-use versus when-not guidance relative to siblings such as tz_convert (time zones) or math_eval, leaving that routing to inference.

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