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

Annotations already declare this is a safe, closed-world read, so the description correctly spends its words elsewhere: it discloses a per-call price ($0.001 USDC via x402) and, crucially, the failure mode — an unpaid call returns payment requirements rather than a result — plus three concrete remediation channels. That is real behavioural information an agent cannot get from the annotations or schema. It stops short of describing the success payload or unit/rounding semantics of the computed output.

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?

Three sentences, no filler: capability first, scope exclusion second, payment mechanics third. The payment sentence is dense with header names and a JSON pointer, but that density is load-bearing rather than decorative. Slight cost for cramming three remediation options into one clause.

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 burden of describing results, and it only covers the failure payload (x402 requirements). It never says what a successful response contains — presumably dimensional kg, actual kg, billable kg after ceiling rounding — so an agent cannot confirm the return shape. The core computation, inputs, and payment contract are otherwise complete.

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%, so the parameters are already fully documented, including "omitted means no carrier rounding" for roundingStepKg and the positivity/max bounds. The description only adds the qualifier "caller-supplied" for the divisor (useful, since it signals the tool does not assume a carrier-standard divisor), which is a marginal value-add over the schema. 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?

"Calculate dimensional and billable kg" gives a specific verb and a precise output resource, and the exclusion "No carrier price lookup" fences off the adjacent pricing use case an agent might otherwise assume. An agent can tell immediately that this is a pure arithmetic/rating-normalisation helper and not a shipping-quote tool.

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 usage (you supply cm, kg, divisor and optionally a rounding step) and gives explicit operational guidance for the payment failure path, including the header names to retry with. What it lacks is any statement of when to prefer this over an alternative, or prerequisites such as whose divisor to supply (carrier-specific vs. standard 5000/6000).

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