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

Annotations already declare readOnlyHint, destructiveHint=false and idempotentHint=false, so safety is covered structurally. The description adds genuinely non-structural facts: it is a pure read/compute, each call is billed at $0.01 via x402 USDC or card key, the first 25 are free, and failed calls are free – which is consistent with idempotentHint=false (repeats cost money). It does not spell out the exact response shape, keeping this from a 5.

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?

The core purpose is front-loaded in the first clause and the billing terms follow, which is the right order. The dangling fragment "Example: bill=84.5." is an unfinished sentence that earns nothing, a minor but real waste.

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 still makes clear what comes back (tip amount, total, and per-person share, optionally rounded) and covers cost and read-only behavior. A 5 would require explicit statement of the returned fields or default handling when people/percent are omitted.

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 bill, people, percent and roundUp are all already documented in the schema, establishing a baseline of 3. The description's note about rounding each share up restates the roundUp schema wording and adds no new constraints (units, defaults, precision) beyond it.

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 names the exact computation (tip and total for a bill at a given percentage, split per person, optional per-share rounding) with a concrete resource and scope. No sibling in the toolset (calc_loan, calc_bmi, calc_age) does this, so an agent can route here without opening the schema.

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?

"Use for receipts, expense apps and group payments" gives clear situational context for when to pick this tool rather than a generic arithmetic path. It stops short of naming exclusions or an alternative (e.g., splitting without a tip), so it is context-rich but not a full when/when-not rule set.

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