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

Annotations already cover safety (readOnly, idempotent, non-destructive, closed-world); the description adds genuinely behavioral context beyond them: 'Pure Decimal arithmetic, no network or model' (deterministic/offline), 'Costs are salt only' (cost scope), and a maker-specific iron rule with the caveat 'other makers differ'.

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?

Inputs and constraints are front-loaded before output and behavior clauses, and the dense parenthetical asides ('no national default used', 'other makers differ') are informative. It runs long with several stacked clauses, but each carries content rather than 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, the description takes on the return-value burden and enumerates the outputs (grains/day, capacity before reserve, paired model ratings, EP42 examples, notes, assumptions, dated sources). For a 10-parameter tool the coverage is strong; only minor details like the salt_lb/capacity_grains pairing are left to the schema.

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 baseline is 3. The description restates the required-vs-optional split and the iron ppm rule, but the schema already documents those (including the 5 gpg/ppm and W4 reference), so added value over structured data is marginal.

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 precise verb+resource+scope: 'Size a water softener from household water use and tested hardness, with yearly salt cost.' Among ~90 calculator siblings, it is unambiguously the softener-sizing one, so no further differentiation is needed.

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?

'For softened water only' and 'gallons per person per day is required (no national default used)' tell the agent the operating conditions and the non-negotiable input. No explicit alternative-tool routing is given, but no sibling overlaps this function, so context is sufficient without exclusions.

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