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

Annotations only declare readOnlyHint and openWorldHint; the description adds real value beyond them by disclosing the external dependency (live EPEX DE-LU spot substitution) and the commercial terms — $0.002 USDC per successful call via x402 on Base/Solana/Arc, with failed calls not charged. That payment/auth detail is exactly the kind of behavior an agent needs before invoking. It doesn't cover latency, rate limits, or precision of the returned number.

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 core definition, then the default/fallback mechanics, then pricing terms — a sensible order. Three dense sentences with only modest duplication of the defaults that appear again in the schema; nothing is padded or vague.

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?

Given 7 optional parameters with full schema coverage, an output schema, and read-only/open-world annotations, the description covers everything an agent needs: what is computed, what units, what defaults apply, where the one external input comes from, and what it costs. No material gap remains.

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 same default values the schema already documents (55 kWh/kg, 1000 €/kW, 8% WACC, 20 years, 4000 hours, 3% O&M), adding only the fallback behavior for electricityPrice, which the schema also states. Little semantic value beyond the structured fields.

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+resource+unit: levelized production cost (LCOH) of green hydrogen per kg from an electrolyser, with the cost components enumerated (capex annuity, fixed O&M, electricity, water). The closing line 'A transparent calculator, not a traded hydrogen price' explicitly separates it from the spot/price-feed siblings in the energy_* family.

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?

Clear context for invocation: works 'from your inputs or defaults', and the all-important conditional is spelled out — omit the electricity price and the latest EPEX DE-LU day-ahead baseload is substituted. It gives no explicit name-based routing to alternatives (e.g. energy_epex_spot_power_europe for the raw price, energy_ammonia_fertilizer_spot for a traded benchmark), so it stops just short of full when/when-not guidance.

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