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

Annotations already declare readOnlyHint, openWorldHint, and destructiveHint=false, so safety is covered. The description adds genuinely useful non-schema behavior: the call costs 2 credits and several fields are gated behind paid plans. It stops short of saying what happens on a free plan if hourly/daily are requested.

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 opening is front-loaded and clear, but it redundantly restates the title ('Ethereum Price: Ethereum Price is...') and then spends most of its length on a marketing list of paid-plan features (supply, all-time highs, 30-day stats) that is low-value for tool selection.

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 an output schema present, return values need no explanation, and annotations cover the safety profile, so the description only needs purpose, cost, and tier constraints — all of which it supplies for this simple three-optional-parameter tool. The main residual gap is behavior when paid-only fields are requested without a paid plan.

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 100%, so a baseline of 3 would be acceptable, but the description actually adds meaning: '172 currencies' maps to the currency parameter, and 'hourly and 30-day daily history' maps to the hourly and daily flags while implying these are paid-tier features. That tier mapping is information the schema does not convey.

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 states a concrete resource and scope: a spot price feed for Ethereum denominated in US dollars, plus movement direction. The resource name distinguishes it implicitly from BitcoinPrice, GoldPrice, and SilverPrice, but it never explicitly routes the agent to those siblings.

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?

There is no when-to-use guidance at all — no mention of when to prefer CurrencyRate, ExchangeRate, or BitcoinPrice, and no explanation of when an agent should set the daily/hourly flags. The only contextual hint is a passing reference to paid plans, which is a pricing note rather than usage guidance.

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