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

Beyond the readOnlyHint/openWorldHint/idempotentHint annotations, it discloses the billing model ($0.002/call via x402 USDC or card key, first 25 free), that failed calls are free, and that repeating a call is charged again — behavior that directly explains the idempotentHint=false signal. It also confirms the operation is read/compute-only, consistent with the annotations.

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 first sentence front-loads what is returned, followed by use cases, an example, then cost/behavior notes. It is dense but every sentence carries information; the pricing clause is the only part that could be trimmed for some audiences, though it is decision-relevant here.

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 enumerates the return fields (per-currency prices, sats per USD, price time) and covers cost, failure handling, and side-effect profile. An agent has everything needed to call it and interpret the result.

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% and the schema already explains that the seven listed currencies come from the source while others are converted at the ECB reference rate. The description adds only an example value (currency=SEK) and the list of currencies, so the baseline of 3 is appropriate.

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 specific resource and scope: current BTC spot price in seven named currencies, averaged across major exchanges, plus satoshis per USD and a timestamp. That is concrete enough for an agent to distinguish it from generic siblings like crypto_price, price_token, or fx without opening a 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?

It gives three explicit use cases (sats/fiat conversion, valuing a Bitcoin or Lightning payment, pre-trade price check) and a concrete call example with currency=SEK. It stops short of naming a fallback tool or stating when not to use it, so it does not reach the 'alternatives' bar of a 5.

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