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

Beyond the read-only/open-world annotations, it discloses the critical hidden behavior: this is a paid call at $0.002 USDC via x402 on four chains, that x402 MCP clients pay automatically, and that an empty input yields a free sample. Cost and payment mechanics are exactly the kind of context annotations cannot convey.

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 return contents, followed by the call trigger and then payment logistics. Every sentence carries information an agent needs, though the payment/x402 sentence is dense and could be trimmed slightly.

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?

There is no output schema, but the description compensates by enumerating the returned fields in detail. Combined with the payment disclosure and the free-sample path, an agent has everything needed to decide and invoke correctly.

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?

The tool takes zero parameters, so the baseline is 4. The description correctly signals this by pointing to the empty example input {} as the free sample invocation, leaving no ambiguity about required inputs.

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 specific resource (Bitcoin fees right now) and enumerates the exact outputs: recommended sat/vB for next block/30min/1hr/economy, USD cost, mempool size, tip height, difficulty adjustment, and BTC price in USD/EUR. It is unmistakably distinct from sibling tools like gas_price (EVM), btc_tx, or crypto_prices.

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?

Gives an explicit trigger: 'Call before sending a Bitcoin transaction.' It also tells the agent how to get a free sample via the empty example input. It does not name an alternative tool or state when-not to use it, so it stops short of a full 5.

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