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

Goes well beyond the annotations by disclosing that the tool reads or computes only and explicitly explaining why readOnlyHint is false (each call is charged), which pre-empts an apparent contradiction. It also states cost ($0.002/call), payment paths (x402 USDC or card key), the 25-free-call allowance, and that failed calls are free.

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 returned data, then usage, then pricing/behavior in a tight three-sentence block with no filler. The opening sentence is long due to its output enumeration, but every clause carries distinct information.

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?

An output schema exists, so return-value structure need not be re-explained, and the description still summarizes the key fields. Combined with the usage triggers, pricing, and the rationale for the non-read-only flag, nothing an agent needs before calling is missing.

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 there is nothing for the description to disambiguate; baseline 4 applies. The description instead characterizes the shape of the response fields, which is a reasonable substitute.

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?

Names the resource (Bitcoin network fee rates) and enumerates the exact outputs: next-block, half-hour, one-hour and economy rates in sat/vB, sats cost of a typical payment, mempool size/pending fees, and latest block height. This is unmistakably distinct from btc_address or any of the chain-specific *_gas 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?

Gives explicit trigger conditions: 'Use before sending a Bitcoin transaction or to decide whether to wait for lower fees.' That is a clear when-to-use, though it never names an alternative tool or a when-not-to-use case (e.g., historical fee lookups or other chains).

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