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

Annotations already mark the tool as read-only, so the bar is lower. The description adds valuable behavioral context by stating return units (BTC/kB and sat/byte) and the -1 sentinel when no estimate is available, which goes beyond the structured metadata.

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?

Three concise sentences, each earning its place: function, return format, and edge-case behavior. No wasted words, information is front-loaded, and the structure is easy to parse.

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 the tool's simplicity (one optional parameter, no output schema), the description is complete: it covers input usage, output units, and error indication. The annotations handle safety context, leaving no critical gaps.

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 for the only parameter 'target' is 100%, but the description enhances it by mapping target values to real-world timeframes (next-block, ~1 hour), making the parameter more actionable than the schema alone.

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 clearly identifies the tool's function with a specific verb and resource: 'Get fee rate estimate on Bitcoin testnet3 for a target number of blocks.' It differentiates from the sibling btc_get_fee_estimate by explicitly naming testnet3, removing ambiguity.

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?

Provides concrete guidance on parameter values ('Use target=1 for next-block, target=6 for ~1 hour'), which helps the agent select appropriate inputs. While it doesn't explicitly contrast with mainnet alternatives, the testnet3 designation in both name and description implies the correct context.

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