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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the description doesn't need to repeat safety. It adds valuable behavior beyond annotations: 'Returns estimated fee in BTC/kB and sat/byte' and 'Returns -1 if no estimate available.' This gives the agent clear expectations for output and error handling.

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 compact, front-loaded sentences. The first sentence states the purpose, the second gives output units, and the third covers usage examples and error behavior. Every word earns its place with no redundancy or filler.

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?

For a simple tool with one optional parameter and strong annotations, this description is complete. It covers purpose, output format, error condition, and parameter interpretation. There is no output schema, but the description explicitly states what is returned, so the agent has full information to invoke and interpret results.

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 schema covers target's meaning (blocks for confirmation) at 100% coverage, so baseline is 3. The description enhances this with practical guidance: 'Use target=1 for next-block, target=6 for ~1 hour.' This adds real-world conversion beyond the schema's dry definition, justifying a 4.

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 states the tool's function: 'Get fee rate estimate on Bitcoin mainnet for a target number of blocks.' It specifies the resource (Bitcoin mainnet), the action (get estimate), and the key input (target blocks). It distinguishes from sibling tools by explicitly noting 'mainnet' and focusing on fee estimation, which is unique among the listed btc_* tools.

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 usage context: 'Use target=1 for next-block, target=6 for ~1 hour.' This tells the agent when to use specific parameter values. While it doesn't explicitly contrast with alternatives like btc_testnet_get_fee_estimate, the 'mainnet' qualifier implies chain selection, and the tool name itself disambiguates.

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