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

Annotations already declare readOnlyHint and openWorldHint. The description adds value by specifying the exact return shape ([fee_rate, cumulative_vsize] pairs) and unit (sat/vB), which is behavior beyond the annotations. It does not note any limitations or edge cases, but for a simple read-only tool this is adequate.

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?

Two concise sentences with no filler. The purpose is front-loaded, and the return format explanation earns its place. Ideal length for this tool.

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 has no parameters and no output schema, the description fully explains what the tool returns and how to interpret it. The mainnet scope is clear, and the return format is sufficiently detailed for an agent to use correctly. No significant 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?

The input schema has zero parameters, so there is no parameter information to add. The baseline for zero-param tools is 4, and the description appropriately focuses on the return value rather than parameters.

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 uses a specific verb ('Get') and resource ('mempool fee histogram') and clearly scopes to 'Bitcoin mainnet', distinguishing it from the testnet sibling btc_testnet_get_mempool_info. It also explains the return format and units, making the purpose unambiguous.

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?

The description clearly indicates this is for the mainnet mempool fee histogram, which sets context. However, it does not explicitly mention when not to use it or mention alternatives like btc_get_fee_estimate, which could be confused for fee-related data. The context is clear but lacks explicit exclusions or alternative guidance.

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