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

Annotations already declare readOnlyHint and openWorldHint, so safety is covered; the description adds genuinely useful context beyond that: the upstream provider, that results include fee rates plus approximate confirmation duration, and the pricing model ($0.002 per successful call via x402, failures uncharged). It does not mention rate limits, freshness/lag of the estimate, or failure modes such as invalid target values.

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?

The purpose and data source are front-loaded in the first sentence, and the return content and pricing follow in logical order. The sibling-routing sentence is slightly verbose with awkward phrasing ("when you need Detailed Bitcoin block summary..."), but no sentence is expendable.

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 single-optional-parameter read-only tool with an output schema, the description covers purpose, source, defaults, and cost — enough to invoke it correctly. The only omission is guidance on estimate staleness or behavior for out-of-range targets, which is minor given the schema's documented max.

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 is 100% and the single optional targets parameter is already documented with an example and a max of 1008. The description restates the default target list (1, 3, 6, 12, 24, 144) and mentions custom targets, adding mild value but no format or behavior detail the schema lacks.

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 a specific resource (Bitcoin fee estimate curve), the unit (sat/vB), the confirmation targets, and the data source (blockstream.info), which is far more concrete than the tool name alone. It also explicitly names the two Bitcoin siblings it is not (crypto_btc_block, crypto_btc_blocks_range), so an agent can route correctly without opening schemas.

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?

There is clear implied context — use this when you need fee rates per confirmation target — and the description explicitly redirects to crypto_btc_block and crypto_btc_blocks_range for block-level data. It stops short of stating when-not to use it (e.g. vs. crypto_btc_mempool_pressure or mempool-based estimators), so it is not a full 5.

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