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

The annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, and the description usefully adds provenance details (source URL, fetched_at in RFC 3339, source_sha256 of the upstream body). However, it omits the most consequential behavior: an unpaid first call returns a 0-sat Lightning invoice and macaroon rather than a price, a fact that appears only inside the parameter schema.

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 short sentences, front-loaded with the core capability, followed by source and freshness, then the provenance fields. No filler or repetition.

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?

With no output schema, the description must convey the return shape: it lists the provenance fields and mentions USD plus other currencies, but does not describe the price field structure or currency keys. It also fails to surface the payment-gated first-call behavior, which is essential to calling the tool successfully.

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 description coverage is 100% and the l402_proof parameter's description fully documents the invoice-then-retry flow, so the schema carries the parameter burden. The tool description itself says nothing about l402_proof, so it adds no meaning beyond the schema; baseline 3 applies.

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 states a specific verb and resource ('Get the current Bitcoin price in USD and other major currencies') plus the upstream source (mempool.space), so an agent can immediately tell it apart from sibling utilities like btc_send_decision. It stops short of naming which 'other major currencies' are supported, leaving the return shape partially open.

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 no explicit when-to-use or when-not-to-use guidance, and no routing to or away from siblings such as btc_send_decision or optimal_send_window. 'Real-time' implies it is for fresh quotes, but that is inference rather than stated guidance.

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