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

Annotations only cover readOnly/openWorld, but the description adds substantial behavioral context: the data source (mempool.space), the fact that unconfirmed balance is included, and a precise pricing model ($0.002 USDC per successful call via x402; failed calls are free). The cost and charging semantics are exactly the kind of non-obvious behavior an agent needs before invoking.

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 core content is densely front-loaded in one sentence that leads with what the tool returns. The trailing 'Information only, not financial advice' is mild boilerplate, and the return-field list overlaps with the output schema, but overall it is efficient and well-ordered.

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-parameter lookup with an existing output schema and rich annotations, the description leaves nothing essential out: source, scope, return contents, and payment terms are all covered. An agent has everything needed to decide to call it and to predict the outcome.

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?

With a single required parameter and 100% schema description coverage, the schema already documents the address field. The description's 'bech32 and legacy only' note slightly reinforces the accepted formats but adds no new syntax, validation, or format detail beyond the schema. Baseline 3 is appropriate.

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 (a Bitcoin mainnet address) and enumerates exactly what is returned: confirmed/unconfirmed balance in BTC and USD, transaction count, funded/spent totals, and last 10 transactions. It is clearly distinguishable from siblings like evm_address, solana_account, btc_tx, or address_parse, since it pins the chain and the entity.

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?

It states a clear scope constraint — 'Mainnet bech32 and legacy addresses only' — which effectively tells the agent when this tool will and will not work, and the address format hint narrows selection further. It stops short of naming alternative siblings (e.g., btc_tx for raw transactions), so it's strong context without explicit routing.

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