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

Beyond the readOnly/idempotent annotations, the description discloses the return payload (wei plus decimal amount, block number, as_of), the cost ($0.001 per call), the error behavior ("Bad input is refused before any charge"), and the two-mode execution model (raw x402 request vs. P0 key where the tool pays and returns data). This is exactly the kind of billing and side-effect context annotations cannot carry.

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?

It is a single dense paragraph that front-loads purpose and scope before payment mechanics, and every sentence carries information. The x402/key-buying clause is long and URL-heavy, which slightly hurts scannability, but it is not 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?

There is no output schema, yet the description states what comes back (wei, decimal amount, block number, as_of), what it costs, what happens on bad input, and how to actually retrieve the data in both auth modes. For a two-parameter paid lookup tool, nothing an agent needs to call it successfully is missing.

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%, with the chain enum and address pattern already documented in the schema, so the baseline is 3. The description adds only tangential meaning (that the chain determines whether the coin is ETH or POL) and no format or constraint details beyond the schema.

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 verb and resource ("native coin balance (ETH or POL) of an address") and enumerates the supported chains. It also explicitly distinguishes itself from the sibling wallet_balance for stablecoins or multi-chain needs, so an agent can route without opening either schema.

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?

"Use when you need the native coin balance" gives the positive trigger, and "Native coin on one chain only; for stablecoins or several chains use wallet_balance" gives both an exclusion and the named alternative. Nothing about selection is left to inference.

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