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

Annotations already declare readOnly/openWorld/non-destructive and non-idempotent, but the description adds materially more: cost ($0.01/call via x402 or card key), first 25 free, failed calls free, and that repeat calls are billed. It also confirms the call reads or computes only and mutates nothing, plus how invalid targets surface as errors.

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?

One front-loaded paragraph: purpose and return shape first, then the sibling routing hint, then input/error/pricing constraints. Every clause carries operational value, though the pricing and error tail-ends make it denser than strictly necessary for a two-parameter read.

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?

No output schema exists, so the description carries return-shape duty and does so (raw amount, decimals, symbol, human-readable). Cost model, error codes, and sibling routing are all covered, leaving nothing an agent needs to call this correctly.

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%, so the schema already documents both parameters (token accepting a ticker or 0x contract address, address as a 0x wallet), making 3 the baseline. The description adds length detail (40 hex characters) but its phrasing "Both inputs are 0x addresses" partially contradicts the schema's ticker option for `token`, adding muddle rather than clarity on that parameter.

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?

States a precise verb+resource: ERC-20 balance of a wallet on Base, resolved via balanceOf, and enumerates the returned fields (raw amount, decimals, symbol, human-readable amount). It explicitly distinguishes itself from base_balance for the native coin, so an agent can route correctly 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?

Gives the concrete use case ("check stablecoin or token holdings before a transfer or trade") and names the alternative for native coin (base_balance). It also specifies error semantics for the address argument (404 for no contract, 422 for non-ERC-20), which is guidance an agent needs to interpret outcomes.

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