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

Goes well past the annotations: no-contract returns 404, non-ERC-20 contract returns 422, both uncharged; pricing is $0.01/call via x402 USDC or card key, first 25 free, failed calls free. The "each call is charged, so repeats are not free" note is consistent with idempotentHint=false and warns about metered repetition.

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?

Front-loads purpose and scope, then folds error codes, pricing and the native-coin alternative into two dense sentences. Slightly over-packed, and "Reads or computes only and changes nothing" restates readOnlyHint/destructiveHint, but no sentence is wasted.

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 still enumerates the return shape (raw amount, decimals, symbol, human-readable amount), plus failure modes and cost. For a 2-parameter read tool this covers everything needed to call it correctly and interpret the result.

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 both parameters are already documented; the description adds the balanceOf source and return fields but little about the inputs. The line "Both inputs are 0x addresses (40 hex characters)" is imprecise for `token`, which the schema and the opening sentence both say accepts a ticker such as USDC — a minor contradiction the agent must resolve from 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?

Names a specific verb+resource+chain: ERC-20 balance of a wallet on Optimism, sourced via balanceOf from the token contract. It explicitly separates itself from the sibling op_balance (native coin) and implicitly from op_token_info/op_allowance, so an agent can route without opening any 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 a concrete when-to-use ("check stablecoin or token holdings before a transfer or trade") and an explicit when-not with the named alternative ("for the native coin use op_balance"). That is exactly the routing information an agent needs among this large sibling set.

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