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

Adds substantial context beyond annotations: malformed address or slot yields a 400 and is uncharged, pricing is $0.01/call with first 25 free and failed calls free, and repeats are billed — which explains the otherwise puzzling idempotentHint=false. It also confirms read-only semantics ('reads or computes only and changes nothing'), consistent with readOnlyHint=true.

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?

Purpose and return shape are front-loaded, followed by usage routing, error behavior, and cost. Slightly overstuffed at the end: the payment-channel detail ('x402 USDC or a card key (/credits)') and the restated charging note ('each call is charged, so repeats are not free') partially repeat the same fact.

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 2-parameter, annotation-covered read with no output schema, the description covers purpose, input formats, return value, error behavior, and cost. Nothing an agent needs to call it correctly 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 coverage is already 100%, so the baseline is 3; the description adds meaning by clarifying that the slot can be a mapping or proxy slot as well as a plain index, and supplies a concrete address example. It stops short of explaining what happens when the slot is empty or how the word is formatted numerically.

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: 'Read one raw storage slot of a contract on Optimism (eth_getStorageAt)', and states the return shape (32-byte word as number, and as address when applicable). This clearly separates it from siblings like op_call and op_proxy without requiring schema inspection.

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?

Explicitly says when to use it ('inspect contract state that has no getter, owners or implementation pointers') and names the alternative for the adjacent case ('for proxies use op_proxy'). Also explains the two accepted slot encodings, so an agent knows what inputs are valid before invoking.

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