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

Beyond the annotations (readOnly, idempotent, openWorld, non-destructive), the description discloses the crucial non-obvious behavior: the default return is an unpaid x402 request (GET /evm/code at $0.002 per call) plus a schema link, and only a P0 access key causes the tool to actually perform the paid call and return data. It also states that bad input is refused before any charge, which is exactly the kind of cost/error semantics an agent cannot infer from the annotations.

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-loaded with a 'Use when' trigger, then the return shape, then pricing and the access-key fork. It is dense but every sentence carries operational information; the parenthetical URL for buying a key is long but necessary for the fallback path.

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 two-parameter read tool with no output schema, the description covers what comes back, the two invocation modes (payment-request generation vs. direct paid call with a P0 key), the price, and error-before-charge behavior. An agent can act correctly with only this text.

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% and the enum already enumerates the five chains with defaults, so the schema does the work. The description adds only marginal meaning by naming those chains in prose and implying the address is the subject of the contract check; it adds no format or edge-case detail beyond the schema's pattern.

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 specific decision the agent needs ('whether an address is a contract') across five named chains, and enumerates the three returned fields: is_contract, bytecode size, sha256 of the bytecode. That clearly separates it from siblings like evm_native_balance, evm_nonce or evm_transaction, which query different on-chain facts.

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 an explicit trigger ('Use when you must know whether an address is a contract') and implicitly scopes it against the other evm_* siblings through that framing. It does not name an alternative tool or state when not to use it, so it stops short of full routing guidance, but the intended context is unambiguous.

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