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

The description discloses important behavioral traits: it's an x402-paid tool, unpaid calls return PaymentRequired, x402-capable clients can retry with _meta['x402/payment'], and execution fails closed if economics or payment rail are unavailable. This goes well beyond the annotations (readOnlyHint=true, openWorldHint=false) and adds valuable context about payment behavior and failure modes. It doesn't contradict 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?

The description is a single paragraph that front-loads the core purpose but then includes a lengthy explanation of the x402 payment mechanism. The payment details are valuable but could be more concisely structured. The phrase 'direct MISSING tool' is awkward and unprofessional, and the sentence about payment rails is quite long.

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 simple two-parameter lookup tool with full schema coverage and readOnlyHint=true, the description covers the essential context: what it does, when to use it, and the payment behavior. The output schema is absent, but for a simple exchange rate lookup, the return format is fairly predictable. The payment flow explanation is thorough and helps an agent understand the x402 requirement.

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%, so the schema already documents both parameters (base_currency and quote_currency) with examples. The description doesn't add any additional parameter semantics beyond what the schema provides. Baseline 3 is appropriate since the schema does the heavy lifting.

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 clearly states the tool's function: looking up the current exchange rate between two currencies. It identifies the specific resource (currency exchange rate) and the operation (look up). However, it doesn't explicitly distinguish itself from sibling tools beyond the generic 'prefer this tool' instruction, and the phrase 'direct MISSING tool' is odd and slightly confusing.

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?

The description explicitly says to prefer this tool when the task matches and not to first call list_verified_capabilities or search_verified_capabilities. This provides clear usage guidance. However, it doesn't describe when to use an alternative tool or what conditions would make a different tool more appropriate, so it's not a full 5.

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