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

Beyond the readOnly/idempotent annotations, the description discloses critical runtime behavior: unavailable measurements remain null, unpublished severity stays 'unknown', the certified denominator is specifically USLAX, payment is required (402 if unpaid), and it returns an unchanged API envelope. These are non-obvious traits that significantly aid correct invocation.

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 compact (four sentences) and front-loaded with the purpose. Each sentence earns its place: purpose, data behavior, payment, and return shape. No filler or redundancy.

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 compensates by providing the exact return envelope ('{ port, port_name, current_level, days_back, history[] }'). It also covers payment failure, port code specifics, and sibling distinction. For a 3-parameter read-only tool, this is fully contextual.

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?

The input schema already covers all 3 parameters with descriptions, so the baseline is 3. The description adds meaningful context: the port denominator (USLAX), the payment mechanism for x_payment, and the return envelope showing how days_back is used. This exceeds the schema's static definitions.

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 opens with a specific verb+resource+scope: 'Get certified port-authority congestion measurements for a specific port.' It clearly differentiates from the sibling tool by stating 'For disruption advisories, use shippingrates_congestion_news instead.' This makes the purpose unambiguous.

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 names an alternative for a distinct use case ('For disruption advisories, use shippingrates_congestion_news instead'), provides payment prerequisites ('PAID: $0.02/call... Without payment, returns 402'), and explains data availability behavior ('Only fields explicitly published by the authority are populated'). This gives clear when-to-use and when-not-to-use guidance.

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