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

Annotations already declare readOnlyHint=true, openWorldHint=true, and destructiveHint=false, so the safety profile is covered. The description adds genuinely non-obvious behavior beyond that: the shape of a result (assigned service, description, range classification) and, importantly, that protocol details are gated behind paid plans plus a 1-credit cost. It does not describe any rate limits or failure behavior, which keeps it short of a 5.

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?

Two sentences plus a cost note, all of which carry content: what it looks up, what comes back, plan-gated fields, and price. The only waste is the redundant title echo at the start ('Port Number Lookup: Port Number Lookup'), which repeats structured fields rather than adding information.

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?

An output schema exists, so return values need not be spelled out, yet the description still summarizes the payload and flags paid-plan gating and credit cost. For a simple two-optional-parameter read-only lookup this covers nearly everything an agent needs; only the both-parameters/at-least-one ambiguity remains unresolved.

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 both parameters carry their own descriptions, so the schema already does the heavy lifting and the baseline is 3. The phrase 'from any port number or service name' confirms the either/or intent of the two optional parameters, but it does not clarify whether one is required, whether both may be passed, or which wins on conflict.

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 names a specific verb and resource: it identifies network services and port ranges from a port number or service name, and it scopes the result to well-known, registered, or dynamic ranges. That is enough for an agent to distinguish this from neighbors such as MIMETypeLookup or HTTPStatusCodeLookup, though it never names or contrasts with a sibling explicitly.

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?

Usage is only implied: 'from any port number or service name' suggests a caller supplies one or the other, and the tool is obviously a lookup. There is no statement of when to prefer this over related sibling lookups, no preconditions, and no note on what happens if both or neither parameter is given.

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