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

Annotations already declare readOnly/idempotent/closed-world, so the description adds real operational context beyond them: the per-call price, the x402/Base mainnet payment rail, and the E.164 normalization behavior. It does not disclose what happens for invalid numbers or unsupported countries, so it is strong but not exhaustive.

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 the core action, then coverage, return value, use cases, and price in a logical order with no filler sentences. The operator brand lists are long, though they do double as coverage evidence, so the verbosity is defensible rather than wasteful.

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, no-output-schema tool, the description covers purpose, coverage, return values, cost, and payment mechanism, which is enough to call it correctly. The only gap is failure behavior for invalid or out-of-scope numbers.

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%, so baseline is 3, but the description adds meaning the schema lacks by enumerating exactly which country codes are supported (PK, BD, ID, VN, LK) rather than leaving the ISO field open-ended. It adds no format guidance for 'number' beyond what the schema already states.

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 verb+resource ('mobile network operator lookup for phone numbers') and enumerates the exact countries and operators covered, which cleanly separates it from the sibling africa_mobile_network_lookup. It also states the return shape (operator + E.164 number), so an agent knows the tool's output without opening a schema.

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 concrete downstream use cases ('before sending airtime, mobile wallet payments (JazzCash, bKash) or SMS'), which tells the agent when this tool is relevant. It stops short of explicit when-not conditions or naming alternatives (e.g., what to do for non-listed countries), so it falls just short of a 5.

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