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

Annotations declare a non-read-only, non-idempotent, destructive write, and the description reconciles that by disclosing the actual cost model: each lookup is billed against the D7 credit balance and requires routing to be enabled. It also clarifies the non-effect (no message is sent) and the underlying REST endpoint, which annotations cannot convey.

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-loads the operation type and cost warning ('WRITE (spends credits)') in the first clause, then packs prerequisites, non-effects, and the endpoint into compact clauses with no filler.

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 one-parameter tool with no output schema, the description covers the safety/billing profile, the routing prerequisite, what is not done, and the backing endpoint. Nothing an agent needs to invoke it correctly is missing.

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% and the single parameter's format (international, with country code, example) is already documented in the schema. The description adds only the emphasis that lookup is for ONE number, which is marginal value over the schema.

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 and resource (look up one phone number) and enumerates exactly what is returned: country, network, MCC/MNC, ported, reachable, mobile vs fixed. It also differentiates from d7_send_sms by explicitly stating it does not message the number.

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 clear preconditions for use (credits are spent, lookup routing must be enabled) and points at d7_get_lookup_report to find the cost, which routes the agent between siblings. It does not, however, state when to prefer a different sibling over this one (e.g., bulk vs single lookup), so it falls short of explicit alternatives.

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