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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds provenance (derived from Google's libphonenumber metadata) and the determinism implicit in "exact digits", which usefully tells the agent the output is authoritative rather than heuristic, but it doesn't add error/edge-case behavior beyond that.

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 core deliverable, then the routing rule, then the parameter combinations, then provenance. The shared-code-zone parenthetical is the only aside and it earns its place by preempting a common NANP confusion. No filler sentences.

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?

Parameter combinations, defaults, sibling routing and the source of the data are all covered, and an output schema exists so return values need not be described. Nothing an agent needs to invoke this 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 coverage is already 100%, so 3 is the baseline, but the description adds genuine conditional semantics the schema lacks: a + international number needs no destination, a local-format number does, and destination-only returns the country rule plus a worked example. That combination logic is the difference between a correct and an incorrect call.

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 concrete verb+resource (produce the exact digits to dial a number abroad) and enumerates the payload: exit code, country code, national number with trunk prefix handling, and the + form. It also distinguishes itself from its only sibling by naming get_call_price as the tool for cost questions.

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?

Explicit routing: use this for any "how do I call this country/number" question, and use get_call_price instead for costs. It goes further by giving the alternative invocation modes (number alone, number+destination, destination only), so the agent knows not just whether but how to call it.

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