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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false. The description adds context about the data source ('Derived from Google's libphonenumber metadata') and the deterministic nature ('Exact dialing instructions'), which is useful for the agent. No contradictions.

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?

Three sentences with no fluff. The first sentence front-loads the core function, the second explains input modes, and the third gives provenance. Every sentence is valuable.

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?

Given the read-only, idempotent annotations and an output schema, the description is sufficiently complete. It explains what the tool returns (digits, + form, step-by-step guidance) and the input scenarios. It misses explicit error handling, but the output schema likely covers that.

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% since each parameter has a clear description. The description enriches semantics by explaining parameter combinations: passing number yields exact sequence, omitting number and passing destination yields rule and example. This goes beyond the schema's simple field 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?

States a specific verb+resource: 'Exact dialing instructions from one country to another' with clear output details (exit code, country code, national number, + form). It distinguishes from sibling get_call_price by focusing on dialing sequence rather than price, though it does not explicitly name the sibling.

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?

Provides clear usage context: pass a specific number for exact sequence, or just destination for rule and example. This tells the agent when to use different parameter combinations. It does not explicitly compare to sibling get_call_price, but the distinct purpose makes the alternative obvious.

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