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

Annotations already declare readOnlyHint/destructiveHint=false, but the description adds non-obvious behavior: numbers come back masked, contact_ref pins the current name-and-number mapping, and full numbers must never be echoed in chat. These privacy and snapshot semantics go beyond the structured metadata.

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 what the tool returns in the first clause, then layers the contact_ref contract, ambiguity rule, and privacy rule in tight sentences. No filler; every sentence constrains agent behavior.

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?

With zero parameters, read-only annotations, and an output schema present, the description covers the remaining gaps: masking, the contact_ref handle, ambiguity handling, and chat privacy. Little more is needed for an agent to call it and use the result correctly.

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?

The tool takes zero parameters, so the baseline applies; there is no parameter surface for the description to clarify. The mention of contact_ref pertains to the output/returned handle rather than an argument, which is appropriate given the output schema exists.

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 ('List the token owner's saved contacts') and adds a meaningful qualifier ('with masked phone numbers') that tells the agent what kind of data comes back. It does not explicitly distinguish itself from save_contact/update_contact/delete_contact, but the read-only listing intent is unambiguous.

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 actionable context: contact_ref values feed call_contact, and ambiguous name matches should be resolved by asking the user for one exact name. That is real when/how guidance. It stops short of naming an alternative tool or an explicit when-not condition, so it is not a 5.

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