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

Annotations already declare readOnlyHint=false and destructiveHint=true, so the safety profile is known; the description adds the crucial scope of that destructiveness ('replacing any contact with the same contactId, other contacts are untouched') and the downstream effect ('contacts feed caller ID'). It stops short of permission/auth or rate-limit details, but that is a minor gap given the annotations.

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 short sentences, front-loaded with the WRITE marker and the destructive scope, then the id convention, then the purpose, then the endpoint. Nothing is redundant and the most decision-critical fact comes first.

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 9-parameter destructive write with no output schema and no annotations beyond hints, the description covers the essential behavior: upsert semantics, blast radius, phonebook id aliasing, and caller-ID impact. What remains thin is the undocumented optional fields and any statement of what the call returns.

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 only 44% with 9 parameters, so the description needs to compensate; instead it largely restates what the schema already says (phonebookId as email, contactId reuse for replacement) and never mentions the name, company, jobTitle or emails fields. The majority of parameters remain undocumented in both places.

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 ('add a contact to a phonebook') and immediately qualifies it with upsert semantics ('REPLACING any contact there with the same contactId'), which distinguishes it from the get/delete phonebook-contact siblings. The PUT endpoint reference reinforces the exact resource addressed.

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?

The description implies when to use it (adding or replacing a contact) and notes that the phonebook id can be a user email for a personal phonebook, which is a useful routing hint. However, it never says when to prefer this over siblings or states any prerequisites/exclusions for a destructive upsert.

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