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

Annotations already declare readOnlyHint=false, destructiveHint=false, idempotentHint=true; the description earns credit by disclosing the merge semantics ('existing tags are kept') that explain why the operation is non-destructive. It stops short of covering permissions or failure behavior, but with annotations present that is a minor gap.

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?

Two short sentences, both front-loaded with the mutation semantics first and contact-location second. No filler or redundancy.

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 rich annotations, a fully described 3-parameter schema, and an output schema, the description covers everything an agent needs: what is merged, what is preserved, and how the contact is identified.

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%, so the schema already documents tags, email, and contact_id in detail. The description restates the either/or locating rule but adds no syntax, limits, or format information beyond the schema; baseline 3 applies.

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 ('Merge one or more tags onto a contact') and adds the distinguishing scope detail that existing tags are kept, which separates it from an update/replace operation like update_contact. An agent knows exactly what the call does without opening the schema.

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 operational context: locate the contact by contact_id or email, and tags are merged rather than replaced. It does not name explicit alternatives or when-not conditions (e.g., how to remove or overwrite tags), which keeps it short of a 5.

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