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

Annotations already declare readOnlyHint=false, idempotentHint=true, and destructiveHint=false, so the safety profile is covered structurally. The description adds a genuinely useful behavioral constraint not in the annotations: this mutation should only be performed on explicit user request.

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?

A single short sentence with the action front-loaded and the caveat placed last. Nothing is wasted and no padding is present.

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 one-parameter, non-destructive, idempotent mutation with rich annotations and a fully documented schema, the description plus structured fields give an agent everything needed. Only the absence of any note about already-favorited voices keeps it from being fully complete.

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?

There is one parameter with 100% schema description coverage, and the schema already explains it is a 'Voice token (v1:...) from list_voices.' The description adds no further meaning about the parameter, so the baseline of 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?

The description states a specific verb and resource ('Save a voice to the user's favorites'), which cleanly separates it from siblings like remove_favorite_voice and list_favorite_voices. It does not explicitly name those siblings as alternatives, but the action 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?

The parenthetical '(only when the user asks)' gives an explicit consent/timing condition for invoking the tool, which is real when-to-use guidance. It stops short of naming alternatives or stating what to do when the voice is already favorited.

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