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

Annotations already declare idempotentHint=true, destructiveHint=false, and readOnlyHint=false, and the second sentence ('Adding a tag the contact already has succeeds') largely restates that idempotency rather than adding new behavior. It does confirm observable semantics concretely, but omits mutation side effects, auth requirements, and any return indication.

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, front-loaded with the action; there is no filler. The second sentence is mildly redundant against the idempotentHint annotation but still readable and useful.

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 simple two-parameter mutation with no output schema and annotations covering the safety/idempotency profile, the description is minimally adequate. It leaves the contactId parameter and any permission or error behavior unexplained, which matters for a write operation.

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 50%: tagRef is richly documented in the schema, but contactId has no description and the tool description adds nothing about either parameter (no ID format, no name-vs-ID resolution detail). With half the parameters undocumented, the description fails to compensate.

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 precise verb+resource (add tag to contact) that matches the tool name and distinguishes it from siblings like contacts_removeTag and tags_create. It is clear but does not explicitly name or contrast with any 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?

There is no guidance on when to use this versus contacts_removeTag, contacts_update, or tags_create, and no prerequisites (e.g., tag must exist, permissions). The only contextual sentence concerns repeated calls, not tool selection.

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