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

Goes well beyond the annotations: discloses that assignments are overwritten (consistent with destructiveHint=true), that a mistyped group ID is silently resolved to a name, and that `shared` discloses associations to corporate members. That is real behavioral context an agent could not infer from the flags alone.

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 tight sentences plus a labeled side-effects line; the critical constraint (names, not IDs) and the overwrite warning are both front-loaded. No filler.

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 destructive, non-idempotent mutation with an output schema, the description covers the mutation semantics, the overwrite risk, the sharing implication, and the ID pitfall. Nothing an agent needs to invoke it safely is missing.

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 baseline is 3, but the description adds value beyond the schema by giving a concrete group-name example, reinforcing the name-vs-ID distinction, and clarifying the shared/personal scope. The ID-resolution fallback is behavior the schema does not express.

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 ('Assign contacts to contact groups') and immediately names the domain concept (Fax.Plus tags), distinguishing it from create-contact-group by warning that group names, not IDs, are required. An agent can tell exactly what this tool 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 operative guidance: pass group names (e.g. Vendors), not IDs, and warns that prior assignments are overwritten. It doesn't explicitly contrast with sibling mutators like update-contact or share-contacts, so the when-not-to-use side is weaker.

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