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

The description discloses the key behavioral consequence up front: 'This overwrites previous group assignments for the selected contacts.' It also adds a separate 'Side effects' line and notes the tool will resolve group IDs to names before calling the API. These details complement the destructiveHint=true annotation without contradicting it.

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?

The description is compact—five short sentences—and front-loads the core purpose before adding warnings and side effects. The 'Side effects' line is slightly redundant with the overwrite statement and the destructiveHint, but it is brief and clearly labeled. Overall, 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?

The description covers purpose, overwrite semantics, group-name requirement, and ID resolution, and an output schema exists, so return values need not be explained. A notable gap is the behavior when groups is omitted—since only contact_ids is required, clearing existing assignments is not documented. This leaves some ambiguity for a valid call shape.

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 description coverage is 100%, so the baseline is 3. The description adds value by warning that group IDs from create-contact-group are not acceptable and that group IDs mistakenly passed will be resolved to names. However, the groups and shared parameters are already well described in the schema, so the extra contribution is modest.

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 starts with a specific verb-object: 'Assign contacts to contact groups,' and clarifies these are Fax.Plus tags. It distinguishes itself from create-contact-group by explicitly saying to pass group names, not group IDs from that tool. This is more than enough for an agent to know what operation this performs.

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 gives no explicit when-to-use or when-not-to-use guidance relative to sibling tools like share-contact-groups or create-contact-group. The need to assign contacts to groups is implied by the first sentence, and the 'pass group names, not IDs' warning is more about parameter format than tool selection. It therefore only implies usage rather than spelling out alternatives.

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