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

Annotations only declare readOnly and idempotent behavior. The description adds meaningful behavioral context: ordering, pagination mechanics, and the exact response shape. It does not cover edge cases like empty results or errors, but the core behavior is well disclosed.

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 sentences deliver the resource, ordering, filters, pagination workflow, and response format with no filler. The most important information is front-loaded in the first sentence.

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 is complete for a read-only paginated list tool: all six optional parameters are documented in the schema, the return shape is explicitly described, and pagination handling is covered. No output schema exists, but the description compensates by naming the response fields.

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. The description adds value by explaining country as an 'ISO-2 custom field' and clarifying that phone performs an exact E.164 single-contact lookup. It also describes the cursor lifecycle in a way that the schema alone does not fully convey.

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 uses a specific verb ('List') with a clear resource ('contacts') and states the sort order ('most recent first'), making the tool's primary behavior immediately identifiable. This differentiates it from sibling tools that operate on campaigns, segments, or templates.

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 explains optional filters and explicitly positions phone as a single-contact lookup tool ('to look up a single contact'). It also provides concrete pagination guidance ('pass the returned next_cursor to fetch the next page'). It does not name alternatives, but the tool's use case is self-contained and sibling list tools target different resources.

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