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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered. The description contributes nothing behavioral beyond restating the HTTP path: no mention of pagination semantics, result shape, empty-field handling, or permission requirements.

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?

It is short and front-loaded with no filler, but the second sentence is a raw endpoint path that adds little beyond what the name and annotations already convey, and the whole text is a fragment rather than a complete statement of behavior.

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?

With no output schema and a 3-parameter tool where contact_id is undocumented, the description would need to explain the return content (which custom fields come back, pagination shape). It does none of this, leaving the agent to infer everything from the name and endpoint.

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 67%: page and limit are documented in the schema, while contact_id has no description anywhere. The description adds no syntax, format, or meaning for any parameter, so it does not compensate for the contact_id gap or explain pagination behavior.

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 names a specific resource and scope ('The custom field values of one contact') plus the underlying endpoint, so an agent can tell it apart from sendlane_list_custom_fields (definitions) and sendlane_set_contact_custom_fields (write). It is a noun phrase rather than a verb-led sentence, but the GET endpoint supplies the implied action.

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 tool versus sendlane_get_contact, sendlane_get_contact_tags, or sendlane_set_contact_custom_fields. The 'one contact' scoping implies it needs a single contact id, but no conditions, prerequisites, or alternatives are stated.

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