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

Annotations already declare readOnlyHint=false, destructiveHint=true, idempotentHint=true, and openWorldHint=true, so the safety profile is covered structurally. The description adds the POST endpoint and the field-id source, but does not disclose what 'add' does to pre-existing values despite destructiveHint=true, leaving the key behavioral question open.

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 fragments, each earning its place: the action marker, the id source, and the HTTP target. Front-loaded with 'WRITE:' so the mutating nature is the first thing read.

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, open-world POST with no output schema and 0% schema coverage, the definition covers the endpoint and id provenance but omits what happens to existing field values and any response behavior. Adequate but with a real gap on the destructive semantics.

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 0%, so the description must compensate but only partially does: it explains where the custom-field 'id' values come from. It says nothing about contact_id, the value format/limits, or the array bounds, leaving most of the parameter surface undocumented.

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+resource: 'add custom field values to a contact', and the 'WRITE:' marker distinguishes it from the read siblings get_contact_custom_fields and list_custom_fields. It is clear, though the phrasing 'add' is slightly at odds with the tool name 'set', so the agent must infer whether values merge or overwrite.

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?

Routes the agent to sendlane_list_custom_fields for obtaining field ids, which is genuinely useful prerequisite guidance. However, it never states when to prefer this over get_contact_custom_fields or add_contact_tags, so usage is only implied.

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