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

Annotations carry only destructiveHint=true, so the description legitimately adds 'additive' as behavioral context (it adds data rather than replacing an existing record). However, it omits key behaviors an agent needs: what happens when the email already exists (error vs update), permission/auth requirements, and rate limits. Note the mild tension between 'additive' and destructiveHint=true, though these are reconcilable (creation is a state-changing write).

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?

Two short sentences, front-loaded with the action. The raw endpoint string ('POST /v1/contacts/create') is low-value reference detail for an agent but consumes minimal space, so it does not seriously hurt.

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 9-parameter write tool with nested objects and no output schema, the description is thin: it never says what is returned (e.g., a contact id) or how duplicate emails are handled, both of which matter for calling and chaining the tool correctly.

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% across all 9 parameters, including the nested 'properties' and 'mailingLists' objects, so the schema does the heavy lifting. Baseline 3 applies; the description adds nothing about parameter formats or interactions.

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?

Specific verb+resource: 'Creates a contact in Loops'. The '(additive)' qualifier signals this adds rather than replaces, which nudges it apart from loops_update_contact, but no sibling is named. Clear enough to select 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?

Usage is only implied: '(additive)' hints that this is for contacts that don't yet exist, as opposed to loops_update_contact. There is no explicit when-to-use, when-not, or named alternative despite nine sibling tools in the same domain.

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