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

The annotations already declare readOnly=false, destructive=false, idempotent=false and openWorld=true, so the mutation profile is covered. The description adds a genuine compliance constraint (no health, financial, government ID, card or password data), which is real value beyond the annotations. It omits notable behavior the agent needs: what happens if the email already exists (duplicate error vs. merge) and that a dry_run mode is available without creating anything.

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 short sentences with the core action front-loaded and no padding. It is arguably slightly under-sized for a 14-parameter nested-object tool, but nothing in it is wasteful.

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 schema is fully described and annotations cover safety and idempotency, so the description need not carry the return-value burden (no output schema exists). The remaining gaps, duplicate-email behavior and the dry_run escape hatch, are meaningful for a 14-parameter create tool but the surrounding structured data covers most ground.

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; every parameter including custom_fields and dry_run is already documented in the schema. The description only restates the two required fields, adding essentially no semantics beyond structured data.

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 states a specific verb and resource ('Create a new contact in a MailerCloud list'), which is unambiguous. It does not, however, distinguish the tool from its close siblings upsert_contact or batch_create_contacts, so an agent must infer the boundary itself.

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?

It states the two required fields and a data-handling constraint, which is useful context for invoking the tool. But there is no guidance on when to choose this over upsert_contact (which presumably also creates) or batch_create_contacts for many contacts, and no mention of prerequisites such as the list existing.

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