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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is covered. The description adds an important behavioral constraint: availability is provider-dependent, since generic IMAP typically has no contact directory. It doesn't mention return format or pagination, but those are minor given the strong annotation coverage.

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 sentences with zero filler. The core verb, resource, and purpose are front-loaded, and the second sentence earns its place by warning about a common misuse case (generic IMAP).

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 simple optional-parameter list tool with no output schema, the description covers the tool's purpose, the key provider limitation, and enough return implication (contact info enabling name-to-address resolution). Pagination mechanics are left to the cursor schema, and the return shape is not spelled out, but those gaps are not critical for correct invocation.

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?

The schema carries most of the meaning: account_id is richly documented (Nilyo connection ID, unipile_account_id, disambiguation instructions), and cursor is described. The description adds no parameter-specific detail, and the undocumented limit parameter is left to inference, though its intent is fairly clear from the name and bounds.

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 ('List'), a well-scoped resource ('Gmail/Microsoft contacts'), and an explicit goal ('resolve a human recipient name to an email address before composing'). It distinguishes itself from generic IMAP and from the sibling whatsapp_list_contacts by naming the provider scope.

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 explicitly tells the agent when to use the tool (before composing, to map a recipient name to an address) and when not to rely on it ('Generic IMAP normally has no provider contact directory'). It doesn't name a specific sibling tool as the alternative, but 'use mailbox context instead' gives a clear fallback strategy, so it earns just below the top score.

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