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

Annotations already declare readOnlyHint=true, so the description only needs to add context beyond that. It does: it reveals that include_pictures triggers lookups on WhatsApp, caps results at 10 per page, and is slower. It also clarifies the data source is the linked phone's saved contacts. This goes beyond what annotations alone convey, though it does not cover every possible edge case.

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?

Four sentences with no filler. The core purpose is front-loaded, the alternative is named, and the include_pictures caveat is compressed into a single useful sentence. Every sentence contributes to correct selection or invocation.

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 an output schema present, return-value documentation is unnecessary. The description covers tool selection (wa_search_contacts alternative), downstream use (wa_send_message), parameter behavior (include_pictures page cap and speed), and scoping (per-account, phone-linked contacts). Nothing an agent needs to call it correctly is missing.

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 schema already documents all four parameters clearly. The description restates the include_pictures trade-off and adds the cross-tool hint to use returned phone numbers with wa_send_message, but it does not substantially add meaning beyond the parameter descriptions. Baseline 3 is appropriate.

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 opens with a specific verb and resource: 'List the address book of one of your WhatsApp accounts — the contacts saved on the phone this account is linked to.' It immediately distinguishes itself from the sibling wa_search_contacts, so an agent can tell them apart without inspecting schemas.

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?

The description explicitly names the alternative for a different need: 'To find a specific person, use wa_search_contacts instead.' It also gives downstream guidance by saying returned phone numbers can be used with wa_send_message, and explains when to set include_pictures. This is clear, decision-relevant context.

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