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

Annotations already declare read-only, idempotent, non-destructive behavior, so the safety profile is covered. The description adds genuine behavioral context beyond that: it explains the dedup resolution rule ("the saved variant wins") when multiple raw contacts share a phone, which the agent could not infer from structured fields.

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?

Front-loaded purpose followed by the dedup caveat and compact examples; nothing is padded. The three examples are slightly redundant with the schema's own format descriptions, but they remain tight and scannable.

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?

An output schema exists, so return values need no explanation. For a two-param, read-only lookup, the description covers purpose, both keys, and the dedup edge case, leaving nothing an agent needs 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 coverage is 100%, so both parameters are already documented. The description still adds value by supplying concrete call examples for each form (JID, phone with +, bare phone), reinforcing the accepted input shapes beyond the schema's format notes.

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 ("Look up") and resource ("a single WhatsApp contact") plus the two lookup keys (JID or phone). The word "single" cleanly distinguishes it from the sibling list_contacts, so an agent can route without opening either 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?

"Pass either parameter — both work" tells the agent the two keys are interchangeable, which is useful. However, it never states when to reach for get_contact versus list_contacts or export_contacts, so the usage context is only implied.

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