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

Annotations already declare readOnlyHint=false, idempotentHint=true, and destructiveHint=false, so the safety and mutation profile is known. The description adds value by explaining that mark_contacted=true records today's date as the check-in date – a behavioral detail not in the schema. It doesn't contradict any annotation and provides useful context beyond what structured fields carry.

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 with no filler. The main action ('Update a contact') is front-loaded, and the mark_contacted note is a targeted, useful addition. Nothing is redundant.

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?

Given 13 parameters (1 required), a complete schema, and no output schema, the description is adequately minimal. It covers the core purpose and one non-obvious parameter behavior. It doesn't mention partial-update semantics, but that is implicit for an update tool and the schema already lists all fields. The description is sufficient for an agent to call it 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%, so all 13 parameters are already documented in the schema. The description only mentions mark_contacted, adding a bit of context for that parameter. Since the schema does the heavy lifting, a baseline of 3 is appropriate; the description adds marginal value but does not compensate for anything missing.

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 clearly states 'Update a contact' – a specific verb and resource – and adds a special-case note about mark_contacted. It is unambiguous and implicitly distinguishes from create_contact and list_contacts among siblings, so an agent can immediately tell what it does.

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 gives a concrete usage hint: 'Use mark_contacted=true to record a check-in with today's date.' This tells the agent a specific scenario where this tool (and this parameter) is appropriate. It does not explicitly contrast with create_contact or other update tools, but the purpose is clear enough that an agent would know to use it for existing contacts. Minor gap: no explicit 'when not to use' guidance.

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