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

Annotations already flag destructiveHint=true, and the description reinforces that with 'this MODIFIES live data', while adding genuinely new context: partial-update semantics (only supplied fields change), the underlying PUT endpoint, and that the updated contact is returned. It does not cover auth/permission requirements or behavior when contact_id matches nothing, which keeps it at a 4.

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?

The warning that this modifies live data is front-loaded, and the three sentences stay tight with no filler. The contact_id MD5 explanation duplicates the schema and could be trimmed, which is the only minor waste.

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 no output schema, the description correctly supplies the return contract ('Returns the updated contact'), plus the endpoint and required identifiers. For a mutation tool it is nearly complete, though it omits what happens to fields not supplied (cleared vs preserved) and error behavior for a missing contact.

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%, and the schema already documents contact_id as the MD5 hash of the lowercased email and enumerates status values, so the description largely restates structured data. Its only added semantic is the 'send only fields you want to change' partial-update contract, so the 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?

States a specific verb and resource ('UPDATES an existing contact in a list') and explicitly contrasts it with creation/read siblings by noting it MODIFIES live data. It also pins the exact endpoint (PUT /lists/{list_id}/contacts/{contact_id}), so an agent can distinguish it from create_contact, get_contact and delete_contact without opening a 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?

Gives clear operating guidance — send only the fields you want to change, listing email_address, fields, tags and status — which tells the agent this is a partial update rather than a replace. It does not explicitly name alternatives or exclusions (e.g. use create_contact when the contact does not yet exist), so it falls short of a 5.

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