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

Annotations already declare the mutation profile (readOnlyHint=false, destructiveHint=true, idempotentHint=true), so the description's real contribution is the partial-update contract: "Only what you send changes; custom fields you leave out keep their values." That is exactly the ambiguity an agent needs resolved before calling an update tool, and it is consistent with the annotations rather than contradicting them.

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, front-loaded with the effect and followed by the merge semantics. No filler, no restatement of the tool name or title.

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 4-parameter mutation tool with no output schema, the description covers the essential behavior: what it changes and that the update is partial. The remaining gap is minor — how to discover valid custom field keys and whether clearing a field is possible — but nothing critical to invoking 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 50%: id and properties are documented, first_name and last_name are not, though their meaning is self-evident. The description adds one useful semantic — that the properties object is a merge, not a replace — but says nothing about whether field keys must pre-exist (list_custom_fields exists as a sibling for that) or how nulls/empty strings behave.

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?

"Changes a contact's name or custom field values" gives a specific verb (changes) plus the exact editable surface, and the id parameter in the schema pins the target. It cleanly separates this from get_contact, tag_contact, or add_contact_to_list, though it never names a sibling to route against.

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?

There is no explicit when-to-use or when-not-to-use guidance, and no pointer to alternatives such as tag_contact or add_contact_to_list for non-field changes. The agent must infer from the verb that this is the mutation path for names and custom fields.

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