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

Annotations already indicate idempotentHint=true and destructiveHint=false, but the description adds no behavioral context beyond the literal 'update fields.' Critically, it does not clarify whether this is a partial update (only provided fields change) or a full replacement, which is essential for a mutation tool. No additional permissions, side effects, or overwrite semantics are disclosed.

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 description is a single, concise sentence with zero fluff. It front-loads the core action and resource, making it scannable and efficient.

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?

The description, combined with the schema and annotations, is adequate for a basic update operation, but it leaves a significant gap: whether omitted fields are preserved or reset. The output schema exists, so return values need no explanation, but the partial-update behavior is undocumented in both description and schema.

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%, with all six parameters having meaningful descriptions, so the baseline is 3. The tool description adds no additional parameter semantics, but the schema handles the burden fully.

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 the action ('Update') and the resource ('an existing contact'), making the purpose immediately understandable. However, it does not explicitly distinguish from sibling tools like create_contact or set_primary_contact, relying on the tool name for context.

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 implies usage for updating already-existing contacts via the word 'existing,' and the required 'contactId' parameter suggests the need for an existing record. However, it provides no explicit guidance on when to prefer this tool over alternatives like create_contact or set_primary_contact, nor does it mention exclusions.

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