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

The description adds meaningful behavioral details beyond the annotations: it explains that the underlying API uses PUT full-replace and that the tool loads the contact, merges fields, and writes the full record back so omitted fields are preserved. The group-name-vs-ID warning and 'modifies contact' side effect also add transparency.

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?

Every sentence earns its place: the first states the operation, the second explains the critical PUT-merge nuance, the third clarifies group input, and the final line flags the side effect. No filler or repetition.

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 two-parameter tool with a nested payload and an output schema, the description fully covers what an agent needs to call it correctly: which fields to supply, how preservation works, how to format groups, and the fact that the operation mutates data. Nothing essential 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 coverage is 100%, so the schema already documents contact_id and payload. The description still adds value by explaining the merge semantics behind 'omit fields to keep' and by reinforcing the group-name requirement with an example ('Vendors'), going slightly beyond the schema's wording.

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 opens with a specific verb and resource: 'Partially update an existing contact.' It also clarifies the partial-update behavior ('Omit fields you want to keep; only set fields to change'), making it distinct from create-contact, delete-contact, and set-contact-groups.

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 clearly establishes when to use the tool: to modify an existing contact by providing only changed fields. It does not explicitly name alternatives or exclusions, but the partial-update framing gives enough context to distinguish it from create/delete/set operations.

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