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

The description explicitly says 'MUTATES,' which aligns with the destructiveHint annotation. It adds valuable behavior beyond the annotation: only passed fields are changed (partial update semantics), the fields parameter acts as a merge/escape hatch, and the API endpoint is PUT /contacts/{id}, and it returns the updated contact. This is strong behavioral context for a mutating tool.

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?

Three tight sentences, front-loaded with the mutating intent and primary operation. Every sentence earns its place: purpose, partial-update semantics, escape-hatch guidance, API mapping, and return value. No filler.

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 mutation tool with no output schema, the description covers the key missing pieces: endpoint, partial update behavior, the generic fields escape hatch, and the return value. It does not detail authorization or error cases, but these are not strongly required given the annotations and simple 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?

The schema covers 100% of parameters, so the baseline is 3. The description adds meaning beyond the schema by clarifying that only provided fields are changed and that 'fields' is the escape hatch for any untyped attribute. This helps an agent understand the update merge behavior and when to use the fields object.

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 states a specific verb ('update'), a clear resource ('an existing contact'), and the exact API operation (PUT /contacts/{id}). This unambiguously differentiates it from siblings like create_contact, get_contact, and list_contacts.

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 phrase 'update an existing contact' clearly implies this is for modifying already-created contacts rather than creating new ones. It does not explicitly name create_contact as the alternative, but the context is clear enough for an agent to select this tool correctly.

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