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

Discloses mutation effect ('MODIFIES live data') matching annotation destructiveHint=true. Adds that it returns the updated contact and explains partial update behavior. Provides API endpoint info.

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 sentences: first states purpose and effect, second gives usage advice and API reference. No wasted words, front-loaded with key action.

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?

Given no output schema, description still mentions return value (updated contact). All parameters are documented, and the tool's effect is clear. Sufficient for an update operation.

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 baseline is 3. Description adds value by explaining the contact_id derivation (already in schema) and advising to send only changed fields, which is helpful beyond what schema provides.

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 ('UPDATES an existing contact') and the resource ('contact in a list'), with the warning 'this MODIFIES live data'. It effectively distinguishes from siblings like create_contact and delete_contact.

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?

Provides guidance to send only fields to change (partial update), and explains contact_id format. Lacks explicit when-not-to-use or alternatives, but context is clear for a focused update tool.

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