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

Annotations already declare the safety profile (destructiveHint=true, idempotentHint=true, openWorldHint=true), and the description adds real operational detail beyond them: the read-modify-write implementation, the API's requirement that first_name be sent on every update, the empty-string clearing rule, and that the updated contact is returned. It doesn't warn about side effects on omitted fields beyond the patch note, or mention permission requirements.

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?

Front-loaded with the WRITE marker, then four tight clauses covering patch semantics, the first-name quirk, clearing, and the return value. Nothing is repeated from the annotations and no sentence is 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 22-parameter mutation with no output schema, the description covers the essential mechanics (partial update, clearing, return value) and the schema documents all parameters. Gaps are minor: error behavior, whether unknown fields are rejected, and auth requirements are unstated.

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 baseline is 3; the description goes beyond it by explaining empty-string clearing for text fields and the special first_name handling that isn't visible in the schema (first_name is not even required in the schema). It doesn't cover the boolean flags' semantics (vip, hidden, archived), which remain thin.

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?

Opens with an explicit WRITE marker plus a specific verb+resource ("change fields of an existing contact"), which cleanly separates it from create_contact, get_contact, and find_contact. An agent can identify the operation without opening the schema.

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?

States the patch semantics clearly ("Only the fields you pass change") and gives the clearing convention ("Pass an empty string to clear a text field"), which is the main ambiguity an agent would face. It doesn't explicitly name alternatives like create_contact or get_contact, so it stops short of full when/when-not routing.

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