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

Beyond the annotations (readOnlyHint=false, destructiveHint=false, idempotentHint=true), the description discloses important behavioral traits: authentication is required, only provided fields change while omitted fields retain values, and the updated contact is returned. This partial-update semantics adds value beyond what annotations alone convey. No contradiction with annotations.

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 four concise sentences with no redundancy. The action is front-loaded ('Update an existing contact's information'), followed by essential context: authentication, partial-update semantics, how to get the ID, and the return value. Every sentence earns its place.

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 13 parameters but full schema coverage, the description does not need to enumerate parameters. It provides critical workflow context (authentication, ID source, partial-update behavior, return value) and is nearly complete. Slightly lacking is explicit guidance on error scenarios or exact return format, but the absence of an output schema is mitigated by stating 'Returns the updated contact'.

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%, so the baseline is 3. The description adds meaning beyond the schema by clarifying the update behavior: 'Only the fields you provide are changed; omitted fields keep their current values.' This explains that all fields except contactId are effectively optional and that omission means no change. It also reinforces the source of contactId.

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 and resource: 'Update an existing contact's information.' It clearly distinguishes this tool from siblings like createContact, deleteContact, and listContacts by emphasizing 'existing' and referencing the source of contactId. The purpose is unambiguous and not a tautology.

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 gives clear context for when to use the tool: it updates an existing contact, requires authentication, and instructs the agent to obtain contactId from listContacts. It does not explicitly name alternatives or say when not to use it, but the 'existing' framing and workflow implicitly separate it from create/delete operations.

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