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

The annotations already declare destructiveHint=true and idempotentHint=false, so the basic safety profile is visible. The description adds the phone-number matching rule, but it does not explain overwrite semantics, such as whether omitted fields are cleared, or why the operation is non-idempotent.

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 a single front-loaded sentence with no wasted words. It communicates the core upsert rule efficiently, even though the tool's complexity may call for additional operational detail elsewhere.

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 destructive 12-parameter upsert with nested objects and no output schema, the description omits key operational details: overwrite behavior, the significance of phone_number as the sole required field, the import-only constraint on opt_in, and any indication of the result shape. It gives the central matching rule but is not complete enough for confident invocation.

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 only 42% across 12 parameters, with several simple fields such as tags, email, notes, and company undocumented. The description only clarifies that phone_number is the identity key; it does not compensate for the remaining undocumented parameters or explain the nested opt_in and fields behavior beyond what the schema already says.

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 create-or-update operation on a contact and names the matching key, phone_number. This distinguishes an upsert from a plain update_contact or create_customer call.

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?

It clearly explains the selection rule: create a new contact, or update the existing contact with the same phone_number. However, it does not name alternatives like update_contact or import_contacts, nor does it state when not to use this tool.

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