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

The destructiveHint=true annotation already flags a write risk, but the description adds non-obvious behavior beyond it: this is an upsert that silently creates the contact when it does not exist. It still omits whether omitted fields are preserved or cleared (partial update vs replace) and how mailingLists/subscribed are reconciled.

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 short sentences with zero filler; the upsert behavior is front-loaded immediately after the core action so the most decision-relevant fact lands first.

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 9-parameter mutation with nested objects and only a single annotation, the description is thin: it says nothing about how custom properties or mailingLists are merged, what happens to omitted fields, or what the call returns. The upsert disclosure covers the biggest ambiguity, but the tool's complexity warrants more.

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 every one of the 9 parameters is already documented, making 3 the baseline. The description adds only the fact that email/userId serve as the identifying keys, which is marginally useful but largely restates the schema.

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?

States a specific verb and resource ('Updates (or creates) a contact by email/userId') and clarifies the upsert semantics, plus the underlying endpoint. It does not explicitly contrast with the nearby loops_create_contact sibling, leaving the agent to infer why one would use this instead of a dedicated create 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?

The description conveys a conditional usage rule — 'creates the contact if not found' — which is useful for deciding whether a pre-existing lookup is needed. However, it never names alternatives (loops_create_contact, loops_find_contact) or states when not to use this tool, so routing guidance remains implicit.

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