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

Beyond the annotations (which already declare readOnlyHint=false, idempotentHint=true, destructiveHint=true), the description explains the optimistic-concurrency contract: version 0 creates, a matching current version permits a deliberate update, and an identical retry is a safe no-op returning the existing record. That is substantial behavioral context the annotations alone do not convey; it does not contradict destructiveHint, since updates overwrite the stored record.

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 tight sentences with the core action front-loaded and no filler. The negative-scope sentence is slightly list-like but every clause carries distinct information.

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?

With an output schema present, return values need not be described, and the description covers the mutation, idempotency, and concurrency semantics that matter for calling correctly. It omits what happens on a version mismatch (conflict error?) and any permission/auth requirement, which are minor gaps for a 5-param nested write.

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 0% across 5 required parameters, so the description must carry the burden. It explains expectedVersion well and hints at the confirmedSave gate via 'explicitly requests storage', but says nothing about key, label, or the nested contact/details fields, leaving the majority of the input surface undocumented.

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 opens with a specific verb and resource ('Stores a private recipient or return-address contact'), making the object unambiguous among siblings like postedly_contacts and postedly_delete_contact. The 'private' qualifier and the recipient/sender role distinguish it from the broader contact and document tools.

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 states a clear precondition ('after the user explicitly requests storage') and negative scope ('Does not validate deliverability, create a quote, or send'), which rules out confusion with postedly_quote and the send/prepare siblings. It stops short of naming a concrete alternative tool or explaining when to prefer an update path over a fresh save.

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