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

Beyond the annotations (readOnly=false, destructive=false, idempotent=false) the description discloses the real behavioural traits an agent needs: free inboxes expire after activeDays, paid inboxes persist only while subscribed, quotas differ by plan, and retired addresses are never reused. The localPart restriction to paid plans is a genuine policy disclosure rather than a restatement of the schema.

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?

Four compact sentences, front-loaded with the action, then constraints, then the mandatory follow-up call; nothing is filler. The opening "Requires a Free account:" reads slightly ambiguously (required prerequisite vs. free-tier offering), which is the only rough edge.

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 non-idempotent creation tool with no output schema, the definition covers prerequisites, quotas, expiry semantics and the follow-up call, which is nearly everything needed. It never states what the call returns (the created address/ID), so an agent must infer the response shape before chaining into wait_for_email.

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 earns an extra point because it explains the interaction between parameters and plan: activeDays governs free inbox lifetime while paid inboxes remain active while subscribed, and localPart only meaningfully applies on paid plans, which the schema states only tersely.

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?

"Create an owned agent email address" gives a specific verb and resource, and the lifetime/quota language makes clear this produces a persistent, owned mailbox rather than a temporary one. It does not explicitly contrast with neighbouring creators such as create_webhook_inbox or create_phone_number, but the resource noun is distinct enough that an agent will not confuse them.

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 the prerequisite (a Free account), the per-plan quotas (1 free / 10 Pro / 50 Team), and hands off to the correct next tool: "Then call wait_for_email." Missing are explicit when-not conditions, e.g. what happens if a quota is already exhausted, but the usage context is otherwise clear.

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