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

Annotations only declare readOnlyHint=false, so the description carries most of the burden and does it well: TTL is measured from NOW rather than added to the existing expiry, the 24 h ceiling is stated twice (description and param), and owner scoping is disclosed. It stops short of saying what happens to an already-expired box or what the call returns.

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?

Two dense sentences, operation and scope front-loaded with the TTL caveat in a parenthetical. No filler; only the parenthetical nesting slightly hurts scanability.

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 two-parameter mutation with no output schema, this covers the critical facts: eligibility (ephemeral only), TTL semantics (from now, max 24 h), and ownership. Missing only return/error behaviour (expired or unknown id) which an agent would otherwise have to discover empirically.

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 both id (UUID sourced from provision/list) and ttl_seconds (default 600, max 86400, exclusive minimum) are already fully documented. The description's 'counted from NOW' echo adds a little semantic nuance about non-additive behaviour but is largely redundant with the schema text.

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: extend the lifetime of an EPHEMERAL agent inbox, by id. The qualifier that persistent boxes never expire and cannot be extended distinguishes it cleanly from the provisioning, listing and release siblings without needing to name 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?

Gives a clear when-not: persistent boxes cannot be extended, so the agent knows only ephemeral boxes qualify. It also flags owner scoping as a precondition, but does not explicitly route to a sibling (e.g. provision) when the box is persistent or already expired.

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