It's back

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

Beyond the annotations (readOnlyHint=false, destructiveHint=false, openWorldHint=false), it discloses real consequences: the loan persists in the item's history, the borrower's reminder emails stop, nothing else about the item changes, and repeat calls are rejected. That tells the agent both what the mutation does and its blast radius, which the hints alone do not.

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 purpose lands in the first clause, followed by trigger, dependency, effects, and the repeat-call caveat, with no filler sentences. The parenthetical example is slightly redundant with the clause it follows, but the length is well matched to a two-parameter mutation.

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 simple two-parameter mutation with no output schema, the description covers trigger, effects, and the idempotency failure mode. It does not say what a successful call returns or what happens with an unknown or already-closed id, which are the remaining gaps.

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 both parameters are already documented, and the description only repeats the id provenance instead of adding to it. It never mentions the optional `on` date at all, so nothing hints at backdating a return. Baseline 3 is appropriate when the schema carries the load.

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?

"Closes a loan: a lent thing is back, or a borrowed one was given back" states a specific verb (closes) plus the resource (loan) and covers both directions of the relationship. It is clearly distinct from save_loan (create/edit) and list_loans (which is named only as the id source), so an agent can pick it without opening a schema.

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?

"Use it when the person says so ('Sam returned the ladder')" plus "the id comes from list_loans" gives a concrete trigger and a pointer to the dependency tool. It also warns that a repeat call is refused because the loan is already back. It stops short of naming save_loan as the tool for correcting a loan or explaining how to undo a mistaken return.

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