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

Goes well beyond destructiveHint=true: it discloses the exact blast radius ('nothing can be booked on or before its last day again, by any tool, invoice or unlock'), the cascade ('every earlier year is closed with it'), irreversibility, the sanctioned workaround, and pricing ('$0.001 per call; nothing if it fails'). This is the kind of consequence detail an agent needs before an irreversible year-end action.

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 tight sentences ordered by importance: what it does, irreversible consequences, the alternative, then cost. No filler; each sentence adds decision-relevant 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?

For an irreversible, non-idempotent mutation with no output schema, the description covers consequences, cascade effects, irreversibility, the alternative tool, and pricing. Nothing an agent needs to decide whether to call it is missing, aside from the minor undocumented 'reason' parameter.

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 only 50%: year_end is well documented in the schema, but 'reason' has no description in either the schema or the description. The description reinforces the meaning of the date implicitly ('its last day'), but adds no explicit parameter guidance, so baseline 3 is right with the undocumented reason field as an open gap.

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?

Names a specific verb and resource ('Close a fiscal year') and bounds the scope precisely ('once its accounts and tax returns are final'), and immediately distinguishes it from the sibling books_lock by scope (year-end vs monthly). An agent can pick this out of the large books_* family without opening the 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?

Explicitly states the prerequisite (accounts and tax returns final) and names the alternative for the common other case ('Monthly work uses books_lock instead'). It also tells the agent what to do later instead of reopening ('a correction found later is booked in the open year').

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