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

Annotations already declare destructiveHint=true and idempotentHint=true, so the bar is lower, and the description still adds substantive behavior: linked billables update atomically and become immutable once reserved or invoiced. That immutability rule is not derivable from the annotations or schema and is the most valuable sentence here. It does not say what happens to an existing expense's unmentioned fields on replace, nor anything about failure/partial-write behavior.

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 create-vs-replace rule front-loaded. The final clause about billable_amount_minor is dense and slightly ambiguous in referent ("one linked billable item"), but nothing is wasted.

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 11-parameter destructive mutation with no output schema, the description covers the highest-risk semantics (atomicity, post-reservation immutability, no double-recording) but leaves most fields and the create-vs-replace field semantics undefined. Adequate, not complete.

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?

With only 18% schema coverage, the description must carry most parameter meaning; it explains three of eleven (expense_id, client_id, billable_amount_minor) but says nothing about vendor, category, description, incurred_on, cost_minor, reference, or which fields are required. request_id idempotency is documented in the schema, not the description. Useful but far from compensating for the coverage 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?

States a specific verb pair (create or replace) plus the resource (business expense) and the key discriminator (expense_id determines which). It is clearly distinct from expenses_list and billable_record, though it never names those siblings explicitly.

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?

"Create or replace ... using expense_id" implicitly tells the agent that supplying expense_id means update, omitting it means create. The closing instruction "do not record it again" steers the agent away from calling billable_record separately for the same amount, which is real routing guidance. No explicit when-not for the tool as a whole.

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