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

Annotations already declare readOnlyHint=false, destructiveHint=false and openWorldHint=false, so the mutation/safety profile is covered. The description adds a genuinely non-obvious behavioral fact not present in the annotations or schema: expenses are auto-created for booked vendors, which prevents duplicate entries. It still says nothing about the response, whether budget totals are recalculated, or what happens when an identical expense exists.

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 sentences, zero filler, with the duplicate-prevention warning placed immediately after the core action. Nothing is repeated from the title or annotations.

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 6-parameter mutation tool with no output schema, the description covers the main failure mode (duplicate vendor expenses) and points to the budget and vendor lookups. It omits any mention of the return value or effects on budget totals, which is acceptable given no output schema is declared.

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 67%, with amount, category, vendor_id and purchased_on already documented in the schema. The description adds meaningful meaning only for vendor_id (link instead of duplicating) and implies amount is a fixed value, while name and notes remain undocumented in both places. That is incremental value over the schema, not a full compensation of 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 and resource ('Adds an expense to the budget with a fixed amount') and concretizes it with examples ('the dress or the rings'). Crucially it distinguishes itself from the vendor-derived expense path by explaining that a booked vendor already produces an expense, so an agent can tell whether to use this tool or vendor_id linking.

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 an explicit when-not rule: do not add an expense that a booked vendor already generates, link it with vendor_id instead. It also points to get-budget for categories and list-vendors for vendor_id through the schema. It does not address update-expense versus add-expense for correcting an existing entry, so the routing guidance is strong but not exhaustive.

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