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

Annotations already declare readOnlyHint=false and destructiveHint=true, so safety is covered. The description adds genuine behavioral context beyond that: the optimistic-concurrency SyncToken requirement and the fact that a new SyncToken is returned. It does not explain what the destructive path actually clears, but that detail lives in the entity schema.

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 tight sentences with the core action front-loaded and the prerequisite/return note following. The parenthetical is slightly dense but every clause carries needed information; 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?

With no output schema, the description correctly discloses the return value (saved record plus new SyncToken), and annotations cover the mutation profile. The main omission is permission/error behavior on a stale SyncToken, which is minor for a well-annotated single-entity update.

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%, and the entity field's schema text already documents Id, SyncToken, sparse semantics, and excluded fields in depth. The description restates the Id/SyncToken requirement but adds no syntax or format detail beyond the schema, so baseline 3 applies.

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 ('Update an existing company currency in QuickBooks Online') and the word 'existing' distinguishes it from the create/delete siblings. It does not name those siblings explicitly, but the create/update/delete/get family makes the intent unambiguous.

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?

The parenthetical tells the agent it needs Id and the current SyncToken 'as the last search or write returned it', which implies a preceding search/get. However, it never states when to use this versus create_company_currency or the other update tools, nor any exclusions, so usage is only implied.

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