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

Annotations already declare destructiveHint=true and readOnlyHint=false, so the safety profile is covered. The description adds useful behavioral context beyond annotations: the concurrency-token requirement (SyncToken) and the fact that the saved record is returned with a new SyncToken. It does not repeat the schema's sparse-update and deletion details, but those are in the structured field.

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, front-loaded with the core action and requirements, with zero wasted words. The prerequisite is embedded parenthetically without disrupting the main clause.

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?

Given the very rich schema description (100% coverage, nested object details) and annotations covering the destructive profile, the description provides enough context to select and invoke the tool: it says what it does, the required SyncToken, and the return value. It omits schema-level nuances like sparse updates and the Active:false retirement workaround, but those are available in the structured fields.

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 the entity and company parameters are already thoroughly documented in the schema. The description only reiterates the Id and SyncToken requirement, adding no syntax, format, or meaning beyond what the schema provides; baseline 3 is appropriate.

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 (Update) and resource (existing customer in QuickBooks Online), and the word 'existing' distinguishes it from create_customer. An agent can tell this is for modifying an already-present customer, not creating or fetching one.

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?

Provides clear context and a prerequisite: it requires the Id and current SyncToken as returned by the last search or write, implying the customer must already exist and have been fetched. It does not explicitly state when not to use it or name alternative tools like create_customer, so it stops short of a 5.

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