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

Annotations declare destructiveHint=true, readOnlyHint=false, and openWorldHint=false; the description adds critical behavioral details: sparse update semantics, that a Line array replaces all lines, that sparse: false clears omitted fields, and that a sparse update must repeat PaymentType and AccountRef. These go beyond what annotations provide. It does not mention permissions or rate limits, but covers the main mutation behaviors.

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 requirement. It is concise and focused, though the second sentence about return value is somewhat redundant with the schema but still useful. No wasted words.

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 tool's complexity (nested entity object, sparse updates), the description along with annotations and schema provides sufficient information: it covers the required SyncToken, sparse update behavior, and return value. An output schema is absent, but the description states the return includes the new SyncToken. The only minor gap is lack of explicit when-to-use vs alternatives, but overall it is complete for correct invocation.

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 baseline is 3. The description adds extra meaning: it explains that SyncToken must be current (as last search or write returned it), and the schema already details the entity fields including sparse update behavior. The description reinforces the critical SyncToken requirement and return value (new SyncToken), adding value beyond the schema.

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 purchase in QuickBooks Online), and distinguishes itself from create_purchase, get_purchase, delete_purchase, and update_purchase_order by naming the target entity precisely.

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 description implies when to use it (to update an existing purchase) and mentions the prerequisite of having Id and SyncToken, but does not explicitly say when to choose this over delete_purchase or create_purchase, nor does it list alternatives. The implicit guidance is adequate but not comprehensive.

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