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

With no annotations, the description carries the full burden; it does disclose a real side effect — status advancement triggers server-side todo creation/completion and installment todos. It does not say whether omitted fields are left unchanged (patch vs. replace), whether transitions are validated, or what permissions are required.

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?

A single front-loaded sentence with the side-effect detail in parentheses; nothing is wasted, though the parenthetical is dense enough that it deserves at least one clarifying clause about field-patch semantics.

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 21-parameter mutation tool with no annotations, no output schema, and 14% schema coverage, the description covers only the status-advancement path. An agent still cannot determine patch semantics, valid status values, or the effect of most 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?

21 parameters with only 14% schema description coverage, and the description explains none of them beyond the existence of a status field. The installments/installmentPaid/installmentStart/installmentAmount cluster, phaseIdx, and projectId (where empty string clears the project link) are all left undocumented in prose.

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?

The description states a specific verb and resource (update purchase fields) and adds the distinct second mode of advancing status. An agent can tell this apart from purchase.create/delete/get, though no sibling is named 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?

It implicitly signals two usage modes (field patching vs. status advancement) but never says when to prefer this over purchase.create, purchase.get, or recurring.update, and gives no preconditions such as which statuses may be advanced.

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