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

readOnlyHint=true already declares this a safe read, and the description adds real value on top: it scopes the payload ('header, lines, payments') and explicitly notes 'The attachment is never requested', which tells the agent an attached document will be absent. It also exposes the underlying endpoint. No auth or rate-limit detail, but that gap is minor for a read.

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?

Three short, front-loaded sentences with no padding; the payload scope leads. The raw HTTP endpoint at the end is marginally useful but borders on implementation noise.

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 single-resource read with one required param and no output schema, the description conveys what comes back and the notable omission of the attachment, which is enough for correct invocation. It stops short of describing any pagination or error behavior, but none is expected here.

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 100%, so the id parameter (including its PIHId origin and UUID pattern) is fully documented in the schema. The description adds no parameter meaning beyond that, so the baseline of 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 and resource ('One purchase invoice in full') and enumerates the returned sections (header, lines, payments), so an agent can tell it retrieves a single fully-hydrated invoice. It does not explicitly name or contrast itself with the sibling merit_list_purchase_invoices, which keeps it from a 5.

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 gives no when-to-use, when-not, or alternative routing. The only hint that this is a follow-up to a listing tool lives in the id parameter's schema description ('PIHId from merit_list_purchase_invoices'), not in the description itself.

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