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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds genuinely useful behavioral context by enumerating the returned payload (line items, totals, due and paid values) and mapping to the underlying endpoint GET /v2/invoices/:id, which an agent cannot infer from the annotations or the single-parameter 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?

A single front-loaded clause delivers the operation, the scoping key, and the returned content, with the API mapping appended compactly. No sentence 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?

For a one-parameter read tool with no output schema, the description correctly compensates by naming the return fields, and annotations handle the read-only safety signal. It falls short only on edge behavior such as not-found or permission errors, which is minor for this tool shape.

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%, and the schema already documents the parameter as 'The invoice's numeric id or full FreeAgent URL.' The description's 'by id or URL' simply mirrors that, adding no format examples or id-discovery guidance beyond the structured field. 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?

The description states a specific verb and resource ('Fetch a single invoice by id or URL') and adds what comes back (line items, totals, due and paid values). 'Single' implicitly contrasts with the sibling freeagent_list_invoices, but no sibling is named explicitly, which keeps this just below the top tier.

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?

Usage is only implied: it fetches one invoice, so the caller must already hold an id or URL. There is no explicit statement of when to prefer this over freeagent_list_invoices or what to do when the identifier is unknown.

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