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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), yet the description adds substantial behavior the annotations cannot: the result is the fax object plus a page-1 image when readable, thumbnail_url should be rendered in image-capable chats, and view_url gives the full page. It also documents failure modes precisely — not-found for a new or still-processing fax, and a specific removal message with 30-day retention and deleted files.

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?

The core identification rule and return shape are front-loaded, with error cases and sample tools following in a logical order. A few sentences are expendable, notably 'A client can draw that image very small,' which reads as an aside rather than actionable instruction.

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-parameter read tool with no output schema, the description covers everything an agent needs: identifier formats, the shape of a successful result (object plus image), rendering guidance for thumbnail_url and view_url, and the distinct error outcomes for pending versus removed faxes. Nothing material is left to inference.

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 baseline is 3, but the description earns credit above it by specifying the slug pattern ('fax_' plus 8 characters from 0-9 and a-f) and noting that view_url may carry a language prefix such as /de. Those details are not in the schema's parameter text and directly reduce invocation errors.

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 ('Get one test fax') and immediately pins down the two accepted identifier forms with a concrete example (fax_1a2b3c4d) and the URL form. It also relates the return value to list_test_faxes ('the same as an item of list_test_faxes'), which lets an agent distinguish it from the listing sibling without opening either schema.

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?

Gives clear context: fetch a single fax by slug or view_url, and use fax_example / fax_example_removed when a sample answer is wanted. That is real routing guidance, but there is no explicit statement of when to prefer list_test_faxes over this tool (e.g. 'to enumerate, use list_test_faxes').

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