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

Goes well beyond the annotations by disclosing the return payload (document_id, upload_url, upload_headers), the 5-minute PUT window, the 1-hour document_id lifetime, and the reassurance that nothing is faxed or charged. This is the rare case where the non-idempotent, non-destructive profile is made concrete rather than merely hinted at.

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?

Four sentences, each load-bearing: eligibility, call ordering, the upload handshake with deadlines, and the fallback path. The critical ordering constraint and the fallback are front-loaded rather than buried.

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?

Even though an output schema exists, the description still explains the three returned fields because the agent must act on them immediately with a PUT. Nothing about the multi-step workflow, deadlines, or failure path is missing for a five-required-parameter mutation.

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 already 100%, so the baseline is 3, but the description adds real constraints absent from the schema: a 20 MB cap on byte_size and the format allowlist (PDF, JPG, PNG, WEBP, HEIC). It also explains that the returned upload_headers must accompany the exact bytes, tying parameters to the follow-up call.

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 (add a file to a fax) plus accepted formats and size ceiling, and explicitly separates it from the sibling create_fax. An agent can distinguish this from create_fax/send_fax without opening a 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 explicit ordering ('Call it once per file, before create_fax, in the order the files should be faxed') and an explicit when-not-to-use branch: if the bytes can't be read, call create_fax without document_ids and let the person upload on the status page. Both the selection condition and the alternative are named.

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