epost_download_letter

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

With no annotations, the description carries the full burden and does well: it discloses the return value (saved path), file permissions (0600), the filename pattern, the API-vs-portal fallback behavior, and the .eml variant for IncaMail letters. It omits auth prerequisites and error behavior, but the write/return semantics are unusually well covered.

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 action and destination are front-loaded, and every clause (0600, filename format, API/portal fallback, index instability) carries information an agent needs. It is a single dense sentence with an em-dash that runs long, but nothing is redundant.

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 download tool with no output schema, the description adequately explains what is returned and where it is written, including the fallback naming case. It is missing auth/permission context and any indication of failure modes, which keeps it short of complete.

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 only 33% (only letter_id is documented in-schema), so the description must compensate, and it does: it explains that index is 'a position in a list that renumbers itself' and that letter_id 'needs the public API'. This adds material meaning beyond the bare schema types for both optional params.

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+resource ('Download one letter to output_dir') and immediately scopes it against the sibling epost_download_all by specifying a single letter. It also names the two addressing modes (index or letter_id), so an agent knows exactly what the tool operates on without opening the 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?

It gives real guidance on choosing between the two addressing modes ('letter_id... needs the public API') and warns that index is unstable across calls, which helps correct invocation. However, it never states when to prefer this tool over alternatives like epost_download_all or epost_letter_pdf, so usage vs siblings is only implied.

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