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

The readOnlyHint annotation already establishes that this is a safe, non-mutating read. The description's one added behavior detail is the set of fields returned, which matters because there is no output schema; it says nothing about permissions, rate limits, or behavior when the fid is unknown.

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?

It is a single front-loaded sentence with no filler and the purpose leads. The trailing clause about attaches is dense and jargon-heavy but still earns its place as routing guidance.

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?

With no output schema, listing the returned fields compensates well, and the read-only annotation covers the safety profile. Both parameters are fully documented in the schema, so an agent has what it needs, though return format details (e.g. null when unknown) are absent.

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 description coverage is 100%, including the nested entity object, so the structured data already carries parameter meaning. The description only echoes 'by fid' and adds no format, constraint, or entity-scoping nuance beyond the schema, making the baseline 3 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?

The description states a specific verb and resource (returns file metadata by fid) and enumerates the returned fields (name, MIME type, kind, size, upload date, uploader). It does not explicitly differentiate itself from the close siblings file_get, file_url, or file_upload, so an agent must still infer which file tool to pick.

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 a clear use condition ('for a fid without known metadata') and an exclusion ('task and feature attachments already carry it in attaches'), which is real routing guidance. It stops short of naming the alternative tools an agent would otherwise consider, e.g. file_get or file_url.

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