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

With no annotations, the description carries the load and does well: it discloses the 25 MB cap, base64 vs raw-bytes tradeoff, and the return value (file_id). It leaves out what happens on re-upload of the same filename, whether uploads are replaceable or immutable, and error/quota behavior.

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?

Front-loaded with the verb and payload, then the constraints, then the return-value routing — a sensible order with no filler. The trailing clause about POST /api/files is slightly terse and references an endpoint outside the MCP surface, which may confuse an agent that cannot call it.

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 three-parameter upload tool with no output schema, the definition is nearly self-sufficient: size limit, encoding, accepted types, return value, and consumption path are all present. Only error handling and re-upload semantics are absent, which most agents would not need before invoking.

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, and the description adds real meaning on top: the data parameter is the deliverable product file, not an asset, and the format list constrains what the filename extension may be. The api_key fallback is documented in the schema itself, so no credit lost there.

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 (upload) and resource (the product file buyers receive), and enumerates the accepted formats plus the 25 MB ceiling, so the agent knows exactly what this does. It does not distinguish itself from the sibling upload_image, which an agent must infer from the format list rather than from an explicit statement.

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 an explicit downstream workflow: the returned file_id goes to create_listing or update_listing, and it names an alternative path (POST /api/files with raw bytes) for files over the limit. It stops short of saying when to prefer this over upload_image, which is the closest sibling.

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