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

Goes well beyond the annotations (readOnlyHint=false, destructiveHint=false, openWorldHint=false) by disclosing the 7MB size cap, supported formats, that the file is stored byte-for-byte with no compression or resizing, and that a media ID is returned. These are non-obvious behavioral facts an agent needs before committing to this path.

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-loads the 'last resort' framing, follows with a compact ordered alternative list, then two short sentences of constraints and return value. Every sentence earns its place with no filler.

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, the description supplies the return value (media ID attachable to a post), the size limit, the accepted formats, and the storage semantics. Nothing an agent needs to call this correctly is missing.

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%, so the parameters are already fully documented, including the MIME enum. The description's format list (JPEG, PNG, GIF, WebP, MP4, MOV, PDF) essentially restates the enum rather than adding syntax or format guidance beyond it. Baseline 3 applies.

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+resource ('upload a file sent as a base64-encoded string') and immediately positions it against named siblings. A reader knows exactly what it does and why it exists as a distinct tool.

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?

Explicitly labels itself a 'last resort' and gives a prioritized ordering of five alternatives with the condition selecting each (attached file, shell commands via create_upload_url + finish_upload, user picking files, public URL). This is the strongest possible usage routing.

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