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

Annotations already cover safety (readOnly, non-destructive, openWorld), but the description adds genuinely new operational context: per-call billing, first-25-free, failed calls free, and that repeats are charged (which explains the non-idempotent-style charging behavior). It also discloses a second return value (inline display).

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?

Purpose is front-loaded in the first clause, followed by semantics, usage and cost. The pricing sentence is long and partly administrative, but it carries decision-relevant cost information rather than 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 correctly discloses what is returned (MIME type plus inline-display behavior) and adds billing/permission-free context. For a one-parameter lookup tool this is close to sufficient, missing only input-format edge rules.

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% with one parameter, so the baseline is 3, but the description goes slightly beyond the schema by restating that either a full file name or a bare extension is accepted and by showing concrete value forms (report.pdf, file=report.xlsx). It still never says whether the extension needs a leading dot.

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: it returns the MIME (content) type for a file name or extension, with a concrete mapping example (report.pdf → application/pdf). No sibling tool overlaps this function, so an agent can select it unambiguously.

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?

"Use when uploading or serving files" gives a clear triggering context, and the inline/attachment note hints at the serving use case. It does not name when-not-to-use or an alternative, but no sibling competes with it, so the gap is minor.

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