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

Annotations already declare readOnlyHint=true, openWorldHint=true, and destructiveHint=false, so the safety profile is covered. The description adds genuinely non-obvious behavior: tiered output on paid plans (gzip compressibility flags, defining registry, all sharing extensions) and a 1-credit cost. The one gap is that it never says what the tool does when invoked with no parameters.

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?

One dense, front-loaded sentence covering both modes, plus a parenthetical on paid-tier extras and cost. The only waste is the redundant restatement of the title at the start ("MIME Type Lookup: MIME Type Lookup matches..."), and the credit note is buried at the end.

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?

An output schema exists, so return values need not be enumerated, and the description correctly focuses on capabilities and paid-tier differences. The missing piece is behavior for the zero-required-parameter case, which is a real risk for a tool where an agent could call it with an empty object.

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%, including the note that charset suffixes are ignored and that extensions omit the leading dot, so the schema already carries the parameter meaning. The description's only added semantic is that the two parameters select different output shapes, which is a marginal contribution over the schema.

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 — matching file extensions to MIME types, categories, and default character encodings — and explicitly covers the reverse direction (returning extensions for a media type). No sibling tool in the list performs MIME/extension resolution, so the agent can distinguish it immediately.

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?

The two lookup modes are implied by the sentence structure (extension→MIME vs. MIME→extensions), which gives the agent a rough idea of when to use it. However, there is no explicit statement of when to prefer this over alternatives, no guidance on what happens when both parameters are supplied, and no note about the fact that neither parameter is required.

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