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

Annotations already cover the safety profile (readOnlyHint, destructiveHint=false, idempotentHint=false), and the description reinforces it while adding genuinely new behavioral context: per-call pricing, the 25-free tier, free failed calls, and the explicit warning that repeats are charged (which meaningfully explains the non-idempotent hint). It stops short of describing error behavior or the return shape.

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, semantics, example, then billing are ordered sensibly and front-loaded. The billing sentences are longer than strictly needed but earn their place because cost-per-call is unusual and affects agent retry behavior.

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 two-parameter, read-only decode tool with full annotation coverage and no output schema, the description covers semantics, an example, and cost. The only meaningful omission is that it never states what the tool returns (the decoded string), which is low-risk but not stated.

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 baseline is 3, but the description goes further by clarifying the form-encoding semantics of plus ('treating + as a space when plus=true') and providing a concrete worked example (text=a%20b%26c, plus=false). That is real added meaning over the terse schema strings, though it doesn't note that plus is typed as a string rather than a boolean.

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 opens with a specific verb+resource: 'Decode percent-encoded text,' which is unambiguous and immediately distinguishable from the url_encode sibling by verb. It does not, however, contrast itself with the related url_parse sibling, so an agent choosing between decoding and parsing gets no explicit routing.

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 to read encoded links and form data' supplies a clear usage context, but there is no when-not guidance and no mention of url_parse or url_encode as the alternative for the adjacent tasks. The guidance is implied rather than comparative.

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