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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false, openWorldHint=true), so the bar is lower, yet the description still adds real behavioral context: the 1-credit cost and the fact that retry/caching behavior and related headers for details=true are gated behind paid plans. It doesn't say how a non-numeric or unassigned code is handled, which keeps it from a 5.

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?

Everything is packed into one long compound sentence with an awkward trailing parenthetical cost note, and the leading repetition of the tool name/title is wasted space. The useful information is present but not cleanly front-loaded or segmented.

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 an output schema present, the description needn't enumerate return values, and for a simple two-parameter lookup tool it covers purpose, cost, and the commercial gating caveat. The only real gap is behavior on invalid/unknown codes, which is minor for this tool's complexity.

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, and the description adds meaning beyond the schema by disclosing the paid-plan gating on the extra details fields and by emphasizing that the code should be numeric. It does not describe the effect of combining the two parameters or any range validation on code, so it stops short of full parameter explanation.

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 names a specific verb and resource — 'decodes any HTTP status code by returning its standard name and detailed description' — which is unambiguous and clearly distinct from neighbors like MIMETypeLookup or PortNumberLookup. It loses a point because the opening repeats the name/title verbatim ('HTTP Status Code Lookup: HTTP Status Code Lookup decodes...') instead of using that space to sharpen the purpose.

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?

It explains how to call the tool ('Provide a numeric code', 'add details=true') but gives no when-to-use/when-not guidance or alternatives. Usage is implied by the parameter instructions rather than stated as decision criteria, which is the minimum viable level.

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