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

With no annotations, the description carries the full burden, and it does disclose a non-obvious behavioral fact: the result contains a calculator_url that must be surfaced as a clickable markdown link. Since there is no output schema, this is information the agent could not otherwise obtain. It still omits edge-case behavior (e.g., division by zero when old=0), which keeps it short of 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?

The mode definitions are compressed into a single front-loaded sentence, followed by the rendering directive. Every clause carries information, though the capitalized 'IMPORTANT: always show...' sentence is somewhat verbose for a single instruction.

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 five-parameter, single-required-field pure-computation tool with no annotations and no output schema, the description covers the mode semantics and the one notable return value (calculator_url). The main residual gap is edge-case/error behavior, which is minor for a deterministic calculator.

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%: x, y, old, new and the mode enum are each documented in the schema itself, including their per-mode meaning. The description's mode formulas largely restate what the schema already provides, so the baseline of 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?

The description names a specific operation (percentage calculation) and enumerates its three distinct modes with plain-language formulas (percent_of, what_percent, percent_change), which immediately separates it from the many domain-specific calculators in the sibling list such as calculate_tip or calculate_capital_gains_tax.

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 what each mode computes, which implicitly tells the agent which mode to pick for a given phrasing ("x percent of y" vs "x is what percent of y" vs "change from old to new"). However, it gives no guidance on when to prefer this generic tool over narrower siblings like calculate_tip, and no exclusions or prerequisites.

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