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

Annotations already cover readOnly/idempotent/non-destructive, and the description adds behavior annotations don't carry: the error contract (never raises a protocol error, returns {"error": ...} with fix guidance), the retry-safety implication, and a worked example of output 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?

Front-loaded with purpose, then example, then usage guidance, then error behavior — all load-bearing. The 'FREE.' fragment is a minor non-sequitur and the error paragraph is dense, but nothing is redundant.

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 pure computation with an output schema, everything an agent needs is present: correct inputs, expected output, error handling, and retry safety.

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 worked example (unit_cost 12 + target_margin_pct 60 → 30.0) demonstrates how the two parameters interact and clarifies margin-not-markup semantics, adding real value over the field docs.

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 first sentence states a specific verb and resource (compute the selling price for a target margin), and the description explicitly frames itself as the inverse of unit_economics, so an agent can discriminate it from siblings without opening schemas.

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 gives both sides of the routing decision: 'Use when the margin is the fixed requirement' and 'Not when the price is already set,' with unit_economics named as the inverse alternative.

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