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

Annotations already declare readOnlyHint/idempotentHint/destructiveHint, so the restatement of read-only and idempotent is largely redundant. However, the description adds genuinely new behavior: the tool never raises a protocol error and instead returns an {"error": ...} payload, plus the retry guidance. That error contract is the valuable addition beyond annotations.

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 and a worked example, then usage, then errors — a sensible order. The error paragraph is somewhat long-winded (repeats the error example twice conceptually), and "FREE." is promotional filler, but nothing is confusing.

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 and the description still sketches the return shape and the error shape, covering the two possible response modes. For a three-parameter, single-purpose calculator, an agent has everything needed to call it and interpret failures.

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%, so the baseline is 3; the description earns above baseline by showing a concrete parameter->output mapping (bill 86.40, tip_pct 20, people 4 -> tip 17.28, per_person 25.92), which clarifies units and rounding intent. Constraint details (bill>0, people>=1) are repeated from the schema rather than added.

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?

Opens with a specific verb+resource: "Compute the tip and per-person split for a bill," and the example input/output makes the exact computation unambiguous. It also names the sibling it is not for (budget_split), so an agent can route without opening either schema.

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?

Explicitly scopes usage ("one bill shared among a group") and provides an exclusion with the alternative named: "Not for recurring household budgeting (budget_split)." That is the when/when-not/alternative triad complete.

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