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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint, establishing behavioral safety. The description adds value by noting the tool is 'keyless, offline', implying no external dependencies or authentication. However, it does not cover edge-case behaviors (e.g., error handling for invalid numbers). No contradiction with 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?

The description is a single sentence with two clauses, clearly stating the main purpose and available styles. It is front-loaded with the core action and includes additional context without extraneous words. Perfectly concise.

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 simple formatting tool with well-documented parameters and good annotations, the description covers the primary functionality and key constraints (keyless, offline). It lacks explicit mention of the return type, which is minor given the tool's simplicity. Could be slightly more complete but mostly adequate.

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 input schema already documents each parameter fully. The description merely restates the style options and adds the 'keyless, offline' context, which does not enhance parameter understanding beyond the schema. Baseline score of 3 is appropriate.

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 clearly states the verb 'Format' and the resource 'a number' with explicit formatting styles (decimal, percent, compact). The note 'keyless, offline' further distinguishes it from similar tools like format_currency, making its purpose specific and unambiguous.

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?

The description provides no guidance on when to use this tool versus alternatives such as format_currency. It lists available styles but does not explain the context or conditions for selecting one style over another, nor does it mention when not to use this tool.

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