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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description reinforces these with 'Every call is read-only and idempotent' and adds critical behavioral context about error handling: 'on invalid, missing, or malformed input this tool never raises a protocol error — it returns {"error": "<what is wrong and how to fix it>"}.' This goes beyond annotations to inform the agent about error recovery. No contradiction.

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 well-structured and each sentence adds value: purpose, example, usage guidance, and error behavior. It is slightly longer than necessary due to the example and the standalone 'FREE.' line, which is somewhat distracting but doesn't harm clarity. Overall, it is concise and front-loaded with the core purpose.

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 3 parameters (1 required), full schema coverage, annotations, and an output schema (evidenced by the example), the description is complete. It covers purpose, usage boundaries, error handling, safety profile, and example output. Nothing critical is missing for an agent to correctly select and invoke this tool.

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% with clear descriptions for each parameter. The description adds a concrete example showing typical input and output ('Typical input {"bill": 86.40, ...} returns {"tip": 17.28, ...}'), which helps the agent understand how parameters map to results, providing meaning beyond the schema.

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 starts with 'Compute the tip and per-person split for a bill,' which clearly states the verb and resource. It explicitly distinguishes from the sibling tool 'budget_split' by saying 'Not for recurring household budgeting (budget_split).' This meets the highest criteria for specificity and differentiation.

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 explicit guidance: 'Use for one bill shared among a group. Not for recurring household budgeting (budget_split).' This tells the agent exactly when to use this tool and when to use the alternative, satisfying the requirements for when-to-use and when-not-to-use.

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