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

Annotations already cover readOnly, idempotent, openWorld, and non-destructive hints. The description adds valuable context beyond annotations: rejection rules for non-terms and leading zeros, input size limit, pricing (paid per successful call), and an alternative HTTPS endpoint. It does not describe the return format (e.g., integer index), but the safety profile is fully covered by 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 front-loaded with the core purpose, then adds indexing details, rejection rules, input size, pricing, and an alternative endpoint. Every sentence adds value, though the pricing and alternative endpoint might be considered slightly extraneous for an MCP tool. Still, it is well-structured and efficient.

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 one-parameter lookup tool with full safety annotations but no output schema, the description is nearly complete: it covers purpose, input constraints, rejection behavior, and invocation cost. The only minor gap is that it does not explicitly state the return type (presumably an integer index), though this is strongly implied by 'smallest index n'.

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?

With 0% schema description coverage, the description must compensate. It does: it explains that value is a decimal integer of at most 600 digits, and that numbers with leading zeros or non-terms are rejected. The schema enforces a pattern and maxLength, but the description adds crucial semantic constraints (leading zero rejection) not captured in the schema. It could explicitly state that value is passed as a string, but the schema already indicates type string.

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 states a specific verb+resource: finding the smallest index n such that the hexanacci number H(n) equals value. It explicitly clarifies indexing (0 is index 0, 1 is index 5), which distinguishes it from sibling tools like hexanacci (which likely computes H(n) from n) and other index tools (e.g., pentanacci_index, tetranacci_index). The purpose is 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 clear context: it rejects numbers that are not terms or have leading zeros, and limits input to a decimal integer of at most 600 digits. This implicitly tells the agent when to use it (valid hexanacci terms only), but it does not explicitly name alternatives or when to prefer other tools. However, the purpose is clear enough that an agent can infer usage.

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