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

The description goes well beyond the readOnly/idempotent annotations by explaining the shift mapping, keyword repetition, case preservation, pass-through of non-letters, and the fact that non-letters do not advance the keyword. This is exactly the behavioral detail an agent needs.

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 compact and front-loaded with the core action, then expands into the few algorithmic details that matter. Every sentence contributes useful information, and the source link is a harmless addition.

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 single-algorithm transform tool, the description covers all necessary context: operation, keyword semantics, character handling, and decode behavior. Since an output schema exists, not detailing the return shape is acceptable.

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?

Although the schema already covers all three parameters, the description adds meaningful semantics: how the keyword repeats, how letters map to shifts, how non-letters are handled, and how decoding reverses the process. This is valuable beyond the schema's short field descriptions.

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 the exact operation (encode or decode) and the exact algorithm (classic Vigenere cipher using a keyword). This is specific enough to distinguish it from cipher siblings like hopi_caesar_cipher and hopi_cipher_identifier.

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?

Usage is only implied by the algorithm name and behavior; there is no explicit statement of when to choose this tool over an alternative such as Caesar cipher. An agent can infer the use case, but the description does not provide explicit routing or exclusions.

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