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

With no annotations supplied, the description carries the full burden, and it does disclose one meaningful trait: 'Pure CPU, instant' tells the agent this is a local, side-effect-free, non-networked operation with no auth or rate-limit concerns. It omits other behavioral details that matter for a cipher, such as how non-alphabetic characters or key casing are handled and whether output is a plain string.

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?

Two short sentences with zero filler, front-loading the operation and following with a cost/performance note. Nothing could be removed without losing information.

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 three-parameter tool with no output schema and no annotations, the description covers purpose and cost but leaves the return value and edge-case behavior unexplained. An agent can call it correctly, but not predict its output shape or how it treats non-letter characters.

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 all three parameters (key, text, decode) are already documented, which sets the baseline at 3. The description reinforces that the key is a Vigenere keyword rather than an arbitrary secret, but adds no format, length, or validation detail 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 names a specific verb pair (encrypt/decrypt) and a specific resource (Vigenere keyword cipher), so an agent can distinguish it from caesar_cipher, atbash, rot13, and morse by the cipher name alone. It stops short of explicitly contrasting with those siblings, but the cipher name is itself a strong discriminator.

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?

There is no when-to-use guidance, no mention of prerequisites such as the key being required for meaningful encryption, and no routing to or away from the many sibling cipher tools. The only implicit signal is that the tool both encrypts and decrypts, which the schema's decode flag already conveys.

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