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

Annotations already declare readOnly/idempotent/non-destructive, and the description goes well beyond them: exact-one-of input constraint with 422 invalid_request, 422 invalid_base64 for malformed input, no echo/no logging guarantees, latency profile (keccak256 ~2 s per MB), pricing and a 10-calls/day free quota. That is unusually rich behavioral context.

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?

Front-loaded with purpose and the sibling redirect, with each clause carrying distinct information. It is dense with pricing/quota/docs-link tail matter, but nothing is genuinely redundant.

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?

Given an output schema exists, the description need not detail return shape, yet it still notes the digest format (hex of UTF-8 text, or base64 bytes). Input constraints, errors, timing and quotas are all covered, leaving no material gap for calling the tool correctly.

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%, so the baseline is 3; the description still adds value by stating the mutual-exclusion rule ('Send exactly one of text or base64') and the failure modes, which the schema's maxLength/anyOf does not convey.

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?

States a specific verb+resource ('Hash text or bytes') and enumerates the algorithm family, then explicitly names the sibling it is not (hmac_sign_verify). An agent can distinguish it from hmac_sign_verify, image_hash and text_encode without opening any schema.

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?

Routes the agent away to the alternative with the selecting condition ('For a keyed HMAC use hmac_sign_verify') and adds a suitability caveat ('md5 and sha1 are for checksums, not security') that steers algorithm choice. This is explicit when-to-use and when-not guidance.

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