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

Annotations already declare readOnly, idempotent, non-destructive and open-world, so the safety profile is covered; the description adds genuinely new context by flagging that JWT decoding is unverified and by disclosing the rate limit and payment mechanism (x402 via _meta or X-API-Key). It does not cover error behavior for unsupported ops, which keeps it below a 5.

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 capability list is front-loaded and dense, but it is a single run-on sentence of slash-separated tokens with the commercial terms appended in brackets, which reads more like a marketing tagline than structured documentation. No sentence is truly wasted, yet the layout makes scanning harder.

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 an output schema present, return values need no description, and the definition covers the op vocabulary, pricing constraints, and a key behavioral caveat (unverified JWT). For a 4-parameter multi-operation tool it is nearly complete, missing only error-handling and per-op input expectations.

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, and the description goes beyond the schema by enumerating many more valid 'op' values than the schema's single example ('selector') and implicitly scoping 'key' to HMAC operations. It still does not explain the interaction of inputEncoding with specific ops.

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 enumerates concrete capabilities — sha256/sha1/sha512/md5/keccak256, function selector, HMAC-SHA256, base64(url), hex, URL encoding, UUID, JWT decode, EVM checksum — so an agent can tell it is a general hashing/encoding utility rather than an ABI tool like abi_encode or a file digest tool like content_hash. It never states a clean verb+resource phrase, but the capability list functions as one.

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 or when-not-to-use guidance and no reference to siblings such as abi_encode, calldata_decode, or content_hash, which overlap with 'selector' and hashing operations. The only routing information given is commercial (free tier, then paid), not semantic.

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