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

Goes well past the annotations: states the key is ephemeral and not stored, discloses per-call pricing ($0.01 via x402 USDC or card key, first 25 free, failed calls free), and explains the consequence of idempotentHint=false by warning that repeats are not free. The read-only/no-side-effect claim is consistent with readOnlyHint=true and destructiveHint=false.

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 the core operation and algorithm list, then the use case, then cost. The billing sentences are long and slightly rushed together ('Price $0.01/call... first 25 free; failed calls free'), but every clause carries decision-relevant 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?

With no output schema, the description usefully discloses that digests are returned in hex and base64, and it covers algorithms, key handling, cost, and side-effect profile. It leaves minor ambiguity about whether both encodings are returned in a single response or one is selectable, since no encoding parameter exists.

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 the three parameters are already documented, and the description only restates the sha256 default and adds a trivial 'message=hello, key=secret' example. The genuinely useful addition — that digests come back in hex and base64 — is about output rather than parameters, so the baseline of 3 is right.

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 and resource ('Compute an HMAC of a message with a key') plus the supported algorithms and output encodings, which cleanly separates it from the unkeyed `hash` sibling. It never names `hash`/`pack_hash_toolbox` explicitly, so routing is inferred rather than declared.

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?

Offers one concrete use case ('Use to check webhook signatures') and shows an example invocation, which implies when to reach for it. There is no when-not guidance and no named alternative for plain (unkeyed) hashing, so the agent must infer the boundary with `hash`.

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