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

With no annotations, the description carries the full burden. It does add genuinely useful non-structural context—per-call pricing of 0.001 USDC via x402—but it says nothing about secret handling (storage/retention/logging), determinism, or which encoding is returned when, all material for a signing tool that takes a secret.

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 sentences, zero filler, with the operation stated first and the cost/payment caveat trailing where it belongs. Nothing is 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?

For a four-parameter tool with no output schema, the description adequately covers what is produced (hex and base64 HMACs) and the payment requirement. It leaves a small gap around whether both encodings are always returned together versus selected, and gives no guidance on failure modes.

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% and the schema itself documents algo options and the paymentProof 402 flow, so the baseline is 3. The description only echoes the message/secret pairing and output encodings; it adds no syntax, format, or default details 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?

States a specific verb and resource ("Computes an HMAC of a message with a secret") plus the output encodings, which cleanly separates it from siblings like hash, base64, hex_codec and evm_signature_verify. An agent knows exactly what operation it is selecting without opening the 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?

"for verifying webhooks and API requests" gives a concrete context of use, which is more than an implied purpose. It stops short of explicit routing (e.g. "if you have no secret, use hash instead") or any when-not condition, so it does not reach a 5.

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