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

Annotations already declare readOnlyHint=true, destructiveHint=false, and idempotentHint=false; the description adds context the annotations cannot: the key is used only for this call and not stored, each call costs $0.002 via x402 USDC or a card key, the first 25 are free, and failed calls are free. The cost-and-repeat warning also explains in plain language why idempotentHint is 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 operation and algorithms, then usage, then privacy and pricing – a sensible order. The pricing sentence is dense but carries real decision-relevant information; only 'Reads or computes only and changes nothing' is largely redundant with the readOnlyHint annotation.

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 partially compensates by stating the digest is returned in hex and base64, though it is slightly ambiguous whether both are always returned or one is selectable. Combined with full parameter documentation and the cost/key-retention disclosure, an agent has enough to call it 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 all three parameters are already documented in the schema, including the "sha256 (default)" enumeration. The description only restates the default algorithm and adds a trivial message=hello, key=secret example, which is marginal value beyond the structured fields. Baseline 3 applies.

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 gives a specific verb+resource (compute an HMAC of a message with a key) and enumerates the supported algorithms and output encodings, so an agent knows exactly what it produces. It does not explicitly distinguish itself from the sibling `hash` tool (keyed vs unkeyed), which is the one remaining ambiguity in a dense sibling list.

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?

"Use to check webhook signatures" gives a concrete triggering context, which is more than most siblings offer. It stops short of naming alternatives (e.g., use `hash` when no key is involved) or stating exclusions, so the routing guidance is clear but not complete.

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