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

Annotations already cover the safety profile (readOnlyHint=true, openWorldHint=true), so the bar is lower. The description adds genuinely useful behavioral context not in the annotations: the per-call price, the x402/eip155:8453 payment rail, and the shared 5-call/day free quota, all of which affect whether an agent should invoke it.

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?

A single dense sentence front-loads the core operation and algorithm list before the pricing clause; nothing is padded. The semicolon-chained fragments are efficient, though slightly compressed.

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 6-parameter, no-output-schema utility with light annotations, the description covers the operation, algorithm and encoding options, and the billing model. The one omission is that it never states what is returned (the digest string), but that is minor for a tool whose contract is self-evident.

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 schema already documents all six parameters, including defaults (sha256, hex, utf8). The description largely restates the algorithm and encoding options without adding syntax or format detail beyond the schema, so the 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?

States a specific verb (hash or HMAC) and resource (a string), and enumerates the supported algorithms and encodings, which lets an agent distinguish it from encoding siblings like base64 or uuid. It stops short of explicitly naming any sibling it should not be confused with, so it is clear but not fully differentiated.

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?

Usage is implied by the operation and its parameter list, and the description usefully discloses the commercial terms ($0.001 per successful call, 5 free calls/day). However, it gives no when-to-use vs when-not guidance and never routes the agent to or away from a sibling tool.

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