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

Annotations already declare readOnlyHint, idempotentHint and closed-world behavior, and the description adds genuinely new context: the secret is used for this call only and is not stored, and decode performs temporal claim validation (exp, nbf, iat). That non-persistence guarantee is the key behavioral fact for a tool that accepts a signing key. It does not say what a failed verification returns (error vs. a flag), which keeps it below 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?

Three short sentences, front-loaded with the core purpose and then split cleanly by operation. The closing "No API key, no account and no sign up" is slightly promotional but earns its place by answering an auth prerequisite question. No filler or repetition of the title.

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?

There is no output schema, and the description compensates by naming both return keys, jwt for encode and decoded_payload for decode. Combined with the secret non-persistence note and the claim-validation behavior, an agent has what it needs to call this correctly. The nested payload object is described only as "the claims" in both schema and description, a minor remaining gap.

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 all four parameters are already documented, including the encode/decode enum and its meaning. The description largely restates that mapping (payload for encode, token for decode, secret as the HS256 key) rather than adding format or constraint detail beyond the schema. Baseline 3 applies when the schema carries the weight.

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 states a specific verb pair and resource ("Create or check an HS256 JSON Web Token") and immediately decomposes it into the two operations, encode and decode, with what each produces. No sibling in the list offers JWT handling, so there is no ambiguity about which tool to reach for.

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?

It gives clear operational routing: encode when you have a payload and want a token, decode when you have a token and want its claims, and it notes decode enforces exp, nbf and iat. It also removes a common pre-flight question by stating no API key or account is needed. It stops short of naming exclusions or an alternative tool, so it is a 4 rather than a 5.

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