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

Annotations cover safety (readOnlyHint, destructiveHint=false) but the description adds real behavioral context beyond them: the token is not stored, iat/nbf/exp are surfaced as dates with an expired flag, calls are charged so repeats are not free (consistent with idempotentHint=false), and failed calls are free. It also states the compute-only, changes-nothing semantics explicitly.

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 purpose, then warning, usage, example, and cost. The pricing/key mechanics sentence ('$0.003/call via x402 USDC or a card key...') is long and commercial in tone, but it does encode a real constraint (per-call cost, 25 free).

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?

No output schema exists, so the description must carry the return burden — and it does describe what is returned (header, payload, dates, expiry status). It does not say what happens on a malformed token beyond implying failures are free, a minor gap for a single-param tool.

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?

Only one parameter and schema description coverage is 100%, so the schema already carries the semantics; baseline is 3. The description contributes a concrete example token, which helps with format expectations but adds no new constraint beyond what the schema states.

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 ('Decode a JSON Web Token's header and payload') plus the key scope qualifier ('without verifying the signature'). This clearly separates it from the sibling base64_decode, which would otherwise look like a substitute.

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?

Gives an explicit when-to-use ('Use to debug auth flows') and a strong when-not/warning ('Do not use the result to trust a token'). It stops short of naming the alternative sibling (e.g. base64_decode) for raw payload extraction, so it is clear context rather than full routing.

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