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

Annotations cover the safety profile (readOnly, idempotent, closed-world), and the description layers on real behavioral detail beyond them: the exact return contents, structural-failure reporting (missing part, invalid base64url, alg none), and the commercial constraint of $0.05 per successful call over x402. The cost/auth disclosure is especially valuable for an agent deciding whether to invoke.

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?

The critical scope qualifier (no signature verification) is front-loaded in the first clause, and the price/auth note is correctly placed last. It is dense with parentheticals but every clause carries information; nothing is padding.

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 carries the full burden of describing returns, and it does: header, claims, signature length and algorithm, computed time-claim view, and structural error classes. Nothing an agent needs to call and interpret this correctly is missing.

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 both parameters are already documented, giving a baseline of 3. The description adds the semantic tie between the 'at' input and the exp/nbf evaluation ('against a supplied or current instant'), which is mildly useful but largely restates 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?

The description gives a specific verb (decode) and resource (JSON Web Token) and immediately scopes it ('without verifying it'), which no sibling tool overlaps. An agent can distinguish this from hash_generate or crypto_address_validate 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?

It states the decisive when-not condition explicitly: 'It does not and cannot verify the signature; do not use it to decide trust.' That prevents the most dangerous misuse. It does not name an alternative tool, but no sibling in the list performs JWT signature verification, so there is no better route to point at.

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