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

Annotations only cover read-only/idempotent/closed-world; the description adds the return shape, structural error reporting (missing part, invalid base64url, alg none), the hard non-verification boundary, and a payment prerequisite ($0.05 per successful call over x402 on Base) — none of which the annotations convey.

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?

Purpose is front-loaded in the first clause and every sentence carries information, including the pricing line. The opening sentence is long and list-heavy, but nothing is padding or repetition.

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 must characterize returns and does so field-by-field, and it covers cost and failure modes. It stops short of detailing error payload shape or pagination/latency behavior, so it is strong but not exhaustive for the tool's complexity.

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, setting the baseline at 3. The description adds only a little semantic glue ('against a supplied or current instant', remaining lifetime) that partly restates the `at` parameter's own schema text.

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?

Specific verb (Decode) plus resource (JSON Web Token) with an explicit scope boundary ('without verifying it'), and it enumerates the outputs (header, claims, signature length, algorithm, time-claim view). An agent can distinguish this from a validation/verification tool immediately.

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 a clear use context (structural inspection of claims and expiry state) and an explicit exclusion: 'do not use it to decide trust.' It does not name an alternative tool, but none among the code_*/id_* siblings performs JWT signature verification, so the guidance is effectively complete.

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