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

Annotations already declare readOnly/destructive/openWorld/idempotent, so the safety bar is low, and the description adds genuinely useful behavior: results are unauthenticated ("never verifies a signature"), it is free during validation, and no key/credits/wallet are required. Input-limit behavior is only gestured at vaguely ("Existing source and input limits apply"), which keeps it from a 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?

Front-loaded with the core action and the critical "never verifies a signature" caveat, and it is short. The cost information is split across two sentences ("FREE during validation" / "No key, credits or wallet") with mild redundancy, but nothing is padded.

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?

An output schema exists so return values need no explanation, and annotations cover the safety profile; the description fills the remaining gaps (no signature verification, free/credit-free operation). The only omission is a concrete statement of the "existing source and input limits" it references, which is too vague to act on.

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 coverage is 67%: requestId and maxPriceMicroUsd carry "legacy, ignored in free mode" descriptions, while token and atUnixSeconds have none. The description partially compensates by tying the optional timestamp comparison to atUnixSeconds, but adds no format or semantic detail (e.g., what timestamps are compared, JWT format expectations). Baseline 3 is appropriate.

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 untrusted JWT claims") plus a scope qualifier ("optionally compare timestamps"). It is clearly distinct from the other security_* siblings (secret_scan, cookie_audit, csp_audit), though it never names or contrasts an alternative explicitly.

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?

The line "never verifies a signature" implicitly tells the agent not to use this for trust/verification decisions, and the cost framing ("FREE during validation. No key, credits or wallet") signals a low-friction context. But there is no explicit when-to-use/when-not-to-use guidance or named alternative.

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