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

The annotations already establish readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuinely new behavioral facts: a hard refusal of private keys, chain (multi-certificate) handling with order verification, and the shape of the result set. With the annotation bar already met, this is solid added context, short only of failure-mode detail.

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?

Two sentences, front-loaded with the verb and resource, then the output inventory, then the one exclusion. No filler, no restatement of the title, and the most important constraint (private keys refused) is placed last where it reads as a hard rule rather than an afterthought.

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, so the description correctly carries the burden of enumerating return fields, which it does comprehensively. The only soft spot is the undefined term 'findings' and the absence of any statement about what happens when PEM parsing fails or a chain is out of order, which matters for a decoder that explicitly performs an order check.

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% and the single 'certificate' parameter is already documented down to the BEGIN/END delimiters and the 10-certificate cap. The description reinforces the chain case and adds the private-key refusal, but contributes no format, encoding, or length semantics beyond what the schema already states. 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?

The description uses a precise verb ('Decodes') with a specific resource ('pasted PEM certificate or chain') and enumerates exactly what is extracted: subject, issuer, validity dates, names, key, signature algorithm, fingerprints, extensions, findings, plus a chain order check. This is far more specific than the title alone. It does not explicitly contrast with the closest sibling, csr_decoder, so an agent must infer that distinction from the resource noun rather than being told.

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?

Usage is implied rather than stated: 'pasted' plus the PEM BEGIN/END format indicates the agent must already hold the certificate text, and 'Private keys are refused' rules out one input class. There is no explicit when-to-use versus alternatives such as ssl_check (live validation) or csr_decoder, and no guidance on what to do if decoding fails.

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