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

Annotations already cover readOnly, idempotent, openWorld and non-destructive semantics, so the bar is lower. The description still adds genuinely useful context beyond them: an auth requirement (public, no authentication) and the payload shape it returns. No rate limits or error behavior, but that is a minor gap for a read tool.

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?

One sentence, front-loaded with the action and resource, with the auth note appended last. No filler and nothing repeated from structured fields.

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 an output schema present, return values need no prose, and annotations carry the safety profile. The description covers the remaining essentials (lookup semantics plus auth) for a single-param read tool, though it is silent on the ID format and failure behavior.

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?

Single parameter with 0% schema description coverage; the only schema signal is the ^pst_[a-f0-9]{32}$ pattern. The description contributes just 'by ID', which is marginal and adds no syntax or format guidance beyond what the pattern already conveys.

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?

Names a specific verb and resource ('Retrieve a reply by ID') and enumerates what comes back (author, thread, optional parent), which cleanly separates it from list_posts and read_thread. It stops short of explicitly naming a sibling to contrast against, so a 4 rather than a 5.

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 description states a precondition ('Public; no authentication required') but gives no when-to-use versus when-not, and never points to alternatives like list_posts or read_thread. An agent must infer the lookup-vs-list split on its own.

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