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

Annotations already declare readOnlyHint/idempotentHint, but the description adds non-obvious context: bodies are returned as plain text, and the call 'records a read signal' — a side effect an agent would never guess from the schema. It does not disclose auth requirements or what happens for missing/deleted IDs. Note the mild tension between 'record a read signal' (a state change) and readOnlyHint=true, but this is a view-tracking side effect rather than a data mutation, so it reads as helpful disclosure rather than a contradiction.

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?

A single front-loaded sentence with no filler. The batch limit and output format appear early where they matter most.

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 needed to say more about the return shape beyond 'plain-text bodies' — ordering, behavior for IDs that no longer exist, and how `include_replies` alters results are all missing. For a 3-parameter tool with zero schema documentation, this leaves real gaps, though the core contract is conveyed.

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 0%, so the description must carry the parameter burden — and it only partially does. It explains `ids` (capped at 20, plain-text bodies) but says nothing about `useful_ids` (a highly non-obvious feedback-style parameter) or `include_replies`, both of which an agent must guess at.

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 ('fetch'), a specific resource ('full plain-text bodies'), and the scope ('up to 20 post IDs selected from timeline'). An agent can distinguish this from the sibling `timeline` tool, which presumably lists posts rather than hydrating bodies. It is not explicitly contrasted with any sibling, which keeps it from 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?

'selected from timeline' implies the intended workflow (call `timeline` first, then hydrate bodies here), but this is inference rather than an explicit statement. There is no when-not guidance, no mention of what to do for posts not in the timeline, and no named alternative.

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