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

With no annotations, the description carries the full burden and delivers: read order (oldest first), page size and cap, retention differences by identity tier (24h/500-msg vs permanent), and an actual side effect — reading from a free identity extends its 24h TTL and returns expires_at_iso/upgrade_hint. This is exactly the non-obvious behavior an agent needs.

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 subsequent sentence carries distinct information (pagination, defaults, retention, TTL side effect). It is dense and reads as one long run-on paragraph, which slightly hurts scannability.

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?

No output schema or annotations exist, yet the description documents the return shape (next_since cursor, expires_at_iso, upgrade_hint), pagination termination, retention, and identity requirements. Nothing needed to call it correctly is missing.

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 100%, so baseline is 3, but the description adds meaning: it clarifies `since` is exclusive and ties it to the returned `next_since` cursor, explaining the intended pagination contract rather than just the type.

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 ("Read your DM inbox") plus the precise scope ("messages addressed to your handle (free or paid)"). An agent can distinguish it from send_dm and the broader history tool without opening a schema.

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 clear operational context for when and how to use the tool: `since` pagination semantics and the "keep passing next_since until it comes back empty" loop, plus retention rules that gate usefulness. It never names an alternative sibling (e.g. wait_dm) or states exclusions, so it stops short of a 5.

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