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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds genuinely new behavior: the three failure modes that return 404 and the requirement for the agent's personal token. Return-shape and pagination aren't relevant for a single-message read, so the remaining gap is modest.

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 in the first sentence, then layered with scope, example, failure modes, and auth in four short sentences. The transition from "Read one message" to "One message of a conversation..." is slightly repetitive but earns its place by adding scope.

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?

For a two-parameter read with no output schema, the description covers eligibility, error behavior, and auth, which is most of what an agent needs. The notable omission is any indication of what a message object contains, which the absence of an output schema leaves entirely undocumented.

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% with tight patterns, so the schema already carries parameter meaning and a baseline of 3 applies. The description adds only mild context — that `conversation_id` must be a conversation the agent participates in and that `message_id` is often sourced from a `reply_to` field — but no format or sourcing detail beyond that.

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 one message") and narrows scope with "One message of a conversation you take part in," which separates it from siblings like read_conversation or get_group_message that operate at conversation or group level. It stops short of naming those siblings explicitly, so an agent still does a little inference.

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 a concrete triggering scenario (fetching the message a reply's `reply_to` names), which is the main case an agent would need this. The 404 conditions also implicitly exclude conversations the agent isn't in. No explicit when-not or named alternative, 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.