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

Annotations already declare non-read-only, non-destructive, non-idempotent, closed-world, so the safety bar is partly covered. The description adds genuine context beyond them: the audience who will see the message (owner and other agents in a group), that basic markdown renders, and that a shared connection requires sending agent_key on every call. It omits failure behavior and return content, which keeps it from a 5.

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?

The action is front-loaded, followed by audience, the alternative-channel rule, formatting behavior, and the auth note. Every sentence carries information, though the formatting aside is the least essential of the four.

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 three-parameter, fully documented write tool with no output schema, the description covers the key unknowns: who sees the message, how the text is rendered, and the shared-connection auth rule. It does not describe what a successful call returns or what happens on an invalid chat_id, so it is strong but not exhaustive.

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 the per-field meaning of text, chat_id and agent_key is already documented, and the baseline of 3 applies. The description only reinforces the agent_key requirement for shared connections and hints at text formatting, adding marginal meaning rather than new semantics.

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 first sentence gives a concrete verb and resource: "Post your answer into one of your AgentMsgr chats," which clearly separates it from the read-oriented siblings (read_chat, get_new_messages, list_chats). It never names a sibling explicitly to contrast with, so it falls short of 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?

It supplies a real decision rule — "Use this instead of email when you have it" — plus a style directive ("Lead with the answer or result"). It does not say when not to use it or which sibling to prefer if a chat is unknown (e.g., list_chats first), so it stops short of full routing guidance.

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