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

Annotations already flag destructiveHint=true and openWorldHint=true, and the description adds real value beyond them: threading headers, 'Re:' subject, recipients and quoted original are set server-side, plus the two gating conditions that determine whether the call is allowed at all. It stops short of stating irreversibility or what happens on allowlist failure, so not 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?

Three dense sentences, front-loaded with the action, then the gating rules, then the consent trigger. Each sentence earns its place, though the final 'Only when the owner says "send"' fragment is terse enough to read as a tag rather than a complete rule.

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 9-parameter mutation tool with no output schema, the description covers the action, the alternates, the authorization gates and the server-derived fields an agent would otherwise try to set. Nothing essential to calling 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 description coverage is 89%, so the schema already carries most parameter meaning (baseline 3). The description adds non-schema context by stating that recipients, subject and quote are populated by the server, telling the agent there is no recipient/subject parameter to supply. It does not clarify reply_all vs. sender-only beyond what the schema says.

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+resource ('Sends a reply to a message in the same thread') and immediately distinguishes itself from the sibling reply_draft by describing the server-side threading behavior. An agent can tell it apart from send_message and reply_draft 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 explicit preconditions for use (owner enabled sending AND all recipients match send_allowlist) and names the fallback tool ('otherwise use reply_draft'). It also encodes the consent condition (owner must say 'send'). Nothing about when-to-use is left to inference.

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