Reply to a received message

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

Annotations only cover the generic safety profile (readOnly=false, destructive=false, openWorld=true, idempotent=false). The description adds real behavioral context beyond them: recipients and Re: subject are auto-filled, the original is not quoted, and the same allow-list/review/send-limit regime applies. It even discloses the return shape, which is non-trivial given no output schema.

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 and dense with useful facts; nothing is padding. The single long first sentence with two nested parentheticals is slightly cramped to parse, which keeps it just short of ideal.

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 mutation tool with no output schema, the description covers the pieces an agent actually needs: what gets auto-populated, what is not included, the send-limit/permission regime, and the return keys ({ message_id, warnings }). Attachment semantics are fully handled by the schema, so nothing material 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 100%, so the baseline is 3. The description still adds meaning: it explains the reply_all side effect precisely ('also To and Cc, except you') and reinforces that body_markdown is not auto-quoted, both of which go slightly beyond the schema wording.

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+resource (reply to a received message) and immediately scopes it (in the same thread). It names the sibling it supersedes ('Prefer this over send_message') and is clearly distinguishable from forward_message, so an agent can route without opening any 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 an explicit preference rule ('Prefer this over send_message for answers') plus the operating constraints that gate use: same allow list, review and send limit as send_message. Both when-to-use and the practical preconditions are stated rather than inferred.

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