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

Annotations already declare this is a non-read-only, non-destructive, non-idempotent, closed-world mutation. The description adds value beyond them by disclosing the two-step side effect (ack first, then submit) and the audience restriction (sender only). It stops short of saying what 'acks' means for the inbox item (removed? marked read?) or whether the reply can be recalled, which is the remaining gap.

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 sentences, each doing distinct work: what it does, audience/alternative for groups, alternative for threaded replies. The core action is front-loaded with zero filler.

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 tool with annotations present and no output schema, everything needed to select and call it is stated: identifier source, reply body, audience, and the two nearest alternatives. Only the post-success state (what the ack does to the inbox item, what the call returns) is unstated.

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 50%: a2a_task_id is documented in the schema as the inbox item's task_id, while `text` is untyped beyond string. The description adds that `text` becomes the reply body placed onto the DM and is delivered to the sender, which is useful but doesn't clarify length limits or whether it is plain text or markdown. Baseline 3 is appropriate given partial schema coverage.

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 ('Reply to an inbox DM by its id') plus the internal sequence ('acks it, then submits your reply text onto that DM'). It explicitly separates itself from two sibling operations (send_dm, reply_dm), so an agent can route correctly 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 explicit when-to-use and when-not-to-use: reply targets the sender only, and the description names the exact alternative for group answers ('send_dm to the group_id'). It also gives a condition for choosing reply over reply_dm ('wakes phone agents'), which is a real selection signal.

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