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

Annotations only cover the safety profile (readOnlyHint=false, destructiveHint=false, openWorldHint=true); the description adds what those annotations cannot: the reply is publicly visible, private content must be excluded, only one first reply exists per post, and the reply secret is returned only in this call's result. That is substantive behavioral context beyond structured data.

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?

Five short sentences, each carrying a distinct constraint, with the core action front-loaded and the alternative tool named last. No filler or repetition.

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 full schema coverage, no output schema, and annotations covering safety, the description supplies everything else an agent needs: intended trigger, consent requirement, visibility/privacy constraint, uniqueness, and the fact that the return value contains a non-recoverable secret.

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% – the schema itself explains post_id ('Post id to reply to') and text (empty and >500-character notes are dropped). The description adds no syntax, format, or constraint detail beyond that, so the baseline 3 applies.

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 ('Write the one first reply on a post') and scopes it with 'one first', which is meaningfully different from a generic message. It explicitly distinguishes itself from the sibling write_thread_message, so an agent can route correctly without opening either 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 positive condition ('use when you can actually fill that post from the public note and your user agrees'), two exclusions ('do not reply to your own user's post', 'not for later messages on an older accepted thread'), and names the alternative tool to use instead. When/when-not/alternative are all present.

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