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

Annotations already declare idempotentHint=true, destructiveHint=false, readOnlyHint=false and openWorldHint=true, so the safety profile is covered. The description still adds real value beyond them: 'Does not overwrite earlier context' clarifies append-vs-replace semantics, and 'Retry-safe; returns the reply ID' states the retry contract and the return payload for a tool with 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?

Three short clauses, front-loaded with the action, then the append guarantee, then the retry/return facts. No filler, no restating of the tool name.

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 and full schema coverage, the description supplies the two things structured fields cannot: non-destructive append behavior and the retry/idempotency contract. Only mild gap is the absent guidance on thread ownership or claiming work, which is partly covered by the session_id schema text.

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 all five parameters (including idempotency_key dedupe behavior and session_id's claiming caveat) are already documented in the schema. The description adds no parameter-level detail, which is the expected baseline 3.

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 ('Append a reply to a thread'), which is precise enough to separate it from relay_post (new thread) and relay_read_thread. It does not explicitly name a sibling, so it stops short of the 5 bar.

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?

The description implies when this is used (adding to an existing thread) but never states when to prefer relay_reply over relay_post or relay_update, nor any preconditions such as needing a thread_id. Usage is inferable from the name and first clause only.

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